Cellular Reproduction: Cells from Cells

Cellular Reproduction: Cells from Cells
Chapter 8 Cellular Reproduction: Cells from Cells

Biology and Society: Rain Forest Rescue
Endangered species of plants that normally reproduce sexually can by propagated by asexual reproduction. Cell division is at the heart of organismal reproduction, whether by sexual or asexual means. © 2010 Pearson Education, Inc.

WHAT CELL REPRODUCTION ACCOMPLISHES
May result in the birth of new organisms More commonly involves the production of new cells © 2010 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. You might wish to point 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 upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. 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 and cells are assembled. In our society, few non-living products are generated only from 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.

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. © 2010 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. You might wish to point 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 upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. 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 and cells are assembled. In our society, few non-living products are generated only from 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.

Before a parent cell splits into two, it duplicates its chromosomes, the structures that contain most of the organism’s DNA. During cell division, each daughter cell receives one set of chromosomes. © 2010 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. You might wish to point 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 upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. 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 and cells are assembled. In our society, few non-living products are generated only from 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.

Cell division plays important roles in the lives of organisms.
Replaces damaged or lost cells Permits growth Allows for reproduction 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. You might wish to point 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 upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. 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 and cells are assembled. In our society, few non-living products are generated only from 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.

In asexual reproduction:
Single-celled organisms reproduce by simple cell division There is no fertilization of an egg by a sperm 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. You might wish to point 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 upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. 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 and cells are assembled. In our society, few non-living products are generated only from 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.

Some multicellular organisms, such as sea stars, can grow new individuals from fragmented pieces.
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. You might wish to point 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 upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. 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 and cells are assembled. In our society, few non-living products are generated only from 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.

Growing a new plant from a clipping is another example of asexual reproduction.
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. You might wish to point 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 upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. 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 and cells are assembled. In our society, few non-living products are generated only from 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.

Mitosis is the type of cell division responsible for:
In asexual reproduction, the lone parent and its offspring have identical genes. Mitosis is the type of cell division responsible for: Asexual reproduction Growth and maintenance of multicellular organisms 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. You might wish to point 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 upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. 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 and cells are assembled. In our society, few non-living products are generated only from 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.

Thus, sexually reproducing organisms use:
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 Mitosis for growth and maintenance 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. You might wish to point 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 upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. 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 and cells are assembled. In our society, few non-living products are generated only from 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.

THE CELL CYCLE AND MITOSIS
In a eukaryotic cell: Most genes are located on chromosomes in the cell nucleus A few genes are found in DNA in mitochondria and chloroplasts 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

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. 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

Number of chromosomes in body cells Species 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 Figure 8.2 The number of chromosomes in the cells of selected mammals.

Chromosomes: Are made of chromatin, a combination of DNA and protein molecules Are not visible in a cell until cell division occurs 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

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

Animation: DNA Packing
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. Animation: DNA Packing 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

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

DNA double helix Histones “Beads on a string” Nucleosome Figure 8.4a
TEM Nucleosome Figure 8.4a Figure 8.4a DNA packing in a eukaryotic chromosome.

Duplicated chromosomes (sister chromatids) TEM
Tight helical fiber Looped domains Duplicated chromosomes (sister chromatids) TEM Centromere Figure 8.4b Figure 8.4b DNA packing in a eukaryotic chromosome.

Before a cell divides, it duplicates all of its chromosomes, resulting in two copies called sister chromatids. Sister chromatids are joined together at a narrow “waist” called the centromere. 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

When the cell divides, the sister chromatids separate from each other.
Once separated, each chromatid is: Considered a full-fledged chromosome Identical to the original chromosome 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

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

The Cell Cycle A cell cycle is the orderly 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 The mitotic phase 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

Most of a cell cycle is spent in interphase.
During interphase, a cell: Performs its normal functions Doubles everything in its cytoplasm Grows in size Video: Animal Mitosis 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

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

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

Video: Sea Urchin (time lapse)
The mitotic (M) phase includes two overlapping processes: Mitosis, in which the nucleus and its contents divide evenly into two daughter nuclei Cytokinesis, in which the cytoplasm is divided in two Video: Sea Urchin (time lapse) 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

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 Figure 8.6 The eukaryotic cell cycle.

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 two centrosomes, clouds of cytoplasmic material that in animal cells contain centrioles. 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

Mitosis consists of four distinct phases:
(A) Prophase 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

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

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

(A) Prophase (B) Metaphase 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

(A) Prophase (B) Metaphase (C) Anaphase
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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

(A) Prophase (B) Metaphase (C) Anaphase (D) Telophase
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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

Blast Animation: Cytokinesis in Plant Cells Animation: Cytokinesis
Cytokinesis typically: Occurs during telophase Divides the cytoplasm Is different in plant and animal cells Blast Animation: Cytokinesis in Plant Cells Animation: Cytokinesis 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

Cleavage furrow Cleavage furrow Contracting ring of microfilaments
SEM Cleavage furrow Cleavage furrow Contracting ring of microfilaments Daughter cells Figure 8.8a Figure 8.8a Cytokinesis in animal cells.

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

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. 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

What Is Cancer? Cancer is a disease of the cell cycle.
Cancer cells do not respond normally to the cell cycle control system. 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

Cancer cells can form tumors, abnormally growing masses of body cells.
The spread of cancer cells beyond their original site of origin is metastasis. Malignant tumors can: Spread to other parts of the body Interrupt normal body functions A person with a malignant tumor is said to have cancer. 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

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 Figure 8.9 Growth and metastasis of a malignant tumor of the breast.

Cancer Treatment Cancer treatment can involve:
Radiation therapy, which damages DNA and disrupts cell division Chemotherapy, which uses drugs that disrupt cell division 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

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 Regularly visiting a doctor to identify tumors early 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 replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and 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. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph. 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. 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. 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 (1) a chromosome before DNA replication and (2) 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 like the drawstring just beneath the surface of the sweatpants, 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.

Meiosis, the Basis of Sexual Reproduction
Uses meiosis Uses fertilization Produces offspring that contain a unique combination of genes from the parents © 2010 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

Homologous Chromosomes
Different individuals of a single species have the same number and types of chromosomes. 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

A human somatic cell: Is a typical body cell Has 46 chromosomes
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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

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. 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

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

Humans have: Two different sex chromosomes, X and Y
Twenty-two pairs of matching chromosomes, called autosomes 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

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. 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

Haploid gametes (n  23) Egg cell n 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 Figure 8.12 The human life cycle

Humans are diploid organisms in which:
Their somatic cells contain two sets of chromosomes Their gametes are haploid, having only one set of chromosomes 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

In humans, a haploid sperm fuses with a haploid egg during fertilization to form a diploid zygote.
Sexual life cycles involve an alternation of diploid and haploid stages. 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

Meiosis produces haploid gametes, which keeps the chromosome number from doubling every generation.

Figure 8.13-3 Homologous chromosomes separate. Chromosomes duplicate.
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 Figure 8.13 How meiosis halves chromosome number. (Step 3)

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 Crossing over occurs 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Pairs of homologous
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 Centromere Pair of homologous chromosomes Chromatin Homologous chromosomes pair up and exchange segments. Pairs of homologous chromosomes line up. Pairs of homologous chromosomes split up. Chromosomes duplicate. Figure 8.14a Figure 8.14a The stages of meiosis. (Part 1)

INTERPHASE Centrosomes (with centriole pairs) Nuclear envelope
Chromatin Figure 8.14aa Figure 8.14aa The stages of meiosis. (Part 1)

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

Microtubules attached to chromosome Sites of crossing over
PROPHASE I METAPHASE I Microtubules attached to chromosome Sites of crossing over Spindle Sister chromatids Centromere Pair of homologous chromosomes Figure 8.14ac Figure 8.14ac The stages of meiosis. (Part 1)

TELOPHASE I AND CYTOKINESIS
ANAPHASE I TELOPHASE I AND CYTOKINESIS Sister chromatids remain attached TELOPHASE I AND CYTOKINESIS Cleavage furrow Figure 8.14ad Figure 8.14ad The stages of meiosis. (Part 1)

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 Figure 8.14b The stages of meiosis. (Part 2)

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

PROPHASE II METAPHASE II Figure 8.14bb
Figure 8.14bb The stages of meiosis. (Part 2)

LM Figure 8.14bc Figure 8.14bc The stages of meiosis. (Part 2)

TELOPHASE II AND CYTOKINESIS
ANAPHASE II TELOPHASE II AND CYTOKINESIS Haploid daughter cells forming Sister chromatids separate Figure 8.14bd Figure 8.14bd The stages of meiosis. (Part 2)

Review: Comparing Mitosis and Meiosis
In mitosis and meiosis, the chromosomes duplicate only once, during the preceding interphase. 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

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. 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

(before chromosome duplication)
MITOSIS MEIOSIS Prophase Prophase I MEIOSIS I Chromosome duplication Chromosome duplication Duplicated chromosome (two sister chromatids) Parent cell (before chromosome duplication) 2n  4 Homologous chromosomes come together in pairs. Site of crossing over between homologous (nonsister) chromatids Metaphase Metaphase I Chromosomes align at the middle of the cell. Homologous pairs align at the middle of the cell. Anaphase Telophase Anaphase I Telophase I Chromosome with two sister chromatids Sister chromatids separate during anaphase. Homologous chromosomes separate during anaphase I; sister chromatids remain together. Haploid n  2 2n 2n Daughter cells of meiosis I Daughter cells of mitosis MEIOSIS II Sister chromatids separate during anaphase II. n n n n Daughter cells of meiosis II Figure 8.15 Figure 8.15 Comparing mitosis and meiosis

(before chromosome duplication)
MITOSIS MEIOSIS Prophase Prophase I MEIOSIS I Chromosome duplication Chromosome duplication Duplicated chromosome (two sister chromatids) Parent cell (before chromosome duplication) 2n  4 Site of crossing over between homologous (nonsister) chromatids Homologous chromosomes come together in pairs. Metaphase Metaphase I Chromosomes align at the middle of the cell. Homologous pairs align at the middle of the cell. Figure 8.15a Figure 8.15a Comparing mitosis and meiosis

Daughter cells of meiosis I Daughter cells of mitosis
Anaphase Telophase Anaphase I Telophase I Chromosome with two sister chromatids Sister chromatids separate during anaphase. Homologous chromosomes separate during anaphase I; sister chromatids remain together. Haploid n  2 2n 2n Daughter cells of meiosis I MEIOSIS II Daughter cells of mitosis Sister chromatids separate during anaphase II. n n n n Daughter cells of meiosis II Figure 8.15b Figure 8.15b Comparing mitosis and meiosis

The Origins of Genetic Variation
Offspring of sexual reproduction are genetically different from their parents and one another. 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

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 the others during meiosis. 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

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. For a human: n = 23 223 = 8,388,608 different chromosome combinations possible in a gamete Animation: Genetic Variation Blast Animation: Genetic Variation: Independent Assortment 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

Metaphase of meiosis I Metaphase of meiosis II
POSSIBILITY 1 POSSIBILITY 2 Metaphase of meiosis I Metaphase of meiosis II Gametes Combination a Combination b Combination c Combination d Figure Figure 8.16 Results of alternative arrangements of chromosomes at metaphase of meiosis I. (Step 3)

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. 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

Crossing Over In crossing over:
Homologous chromosomes exchange genetic information Genetic recombination, the production of gene combinations different from those carried by parental chromosomes, occurs Animation: Crossing Over Blast Animation: Genetic Variation: Fusion of Gametes 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

Figure 8.18-1 Prophase I Duplicated pair of of meiosis homologous
chromosomes Figure Figure 8.18 The results of crossing over during meiosis for a single pair of homologous chromosomes. (Step 1)

chromosomes Homologous chromatids exchange corresponding segments. Chiasma, site of crossing over Figure Figure 8.18 The results of crossing over during meiosis for a single pair of homologous chromosomes. (Step 2)

chromosomes Homologous chromatids exchange corresponding segments. Chiasma, site of crossing over Metaphase I Spindle microtubule Sister chromatids remain joined at their centromeres. Figure Figure 8.18 The results of crossing over during meiosis for a single pair of homologous chromosomes. (Step 3)

chromosomes Homologous chromatids exchange corresponding segments. Chiasma, site of crossing over Metaphase I Spindle microtubule Sister chromatids remain joined at their centromeres. Metaphase II Figure Figure 8.18 The results of crossing over during meiosis for a single pair of homologous chromosomes. (Step 4)

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 Figure 8.18 The results of crossing over during meiosis for a single pair of homologous chromosomes. (Step 5)

The Process of Science: Do All Animals Have Sex?
Observation: No scientists have ever found male bdelloid rotifers, a microscopic freshwater invertebrate. Hypothesis: Bdelloid rotifers have thrived for millions of years using only asexual reproduction. Prediction: Bdelloid rotifers would display much more variation in their homologous pairs of genes than most organisms. © 2010 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

Experiment: Researchers compared sequences of a particular gene in bdelloid and non-bdelloid rotifers. Results: Non-bdelloid sexually reproducing rotifers had nearly identical homologous genes Bdelloid asexually reproducing rotifers had homologous genes that differed by 3.5–54%. Conclusion: Bdelloid rotifers have evolved for millions of years without any sexual reproduction. 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

LM Figure 8.19 Figure 8.19 A bdelloid rotifer.

When Meiosis Goes Awry What happens when errors occur in meiosis?
Such mistakes can result in genetic abnormalities that range from mild to fatal. 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

How Accidents during Meiosis Can Alter Chromosome Number
In nondisjunction, the members of a chromosome pair fail to separate during anaphase, producing gametes with an incorrect number of chromosomes. Nondisjunction can occur during meiosis I or II. © 2010 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

Figure 8.20-3 NONDISJUNCTION IN MEIOSIS I NONDISJUNCTION IN MEIOSIS II
Pair of homologous chromosomes fails to separate. Meiosis II Nondisjunction: Pair of sister chromatids fails to separate. Gametes Number of chromosomes n  1 n  1 n – 1 n – 1 n  1 n – 1 n n Abnormal gametes Abnormal gametes Normal gametes Figure Figure 8.20 Two types of nondisjunction. (Step 3)

If the organism survives, it will have an abnormal number of genes.
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 number of genes. 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

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

Down Syndrome: An Extra Chromosome 21
Is also called trisomy 21 Is a condition in which an individual has an extra chromosome 21 Affects about one out of every 700 children 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

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

The incidence of Down Syndrome increases with the age of the mother.

Infants with Down syndrome
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 Figure 8.23 Maternal age and Down syndrome.

Abnormal Numbers of Sex Chromosomes
Nondisjunction can also affect the sex chromosomes. 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

Table 8.1 Table 8.1 Abnormalities of Sex Chromosome Number in Humans

Evolution Connection: The Advantages of Sex
Asexual reproduction conveys an evolutionary advantage when plants are: Sparsely distributed Superbly suited to a stable environment © 2010 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

Sexual reproduction may convey an evolutionary advantage by:
Speeding adaptation to a changing environment Allowing a population to more easily rid itself of harmful genes 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 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 310 million humans living in the United States. If every person in the United States received $225,806, 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, 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 upon 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.

Distribution via mitosis Duplication of all chromosomes Genetically
identical daughter cells Figure 8.UN1 Figure UN 8.1 Summary: Cell division

Duplicated chromosome
Chromosome (one long piece of DNA) Centromere Sister chromatids Duplicated chromosome Figure 8.UN2 Figure UN 8.2 Summary: Duplicated chromosome

DNA synthesis; chromosome duplication chromosome duplication
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.UN3 Figure UN 8.3 Summary: The cell cycle

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.UN4 Figure UN 8.4 Summary: Human life cycle

MITOSIS MEIOSIS Parent cell (2n) Parent cell (2n) MEIOSIS I Pairing of
Chromosome duplication Pairing of homologous chromosome Chromosome duplication Crossing over Daughter cells 2n 2n MEIOSIS II n n n n Daughter cells Figure 8.UN5 Figure UN 8.5 Summary: Comparing Mitosis and Meiosis

(a) (b) (c) (d) Figure 8.UN6
LM (a) (b) (c) (d) Figure 8.UN6 Figure UN 8.6 Question 14: Slide of onion root tip

Cellular Reproduction: Cells from Cells

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Cellular Reproduction: Cells from Cells

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Presentation on theme: "Cellular Reproduction: Cells from Cells"— Presentation transcript:

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