1. Cellular Reproduction: Cells from Cells
Chapter 8
Cellular Reproduction: Cells from Cells

2. Biology and Society: Virgin Birth of a Dragon
In 2002, zookeepers at the Chester Zoo were surprised to discover that their Komodo Dragon laid eggs.
The female dragon had not been in the company of a male.
The eggs developed without fertilization, in a process called parthenogenesis.
DNA analysis confirmed that her offspring had genes only from her.
© 2013 Pearson Education, Inc. 2

3. Biology and Society: Virgin Birth of a Dragon
A second European Komodo dragon is now known to have reproduced
asexually, via parthenogenesis, and
sexually.
© 2013 Pearson Education, Inc. 3

4. Figure 8.0
Figure 8.0 Komodo dragons can reproduce using two different types of cell division. 4

5. WHAT CELL REPRODUCTION ACCOMPLISHES
may result in the birth of new organisms but
more commonly involves the production of new cells.
When a cell undergoes reproduction, or cell division, two “daughter” cells are produced that are genetically identical
to each other and
to the “parent” cell.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling can be prevented if each gamete has only half the genetic material of the adult cells.
2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells holds great potential but is variously restricted and regulated.
3. As the authors note, biologists use the term daughter to indicate offspring and not gender. Students with little experience in this terminology can easily become confused.
Teaching Tips
1. The authors do not use the word clone in this chapter. You might wish to point out to your students that asexual reproduction produces clones.
2. Consider pointing out that asexual reproduction is common in prokaryotes and single-celled organisms.
3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends on the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg, donate a nucleus, and activate development of the egg. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus, eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and support growth.)
4. Virchow’s principle of “Every cell from a cell” (not specifically addressed in this chapter) is worth thinking through with your class. Students might expect that like automobiles, computers, and cell phones, cell parts are constructed de novo and cells are assembled. In our society, few nonliving products are generated from only existing products (try to think of such examples). For example, you do not need a painting to paint or a house to construct a house. Yet, this common expectation exists in biology. 5

6. WHAT CELL REPRODUCTION ACCOMPLISHES
Before a parent cell splits into two, it duplicates its chromosomes, the structures that contain most of the cell’s DNA.
During cell division, each daughter cell receives one identical set of chromosomes from the lone, original parent cell.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling can be prevented if each gamete has only half the genetic material of the adult cells.
2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells holds great potential but is variously restricted and regulated.
3. As the authors note, biologists use the term daughter to indicate offspring and not gender. Students with little experience in this terminology can easily become confused.
Teaching Tips
1. The authors do not use the word clone in this chapter. You might wish to point out to your students that asexual reproduction produces clones.
2. Consider pointing out that asexual reproduction is common in prokaryotes and single-celled organisms.
3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends on the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg, donate a nucleus, and activate development of the egg. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus, eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and support growth.)
4. Virchow’s principle of “Every cell from a cell” (not specifically addressed in this chapter) is worth thinking through with your class. Students might expect that like automobiles, computers, and cell phones, cell parts are constructed de novo and cells are assembled. In our society, few nonliving products are generated from only existing products (try to think of such examples). For example, you do not need a painting to paint or a house to construct a house. Yet, this common expectation exists in biology. 6

7. WHAT CELL REPRODUCTION ACCOMPLISHES
Cell division plays important roles in the lives of organisms.
Cell division
replaces damaged or lost cells,
permits growth, and
allows for reproduction.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling can be prevented if each gamete has only half the genetic material of the adult cells.
2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells holds great potential but is variously restricted and regulated.
3. As the authors note, biologists use the term daughter to indicate offspring and not gender. Students with little experience in this terminology can easily become confused.
Teaching Tips
1. The authors do not use the word clone in this chapter. You might wish to point out to your students that asexual reproduction produces clones.
2. Consider pointing out that asexual reproduction is common in prokaryotes and single-celled organisms.
3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends on the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg, donate a nucleus, and activate development of the egg. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus, eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and support growth.)
4. Virchow’s principle of “Every cell from a cell” (not specifically addressed in this chapter) is worth thinking through with your class. Students might expect that like automobiles, computers, and cell phones, cell parts are constructed de novo and cells are assembled. In our society, few nonliving products are generated from only existing products (try to think of such examples). For example, you do not need a painting to paint or a house to construct a house. Yet, this common expectation exists in biology. 7

8. 8 FUNCTIONS OF CELL DIVISION Cell Replacement Growth via Cell Division
Figure 8.1a
FUNCTIONS OF CELL DIVISION
Cell Replacement
Growth via Cell Division
Colorized SEM LM
Human kidney cell
Early human embryo
Figure 8.1 Three functions of cell division (part 1) 8

9. WHAT CELL REPRODUCTION ACCOMPLISHES
In asexual reproduction,
single-celled organisms reproduce by simple cell division and
there is no fertilization of an egg by a sperm.
Some multicellular organisms, such as sea stars, can grow new individuals from fragmented pieces.
Growing a new plant from a clipping is another example of asexual reproduction.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling can be prevented if each gamete has only half the genetic material of the adult cells.
2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells holds great potential but is variously restricted and regulated.
3. As the authors note, biologists use the term daughter to indicate offspring and not gender. Students with little experience in this terminology can easily become confused.
Teaching Tips
1. The authors do not use the word clone in this chapter. You might wish to point out to your students that asexual reproduction produces clones.
2. Consider pointing out that asexual reproduction is common in prokaryotes and single-celled organisms.
3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends on the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg, donate a nucleus, and activate development of the egg. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus, eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and support growth.)
4. Virchow’s principle of “Every cell from a cell” (not specifically addressed in this chapter) is worth thinking through with your class. Students might expect that like automobiles, computers, and cell phones, cell parts are constructed de novo and cells are assembled. In our society, few nonliving products are generated from only existing products (try to think of such examples). For example, you do not need a painting to paint or a house to construct a house. Yet, this common expectation exists in biology. 9

10. 10 FUNCTIONS OF CELL DIVISION Asexual Reproduction
Figure 8.1b
FUNCTIONS OF CELL DIVISION
Asexual Reproduction LM
Division of an amoeba
Regeneration of a sea star
Growth of a clipping
Figure 8.1 Three functions of cell division (part 2) 10

11. WHAT CELL REPRODUCTION ACCOMPLISHES
In asexual reproduction, the lone parent and its offspring have identical genes.
Mitosis is the type of cell division responsible for
asexual reproduction and
growth and maintenance of multicellular organisms.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling can be prevented if each gamete has only half the genetic material of the adult cells.
2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells holds great potential but is variously restricted and regulated.
3. As the authors note, biologists use the term daughter to indicate offspring and not gender. Students with little experience in this terminology can easily become confused.
Teaching Tips
1. The authors do not use the word clone in this chapter. You might wish to point out to your students that asexual reproduction produces clones.
2. Consider pointing out that asexual reproduction is common in prokaryotes and single-celled organisms.
3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends on the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg, donate a nucleus, and activate development of the egg. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus, eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and support growth.)
4. Virchow’s principle of “Every cell from a cell” (not specifically addressed in this chapter) is worth thinking through with your class. Students might expect that like automobiles, computers, and cell phones, cell parts are constructed de novo and cells are assembled. In our society, few nonliving products are generated from only existing products (try to think of such examples). For example, you do not need a painting to paint or a house to construct a house. Yet, this common expectation exists in biology. 11

12. WHAT CELL REPRODUCTION ACCOMPLISHES
Sexual reproduction requires fertilization of an egg by a sperm using a special type of cell division called meiosis.
Thus, sexually reproducing organisms use
meiosis for reproduction and
mitosis for growth and maintenance.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling can be prevented if each gamete has only half the genetic material of the adult cells.
2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells holds great potential but is variously restricted and regulated.
3. As the authors note, biologists use the term daughter to indicate offspring and not gender. Students with little experience in this terminology can easily become confused.
Teaching Tips
1. The authors do not use the word clone in this chapter. You might wish to point out to your students that asexual reproduction produces clones.
2. Consider pointing out that asexual reproduction is common in prokaryotes and single-celled organisms.
3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends on the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg, donate a nucleus, and activate development of the egg. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus, eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and support growth.)
4. Virchow’s principle of “Every cell from a cell” (not specifically addressed in this chapter) is worth thinking through with your class. Students might expect that like automobiles, computers, and cell phones, cell parts are constructed de novo and cells are assembled. In our society, few nonliving products are generated from only existing products (try to think of such examples). For example, you do not need a painting to paint or a house to construct a house. Yet, this common expectation exists in biology. 12

13. THE CELL CYCLE AND MITOSIS
In a eukaryotic cell,
most genes are located on chromosomes in the cell nucleus and
a few genes are found in DNA in mitochondria and chloroplasts.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 13

14. Eukaryotic Chromosomes
Each eukaryotic chromosome contains one very long DNA molecule, typically bearing thousands of genes.
The number of chromosomes in a eukaryotic cell depends on the species.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 14

15. 15 Species Number of chromosomes in body cells Indian muntjac deer
Figure 8.2
Species
Number of chromosomes in body cells
Indian muntjac deer 6
Koala 16
Opossum 22
Giraffe 30
Mouse 40
Human 46
Duck-billed platypus 54
Buffalo 60
Dog 78
Red viscacha rat
102
Figure 8.2 The number of chromosomes in the cells of selected mammals 15

16. Eukaryotic Chromosomes
Chromosomes are
made of chromatin, fibers composed of roughly equal amounts of DNA and protein molecules and
not visible in a cell until cell division occurs.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 16

17. Figure 8.3 LM
Chromosomes
Figure 8.3 A plant cell just before division (colored by stains) 17

18. Eukaryotic Chromosomes
The DNA in a cell is packed into an elaborate, multilevel system of coiling and folding.
Histones are proteins used to package DNA in eukaryotes.
Nucleosomes consist of DNA wound around histone molecules.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 18

19. 19 DNA double helix Histones “Beads on a string” Nucleosome
Figure 8.4
DNA double helix
Histones
“Beads on a string”
TEM
Nucleosome
Tight helical fiber
Thick supercoil
Duplicated chromosomes
(sister chromatids)
TEM
Centromere
Figure 8.4 DNA packing in a eukaryotic chromosome 19

20. Eukaryotic Chromosomes
Before a cell divides, it duplicates all of its chromosomes, resulting in two copies called sister chromatids containing identical genes.
Two sister chromatids are joined together tightly at a narrow “waist” called the centromere.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 20

21. Eukaryotic Chromosomes
When the cell divides, the sister chromatids of a duplicated chromosome separate from each other.
Once separated, each chromatid is
considered a full-fledged chromosome and
identical to the original chromosome.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 21

22. Chromosome distribution to
Figure 8.5
Chromosome
duplication
Sister
chromatids
Chromosome distribution to
daughter cells
Figure 8.5 Duplication and distribution of a single chromosome 22

23. The Cell Cycle
A cell cycle is the ordered sequence of events that extend
from the time a cell is first formed from a dividing parent cell
to its own division into two cells.
The cell cycle consists of two distinct phases:
interphase and
the mitotic phase.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 23

24. 24 S phase (DNA synthesis; chromosome duplication)
Figure 8.6
S phase (DNA synthesis; chromosome duplication)
Interphase: metabolism and
growth (90% of time) G1 G2
Mitotic (M) phase:
cell division
(10% of time)
Cytokinesis
(division of
cytoplasm)
Mitosis
(division of nucleus)
Figure 8.6 The eukaryotic cell cycle 24

25. The Cell Cycle Most of a cell cycle is spent in interphase.
During interphase, a cell
performs its normal functions,
doubles everything in its cytoplasm, and
grows in size.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 25

26. The Cell Cycle
The mitotic (M) phase includes two overlapping processes:
mitosis, in which the nucleus and its contents divide evenly into two daughter nuclei and
cytokinesis, in which the cytoplasm is divided in two.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 26

27. Mitosis and Cytokinesis
During mitosis the mitotic spindle, a football-shaped structure of microtubules, guides the separation of two sets of daughter chromosomes.
Spindle microtubules grow from structures within the cytoplasm called centrosomes.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 27

28. 28 INTERPHASE PROPHASE Centrosomes (with centriole pairs)
Figure 8.7a
INTERPHASE
PROPHASE
Centrosomes
(with centriole pairs)
Early mitotic
spindle
Fragments of
nuclear envelope
Centrosome
Chromatin
Centromere
Nuclear
envelope
Plasma
membrane
Chromosome
(two sister chromatids)
Spindle microtubules
Figure 8.7 Cell reproduction: A dance of the chromosomes (part 1) 28

29. 29 METAPHASE ANAPHASE Nuclear Cleavage envelope furrow forming Spindle
Figure 8.7b
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Nuclear
envelope
forming
Cleavage
furrow
Spindle
Daughter
chromosomes
Figure 8.7 Cell reproduction: A dance of the chromosomes (part 2) 29

30. 30 METAPHASE ANAPHASE Nuclear Cleavage envelope furrow forming Spindle
Figure 8.7b
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Nuclear
envelope
forming
Cleavage
furrow
Spindle
Daughter
chromosomes
Figure 8.7 Cell reproduction: A dance of the chromosomes (part 2) 30

31. Mitosis and Cytokinesis
Prophase
Metaphase
Anaphase
Telophase
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 31

32. Mitosis and Cytokinesis
Cytokinesis usually
begins during telophase,
divides the cytoplasm, and
is different in plant and animal cells.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 32

33. Mitosis and Cytokinesis
In animal cells, cytokinesis
is known as cleavage and
begins with the appearance of a cleavage furrow, an indentation at the equator of the cell.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 33

34. 34 Animation: Cytokinesis Right click slide / select “Play”
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 34

35. 35 Cleavage furrow Cleavage furrow Contracting ring of microfilaments
Figure 8.8a
SEM
Cleavage
furrow
Cleavage furrow
Contracting ring of
microfilaments
Daughter cells
Figure 8.8 Cytokinesis in animal cells (part 1) 35

36. Mitosis and Cytokinesis
In plant cells, cytokinesis begins when vesicles containing cell wall material collect at the middle of the cell and then fuse, forming a membranous disk called the cell plate.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 36

37. 37 Cell plate forming Wall of parent cell Daughter nucleus
Figure 8.8b
Cell plate
forming
Wall of parent cell
Daughter nucleus LM
Vesicles containing
cell wall material
Cell wall
Cell plate
New cell wall
Daughter cells
Figure 8.8 Cytokinesis in plant cells (part 2) 37

38. Cancer Cells: Growing Out of Control
Normal plant and animal cells have a cell cycle control system that consists of specialized proteins, which send “stop” and “go-ahead” signals at certain key points during the cell cycle.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 38

39. Cancer is a disease of the cell cycle.
What Is Cancer?
Cancer is a disease of the cell cycle.
Cancer cells do not respond normally to the cell cycle control system.
Cancer cells can form tumors, abnormally growing masses of body cells.
If the abnormal cells remain at the original site, the lump is called a benign tumor.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 39

40. A person with a malignant tumor is said to have cancer.
What Is Cancer?
The spread of cancer cells beyond their original site of origin is metastasis.
Malignant tumors can
spread to other parts of the body and
interrupt normal body functions.
A person with a malignant tumor is said to have cancer.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 40

41. 41 A tumor grows from a single cancer cell. Cancer cells invade
Figure 8.9
Lymph
vessels
Tumor
Blood
vessel
Glandular
tissue
A tumor grows
from a single
cancer cell.
Cancer cells invade
neighboring tissue.
Metastasis: Cancer
cells spread through
lymph and blood
vessels to other parts
of the body.
Figure 8.9 Growth and metastasis of a malignant tumor of the breast 41

42. Cancer treatment can involve
radiation therapy, which damages DNA and disrupts cell division, and
chemotherapy, the use of drugs to disrupt cell division.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 42

43. Cancer Prevention and Survival
Certain behaviors can decrease the risk of cancer:
not smoking,
exercising adequately,
avoiding exposure to the sun,
eating a high-fiber, low-fat diet,
performing self-exams, and
regularly visiting a doctor to identify tumors early.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show duplicated chromosomes appearing like the letter “X.” It remains unclear to many students why (a) chromosome structure is typically different between interphase G1 and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer (such as HPV, human papillomavirus associated with genital warts and cervical cancer).
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times—all with one continuous strand of thread.
3. The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because the drawstring is just beneath the surface of the sweatpants, and the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10.Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. 43

44. MEIOSIS, THE BASIS OF SEXUAL REPRODUCTION
depends on meiosis and fertilization and
produces offspring that contain a unique combination of genes from the parents.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 44

45. Figure 8.10
Figure 8.10 The varied products of sexual reproduction 45

46. Homologous Chromosomes
Different individuals of a single species have the same
number and
types of chromosomes.
A human somatic cell
is a typical body cell and
has 46 chromosomes.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 46

47. Homologous Chromosomes
A karyotype is an image that reveals an orderly arrangement of chromosomes.
Homologous chromosomes
are matching pairs of chromosomes that
can possess different versions of the same genes.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 47

48. One duplicated chromosome
Figure 8.11
Pair of homologous
chromosomes LM
Centromere
One duplicated chromosome
Sister
chromatids
Figure 8.11 Pairs of homologous chromosomes in a male karyotype 48

49. Homologous Chromosomes
Humans have
two different sex chromosomes, X and Y, and
22 pairs of matching chromosomes, called autosomes.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 49

50. Gametes and the Life Cycle of a Sexual Organism
The life cycle of a multicellular organism is the sequence of stages leading from the adults of one generation to the adults of the next.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 50

51. 51 MEIOSIS FERTILIZATION MITOSIS Haploid gametes (n  23) n Egg cell n
Figure 8.12
Haploid gametes (n  23) n
Egg cell n
Sperm cell
MEIOSIS
FERTILIZATION
Multicellular
diploid adults
(2n  46)
Diploid
zygote
(2n  46) 2n
MITOSIS
and development
Key
Haploid (n)
Diploid (2n)
Figure 8.12 The human life cycle 51

52. Gametes and the Life Cycle of a Sexual Organism
Humans are diploid organisms with
body cells containing two sets of chromosomes and
haploid gametes that have only one member of each homologous pair of chromosomes.
In humans, a haploid sperm fuses with a haploid egg during fertilization to form a diploid zygote.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 52

53. Gametes and the Life Cycle of a Sexual Organism
Sexual life cycles involve an alternation of diploid and haploid stages.
Meiosis produces haploid gametes, which keeps the chromosome number from doubling every generation.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 53

54. 54 1 Chromosomes duplicate. 2 Homologous chromosomes separate. 3
Figure 1
Chromosomes
duplicate. 2
Homologous
chromosomes
separate. 3
Sister chromatids
separate.
Pair of homologous
chromosomes in diploid parent cell
Duplicated pair of homologous
chromosomes
Sister
chromatids
INTERPHASE BEFORE MEIOSIS
MEIOSIS I
MEIOSIS II
Figure 8.13 How meiosis halves chromosome number (step 3) 54

55. The Process of Meiosis In meiosis,
haploid daughter cells are produced in diploid organisms,
interphase is followed by two consecutive divisions, meiosis I and meiosis II, and
crossing over occurs.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 55

56. 56 MEIOSIS I: HOMOLOGOUS CHROMOSOMES SEPARATE INTERPHASE PROPHASE I
Figure 8.14a
MEIOSIS I: HOMOLOGOUS CHROMOSOMES SEPARATE
INTERPHASE
PROPHASE I
METAPHASE I
ANAPHASE I
Centrosomes
(with centriole pairs)
Sites of
crossing over
Microtubules
attached to chromosome
Sister chromatids
remain attached
Spindle
Sister
chromatids
Nuclear
envelope
Chromatin
Centromere
Pair of
homologous
chromosomes
Chromosomes
duplicate.
Homologous
chromosomes
pair up and
exchange
segments.
Pairs of
homologous
chromosomes
line up.
Pairs of
homologous
chromosomes
split up.
Figure 8.14 The stages of meiosis (part 1) 56

57. TELOPHASE I AND CYTOKINESIS
Figure 8.14ab
PROPHASE I
METAPHASE I
ANAPHASE I
TELOPHASE I AND CYTOKINESIS
Sites of crossing over
Microtubules attached to chromosome
Sister chromatids
remain attached
Cleavage
furrow
Spindle
Sister
chromatids
Centromere
Pair of
homologous
chromosomes
Figure 8.14 The stages of meiosis (part 1b) 57

58. 58 MEIOSIS II: SISTER CHROMATIDS SEPARATE TELOPHASE I AND CYTOKINESIS
Figure 8.14b
MEIOSIS II: SISTER CHROMATIDS SEPARATE
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II AND
CYTOKINESIS
Cleavage
furrow
Sister
chromatids
separate
Haploid
daughter
cells forming
Two haploid
cells form;
chromosomes
are still doubled.
During another round of cell division, the sister
chromatids finally separate; four haploid
daughter cells result, containing single
chromosomes.
Figure 8.14b The stages of meiosis (part 2) 58

59. 59 PROPHASE II METAPHASE II ANAPHASE II Sister chromatids Haploid
Figure 8.14ba
PROPHASE II
TELOPHASE II
AND
CYTOKINESIS
METAPHASE II
ANAPHASE II
Sister chromatids
separate
Haploid
daughter cells forming
Figure 8.14 The stages of meiosis (part 2a) 59

60. Meiosis II in a lily cell
Figure 8.14bc LM
Meiosis II in a lily cell LM
Figure 8.14 The stages of meiosis (part 2c) 60

61. Review: Comparing Mitosis and Meiosis
In mitosis and meiosis, the chromosomes duplicate only once, during the preceding interphase.
The number of cell divisions varies:
Mitosis uses one division and produces two diploid cells.
Meiosis uses two divisions and produces four haploid cells.
All the events unique to meiosis occur during meiosis I.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 61

62. 62 MITOSIS MEIOSIS Prophase Prophase I Duplicated chromosome
Figure 8.15
MITOSIS
MEIOSIS
Prophase
Prophase I
MEIOSIS I
Duplicated chromosome
Parent cell
Site of crossing over
Metaphase
Metaphase I
Chromosomes
align.
Homologous pairs align.
Anaphase
Telophase
Anaphase I
Telophase I
MEIOSIS I
Sister
chromatids
separate.
Homologous
chromosomes
separate.
Haploid
n  2 2n 2n
MEIOSIS II
Sister chromatids
separate. n n n n
Figure 8.15 Comparing mitosis and meiosis 62

63. The Origins of Genetic Variation
Offspring of sexual reproduction are genetically different from their parents and one another.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 63

64. Independent Assortment of Chromosomes
When aligned during metaphase I of meiosis, the side-by-side orientation of each homologous pair of chromosomes is a matter of chance.
Every chromosome pair orients independently of all of the others at metaphase I.
For any species, the total number of chromosome combinations that can appear in the gametes due to independent assortment is
2n, where n is the haploid number.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 64

65. Independent Assortment of Chromosomes
For a human,
n = 23.
With n = 23, there are 8,388,608 different chromosome combinations possible in a gamete.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 65

66. 66 Animation: Genetic Variation Right click slide / select “Play”
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 66

67. 67 POSSIBILITY 1 POSSIBILITY 2
Figure
POSSIBILITY 1
POSSIBILITY 2
Two equally probable arrangements of chromosomes at metaphase of meiosis I
Metaphase
of meiosis II
Gametes
Combination a
Combination b
Combination c
Combination d
Because possibilities 1 and 2 are equally likely, the four possible types of gametes will be made in approximately equal numbers.
Figure 8.16 Results of alternative arrangements of chromosomes at metaphase of meiosis I (step 3) 67

68. Random Fertilization
A human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote.
If each gamete represents one of 8,388,608 different chromosome combinations, at fertilization, humans would have 8,388,608 × 8,388,608, or more than 70 trillion different possible chromosome combinations.
So we see that the random nature of fertilization adds a huge amount of potential variability to the offspring of sexual reproduction.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 68

69. Figure 8.17
Colorized LM
Figure 8.17 The process of fertilization: a close-up view 69

70. Crossing Over In crossing over,
nonsister chromatids of homologous chromosomes exchange corresponding segments and
genetic recombination, the production of gene combinations different from those carried by parental chromosomes, occurs.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 70

71. 71 Animation: Crossing Over Right click slide / select “Play”
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 71

72. 72 Prophase I of meiosis Duplicated pair of homologous chromosomes
Figure 8.18
Prophase I of meiosis
Duplicated pair of homologous
chromosomes
Homologous chromatids exchange corresponding segments.
Chiasma, site of crossing over
Metaphase I
Spindle microtubule
Sister chromatids remain joined at their centromeres.
Metaphase II
Gametes
Recombinant chromosomes combine genetic information from different parents.
Recombinant chromosomes
Figure 8.18 The results of crossing over during meiosis for a single pair of homologous chromosomes 72

73. The Process of Science: Do All Animals Have Sex?
Observation: No scientists have ever found male bdelloid rotifers, a microscopic freshwater invertebrate.
Question: Does this entire class of animals reproduce solely by asexual means?
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 73

74. The Process of Science: Do All Animals Have Sex?
Hypothesis: Bdelloid rotifers have thrived for millions of years despite a lack of sexual reproduction.
Prediction: Bdelloid rotifers would display much more variation in their pairs of homologous genes than most organisms.
Experiment: Researchers compared sequences of a particular gene in bdelloid and non-bdelloid rotifers.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 74

75. The Process of Science: Do All Animals Have Sex?
Results:
Non-bdelloid sexually reproducing rotifers had a nearly identical homologous gene, differing by only 0.5% on average.
The two versions of the same gene in asexually reproducing bdelloid rotifers differed by 3.5–54%.
Conclusion: Bdelloid rotifers have evolved for millions of years without any sexual reproduction.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 75

76. Figure 8.19 LM
Figure 8.19 A bdelloid rotifer 76

77. When Meiosis Goes Awry What happens when errors occur in meiosis?
Such mistakes can result in genetic abnormalities that range from mild to fatal.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 77

78. How Accidents during Meiosis Can Alter Chromosome Number
In nondisjunction,
the members of a chromosome pair fail to separate at anaphase,
producing gametes with an incorrect number of chromosomes.
Nondisjunction can occur during meiosis I or II.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 78

79. 79 NONDISJUNCTION IN MEIOSIS I NONDISJUNCTION IN MEIOSIS II Meiosis I
Figure
NONDISJUNCTION IN MEIOSIS I
NONDISJUNCTION IN MEIOSIS II
Meiosis I
Homologous
chromosomes fail
to separate.
Meiosis II
Sister
chromatids
fail to separate.
Gametes
n  1
n  1
n – 1
n – 1
n  1
n – 1 n n
Abnormal
Abnormal
Normal
Figure 8.20 Two types of nondisjunction (step 3) 79

80. How Accidents during Meiosis Can Alter Chromosome Number
If nondisjunction occurs, and a normal sperm fertilizes an egg with an extra chromosome, the result is a zygote with a total of 2n + 1 chromosomes.
If the organism survives, it will have
an abnormal karyotype and
probably a syndrome of disorders caused by the abnormal number of genes.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 80

81. 81 Abnormal egg cell with extra chromosome n  1 Normal sperm cell
Figure 8.21
Abnormal egg
cell with extra
chromosome
n  1
Normal
sperm cell
Abnormal zygote with extra chromosome 2n  1
n (normal)
Figure 8.21 Fertilization after nondisjunction in the mother 81

82. Down Syndrome: An Extra Chromosome 21
is also called trisomy 21,
is a condition in which an individual has an extra chromosome 21, and
affects about one out of every 700 children.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 82

83. Figure 8.22 LM
Trisomy 21
Figure 8.22 Trisomy 21 and Down syndrome 83

84. Down Syndrome: An Extra Chromosome 21
The incidence of Down syndrome in the offspring of normal parents increases markedly with the age of the mother.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 84

85. Infants with Down syndrome
Figure 8.23 90 80 70 60
Infants with Down syndrome
(per 1,000 births) 50 40 30 20 10 20 25 30 35 40 45 50
Age of mother
Figure 8.23 Maternal age and Down syndrome 85

86. Abnormal Numbers of Sex Chromosomes
Nondisjunction in meiosis
can lead to abnormal numbers of sex chromosomes but
seems to upset the genetic balance less than unusual numbers of autosomes, perhaps because the Y chromosome is very small and carries relatively few genes.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 86

87. Table 8.1
Table 8.1 Abnormalities of sex chromosome number in humans 87

88. Evolution Connection: The Advantages of Sex
Asexual reproduction conveys an evolutionary advantage when plants are
sparsely distributed and unlikely to be able to exchange pollen or
superbly suited to a stable environment.
Asexual reproduction also eliminates the need to expend energy
forming gametes and
copulating with a partner.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 88

89. Figure 8.24
Runner
Figure 8.24 Sexual and asexual reproduction 89

90. Evolution Connection: The Advantages of Sex
Sexual reproduction may convey an evolutionary advantage by
speeding adaptation to a changing environment or
allowing a population to more easily rid itself of harmful genes.
© 2013 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See Teaching Tips 8–10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species that is diploid has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair—maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited—either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 315 million humans living in the United States. If every person in the United States received $222,222, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, (or a million dollars every 45 seconds) you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending on the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative. 90

91. 91 Distribution via mitosis Duplication of all chromosomes Genetically
Figure 8.UN01
Distribution via
mitosis
Duplication
of all
chromosomes
Genetically
identical
daughter cells
Figure 8.UN01 Summary of Key Concepts: What Cell Reproduction Accomplishes 91

92. Duplicated chromosome
Figure 8.UN02
Chromosome (one
long piece of DNA)
Centromere
Sister
chromatids
Duplicated chromosome
Figure 8.UN02 Summary of Key Concepts: Eukaryotic Chromosomes 92

93. DNA synthesis; chromosome duplication chromosome duplication
Figure 8.UN03
S phase
DNA synthesis; chromosome duplication
Interphase
Cell growth and
chromosome duplication G1 G2
Mitotic
(M) phase
Genetically
identical
“daughter”
cells
Cytokinesis
(division of
cytoplasm)
Mitosis
(division of
nucleus)
Figure 8.UN03 Summary of Key Concepts: The Cell Cycle 93

94. 94 MEIOSIS FERTILIZATION MITOSIS Human Life Cycle
Figure 8.UN04
Human Life Cycle
Haploid gametes (n  23)
Key n
Haploid (n)
Egg cell
Diploid (2n) n
Sperm cell
MEIOSIS
FERTILIZATION
Diploid
zygote
(2n  46)
Male and female
diploid adults
(2n  46) 2n
MITOSIS
and development
Figure 8.UN04 Summary of Key Concepts: Gametes and the Life Cycle of a Sexual Organism 94

95. 95 MITOSIS MEIOSIS MEIOSIS I Parent cell (2n) Parent cell (2n)
Figure 8.UN05
MITOSIS
MEIOSIS
MEIOSIS I
Parent
cell (2n)
Parent
cell (2n)
Chromosome
duplication
Chromosome
duplication
Pairing of
homologous
chromosome
Crossing over
Daughter
cells 2n 2n
MEIOSIS II n n n n
Daughter cells
Figure 8.UN05 Summary of Key Concepts: Comparing Mitosis and Meiosis 95

96. 96 (a) (b) (c) (d) LM Figure 8.UN06
Figure 8.UN06 The Process of Science, question 14 96