CELL DIVISION AND REPRODUCTION © 2015 Pearson Education, Inc.

8.1 Cell division plays many important roles in the lives of organisms is reproduction at the cellular level, produces two “daughter” cells that are genetically identical to each other and the original “parent” cell, requires the duplication of chromosomes, the structures that contain most of the cell’s DNA Student Misconceptions and Concerns • As the textbook authors note in Module 8.1, biologists use the term “daughter” to indicate offspring and not gender. Students with little experience in this terminology can easily become confused. • Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for these topics 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 is variously restricted and regulated by government. Teaching Tips • 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 will produce dogs, cats will produce more cats, and chickens will only produce chickens. Why does like produce like? © 2015 Pearson Education, Inc.

8.1 Cell division plays many important roles in the lives of organisms Living organisms reproduce by two methods. Asexual reproduction produces offspring that are identical to the original cell or organism and involves inheritance of all genes from one parent. Sexual reproduction produces offspring that are similar to the parents but show variations in traits and involves inheritance of unique sets of genes from two parents. Student Misconceptions and Concerns • As the textbook authors note in Module 8.1, biologists use the term “daughter” to indicate offspring and not gender. Students with little experience in this terminology can easily become confused. • Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for these topics 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 is variously restricted and regulated by government. Teaching Tips • 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 will produce dogs, cats will produce more cats, and chickens will only produce chickens. Why does like produce like? © 2015 Pearson Education, Inc.

8.1 Cell division plays many important roles in the lives of organisms Cell division is used for reproduction of single-celled organisms, growth of multicellular organisms from a fertilized egg into an adult, repair and replacement of cells, and production of sperm and eggs. Student Misconceptions and Concerns • As the textbook authors note in Module 8.1, biologists use the term “daughter” to indicate offspring and not gender. Students with little experience in this terminology can easily become confused. • Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for these topics 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 is variously restricted and regulated by government. Teaching Tips • 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 will produce dogs, cats will produce more cats, and chickens will only produce chickens. Why does like produce like? © 2015 Pearson Education, Inc.

8.2 Prokaryotes reproduce by binary fission Prokaryotes (single-celled bacteria and archaea) reproduce by binary fission (“dividing in half”). Binary fission of a prokaryote occurs in three stages: duplicate chromosome and separate copies, elongation of the cell division into two daughter cells. Student Misconceptions and Concerns • Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for these topics 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 is variously restricted and regulated by government. Teaching Tips • The principle that “every cell comes from another cell” is worth thinking through with your class. Students might expect that, like automobiles, computers, and cell phones, parts are constructed and cells are assembled. In our society, few nonliving 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 is a common expectation in biology. Further, students who think through this principle might ask how the first cells formed. They might wonder further whether the same environments that produced these cells are still in existence. The conditions on Earth when life first formed were very different from those we know today. Chapter 15 of the textbook addresses the origin and early evolution of life on Earth. • Consider contrasting the timing of DNA replication and cytokinesis in prokaryotes and eukaryotes. In prokaryotes, addressed in Module 8.2 of the textbook, these processes are overlapping. However, as revealed in the next few modules, these events are separate in eukaryotes. © 2015 Pearson Education, Inc.

THE EUKARYOTIC CELL CYCLE AND MITOSIS © 2015 Pearson Education, Inc.

8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division Eukaryotic cells store most of their genes on multiple chromosomes within the nucleus. Each eukaryotic species has a characteristic number of chromosomes in each cell nucleus. Humans have 46 chromosomes Fruit Flies have 8 chromosomes Student Misconceptions and Concerns • Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic when discussing DNA replication. • Students are often confused by photographs of chromosomes. Such photographs, such as Figure 8.3B, typically show duplicated chromosomes during some aspect of cell division. It remains unclear to many (a) why chromosome structure is typically different between interphase G1 and the stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes duplicate. Teaching Tips • Figure 8.3B is an important point of reference for some basic terminology. Consider referring to it as you distinguish between a DNA molecule and a chromosome, unreplicated and replicated chromosomes, and the nature of sister chromatids. • Consider an analogy between the precise packaging of DNA into chromosomes and packing a home for a move to another home. Tap into the intuitive advantages of packaging DNA using this or any other analogy of highly packaged materials (perhaps a boxed “desk” that requires some assembly). • The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids. We have doubled the DNA, but the molecules remain attached (although not attached in the same way as the wire). You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes. © 2015 Pearson Education, Inc.

8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division To prepare for division, the chromatin (DNA and proteins) becomes highly compact and visible with a microscope. Student Misconceptions and Concerns • Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic when discussing DNA replication. • Students are often confused by photographs of chromosomes. Such photographs, such as Figure 8.3B, typically show duplicated chromosomes during some aspect of cell division. It remains unclear to many (a) why chromosome structure is typically different between interphase G1 and the stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes duplicate. Teaching Tips • Figure 8.3B is an important point of reference for some basic terminology. Consider referring to it as you distinguish between a DNA molecule and a chromosome, unreplicated and replicated chromosomes, and the nature of sister chromatids. • Consider an analogy between the precise packaging of DNA into chromosomes and packing a home for a move to another home. Tap into the intuitive advantages of packaging DNA using this or any other analogy of highly packaged materials (perhaps a boxed “desk” that requires some assembly). • The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids. We have doubled the DNA, but the molecules remain attached (although not attached in the same way as the wire). You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes. © 2015 Pearson Education, Inc.

8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division Before a eukaryotic cell begins to divide, it duplicates all of its chromosomes, resulting in two copies called sister chromatids. The sister chromatids are joined together and cinched at a “waist” called the centromere. When a cell divides, the sister chromatids separate from each other and are then called chromosomes, and sort into separate daughter cells. Student Misconceptions and Concerns • Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic when discussing DNA replication. • Students are often confused by photographs of chromosomes. Such photographs, such as Figure 8.3B, typically show duplicated chromosomes during some aspect of cell division. It remains unclear to many (a) why chromosome structure is typically different between interphase G1 and the stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes duplicate. Teaching Tips • Figure 8.3B is an important point of reference for some basic terminology. Consider referring to it as you distinguish between a DNA molecule and a chromosome, unreplicated and replicated chromosomes, and the nature of sister chromatids. • Consider an analogy between the precise packaging of DNA into chromosomes and packing a home for a move to another home. Tap into the intuitive advantages of packaging DNA using this or any other analogy of highly packaged materials (perhaps a boxed “desk” that requires some assembly). • The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (a) a chromosome before DNA replication and (b) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids. We have doubled the DNA, but the molecules remain attached (although not attached in the same way as the wire). You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes. © 2015 Pearson Education, Inc.

8.4 The cell cycle includes growing and division phases The cell cycle is an ordered sequence of two stages that starts from the time a cell is formed until its own division. Interphase: duplication of cell contents G1—growth, increase in cytoplasm S—duplication of chromosomes G2—growth, preparation for division Mitotic phase: division Mitosis—division of the nucleus Cytokinesis—division of cytoplasm Teaching Tips • The textbook authors note in Module 8.4 that each of your students consists of about 10 trillion cells. It is likely that this number is beyond comprehension for most of your students. Consider sharing several simple examples of the enormity of that number to try to make it more meaningful. For example, the U.S. population in 2014 is about 317 million people. To give every one of those people about $31,500, we will need a total of 10 trillion dollars. Here is another example. If we gave you $32,000 every second, it would take 10 years to give you 10 trillion dollars. The U.S. Debt Clock helps relate these large numbers to the U.S. national debt at www.usdebtclock.org. • In G1, the chromosomes have not duplicated. 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. © 2015 Pearson Education, Inc.

8.5 Cell division is a continuum of dynamic changes A mitotic spindle is required to divide the chromosomes, guides the separation of the two sets of daughter chromosomes Spindle microtubules emerge from two centrosomes, organizing regions in the cytoplasm of eukaryotic cells. Teaching Tips • Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT, which are the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase. Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

Mitosis Interphase Prophase Prometaphase Figure 8.5-1 Mitosis Interphase Prophase Prometaphase Centrosomes Fragments of the nuclear envelope Early mitotic spindle Chromatin Centrosome Kinetochore Figure 8.5-1 The stages of cell division by mitosis: interphase through prometaphase Centromere Nuclear envelope Spindle microtubules Plasma membrane Chromosome, consisting of two sister chromatids

8.5 Cell division is a continuum of dynamic changes Interphase The cytoplasmic contents double. Two centrosomes form. Chromosomes duplicate in the nucleus during the S phase. Teaching Tips • Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT, which are the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase. Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

8.5 Cell division is a continuum of dynamic changes Prophase (chromosomes become present) In the nucleus, chromosomes become more tightly coiled and folded. Teaching Tips • Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT, which are the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase. Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

8.5 Cell division is a continuum of dynamic changes Prometaphase The nuclear envelope breaks into fragments and disappears. Spindle microtubules form and extend from the centrosomes into the nuclear region and attach to kinetochores. Teaching Tips • Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT, which are the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase. Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Mitosis Metaphase Anaphase Telophase and Cytokinesis Metaphase plate Figure 8.5-6 Mitosis Metaphase Anaphase Telophase and Cytokinesis Metaphase plate Cleavage furrow Figure 8.5-6 The stages of cell division by mitosis: metaphase through cytokinesis Nuclear envelope forming Separated chromosomes Mitotic spindle

8.5 Cell division is a continuum of dynamic changes Metaphase The mitotic spindle is fully formed. Chromosomes align at the cell equator (middle). Kinetochores of sister chromatids are facing the opposite poles of the spindle. Teaching Tips • Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT, which are the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase. Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

8.5 Cell division is a continuum of dynamic changes Anaphase (apart) Sister chromatids separate at the centromeres. Daughter chromosomes are moved to opposite poles of the cell as motor proteins move the chromosomes along the spindle microtubules At the end of anaphase, the two ends of the cell have equal collections of chromosomes. Teaching Tips • Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT, which are the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase. Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

8.5 Cell division is a continuum of dynamic changes Telophase The cell continues to elongate. The nuclear envelope forms around chromosomes at each pole, establishing daughter nuclei. Chromatin uncoils. The mitotic spindle disappears. Teaching Tips • Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT, which are the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase. Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

8.6 Cytokinesis differs for plant and animal cells During cytokinesis, the cytoplasm is divided into separate cells. In animal cells, cytokinesis occurs as a cleavage furrow forms and deepens to separate the contents into two cells. In plant cells, cytokinesis occurs as a cell plate forms in the middle and extends to form new cell walls Teaching Tips • 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). This occurs in human development during the formation of the placenta. • The authors make an analogy between a drawstring on a hooded sweatshirt and the mechanism of cytokinesis in animal cells. Students seem to appreciate this association. 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. © 2015 Pearson Education, Inc.

8.7 Anchorage, cell density, and chemical growth factors affect cell division The cells within an organism’s body divide and develop at different rates. Cell division is controlled by anchorage dependence, the need for cells to be in contact with a solid surface to divide, density-dependent inhibition, in which crowded cells stop dividing, the presence of essential nutrients, and growth factors, proteins that stimulate division. Teaching Tips • Students who closely examine a small abrasion on their skin might notice that the wound tends to heal from the outer edges inward. This space-filling mechanism is a natural example of density-dependent density-dependent inhibition, which is also seen when cells in a cell culture dish stop dividing when they have formed a complete layer. © 2015 Pearson Education, Inc.

8.8 Growth factors signal the cell cycle control system The cell cycle control system is a cycling set of molecules that trigger and coordinate key events in the cell cycle. Checkpoints in the cell cycle can stop an event or signal an event to proceed. Teaching Tips • The cell cycle control system depicted in Figure 8.8A is like the control device of an automatic washing machine (which uses a turning dial). Each has a control system that triggers and coordinates key events in the cycle. However, unlike a washing machine, the components of the control system of a cell cycle are not all located in one place. © 2015 Pearson Education, Inc.

8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors Cancer currently claims the lives of 20% of the people in the United States. Cancer cells escape controls on the cell cycle. Cancer cells divide excessively and invade other tissues of the body. Student Misconceptions and Concerns • Students do not typically know that not 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 • Chemotherapy has some disastrous side effects. The drugs used to fight cancer may 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. Many other approaches to attack cancers are under consideration. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. Active Lecture Tips • See the Activity Losing Control of a Car Relates to Unregulated Cell Division on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors A tumor is a mass of abnormally growing cells within otherwise normal tissue. Benign tumors remain at the original site but may disrupt certain organs if they grow in size. Malignant tumors can spread to other locations in a process called metastasis. An individual with a malignant tumor is said to have cancer. Student Misconceptions and Concerns • Students do not typically know that not 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 • Chemotherapy has some disastrous side effects. The drugs used to fight cancer may 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. Many other approaches to attack cancers are under consideration. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. Active Lecture Tips • See the Activity Losing Control of a Car Relates to Unregulated Cell Division on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors Localized tumors can be removed surgically and/or treated with concentrated beams of high-energy radiation. Metastatic tumors are treated with chemotherapy. It is increasingly possible to personalize cancer treatment by sequencing the genome of tumor cells and tailoring treatment based upon the tumor’s specific genetic profile. Student Misconceptions and Concerns • Students do not typically know that not 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 • Chemotherapy has some disastrous side effects. The drugs used to fight cancer may 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. Many other approaches to attack cancers are under consideration. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks. Active Lecture Tips • See the Activity Losing Control of a Car Relates to Unregulated Cell Division on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

Meiosis and Crossing Over © 2015 Pearson Education, Inc.

8.11 Chromosomes are matched in homologous pairs Homologous chromosomes are matched in length, centromere position, and staining pattern. A locus (plural, loci) is the position of a gene. Different versions of a gene may be found at the same locus on the homologous pair. Student Misconceptions and Concerns • Some students might conclude that sex chromosomes function only in determining the sex of an individual. As the authors note, sex chromosomes contain genes not involved in sex determination. Teaching Tips • Students might recall some basic genetics, remembering that for many traits a person receives a separate “signal” from mom and dad. These separate signals for the same trait are carried on the same portion of homologous chromosomes, such as the freckle trait noted in Module 8.11 of the textbook. Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

8.11 Chromosomes are matched in homologous pairs In humans, somatic cells (body cells) have 46 chromosomes forming 23 pairs of homologous chromosomes. The human sex chromosomes X and Y differ in size and genetic composition. The other 22 pairs of chromosomes are autosomes with the same size and genetic composition. Student Misconceptions and Concerns • Some students might conclude that sex chromosomes function only in determining the sex of an individual. As the authors note, sex chromosomes contain genes not involved in sex determination. Teaching Tips • Students might recall some basic genetics, remembering that for many traits a person receives a separate “signal” from mom and dad. These separate signals for the same trait are carried on the same portion of homologous chromosomes, such as the freckle trait noted in Module 8.11 of the textbook. Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

8.12 Gametes have a single set of chromosomes Humans and many animals and plants are diploid (2n), because all somatic cells contain pairs of homologous chromosomes. Gametes are eggs and sperm and are said to be haploid (n) because each cell has a single set of chromosomes. n = chromosomes sets Student Misconceptions and Concerns • 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips • Consider helping students think 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. • In the shoe analogy, females have 23 pairs of matching shoes, while males have 22 matching pairs and 1 odd pair . . . maybe a sandal and a sneaker! • 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 growth.) Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

8.12 Gametes have a single set of chromosomes The human life cycle begins when a haploid sperm fuses with a haploid egg in fertilization. The zygote, formed by fertilization, is now diploid. Mitosis of the zygote generates all the somatic cells of an adult Student Misconceptions and Concerns • 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips • Consider helping students think 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. • In the shoe analogy, females have 23 pairs of matching shoes, while males have 22 matching pairs and 1 odd pair . . . maybe a sandal and a sneaker! • 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 growth.) Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

8.12 Gametes have a single set of chromosomes Meiosis is a type of cell division that produces haploid gametes in diploid organisms in the ovaries and testes. Meiosis reduces the chromosome number by half. Student Misconceptions and Concerns • 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips • Consider helping students think 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. • In the shoe analogy, females have 23 pairs of matching shoes, while males have 22 matching pairs and 1 odd pair . . . maybe a sandal and a sneaker! • 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 growth.) Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

8.13 Meiosis reduces the chromosome number from diploid to haploid Interphase: Like mitosis, meiosis is preceded by an interphase, during which the chromosomes duplicate. Student Misconceptions and Concerns • 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips • Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

8.13 Meiosis reduces the chromosome number from diploid to haploid Meiosis I – key events Homologous chromosomes separate Chromatids of homologous chromosomes exchange segments in a process called crossing over. Student Misconceptions and Concerns • 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips • Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

8.13 Meiosis reduces the chromosome number from diploid to haploid Each of the two haploid products enters meiosis II. Meiosis II – key event Sister chromatids separate Student Misconceptions and Concerns • 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips • Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

8.14 VISUALIZING THE CONCEPT: Mitosis and meiosis have important similarities and differences Mitosis and meiosis both begin with diploid parent cells that have chromosomes duplicated during the previous interphase. However, the end products differ. Mitosis produces two genetically identical diploid somatic daughter cells. Meiosis produces four genetically unique haploid gametes. Student Misconceptions and Concerns • 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips • How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but seldom understood. It can be explained like this. Consider a pair of chromosomes in a cell before any cell divisions. This pair of chromosomes duplicates such that two chromosomes become four (although each pair of sister chromatids are joined at their centromeres). Therefore, mitosis and meiosis each typically begin with four “chromosomes” after replication but before division. Mitosis divides once, producing two cells, each with two chromosomes. Meiosis divides twice, sorting the four chromosomes into four separate cells. • 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 pairs of chromosomes separate. Consider sketching a comparison of the alignment of the chromosomes at mitosis metaphase and metaphase meiosis I. Module 8.14 helps to make this important distinction. You might create a test question in which you ask students to draw several pairs of homologous chromosomes lined up at metaphase in mitosis versus meiosis I. Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

MITOSIS MEIOSIS I MEIOSIS II Figure 8.14-6 Figure 8.14-6 Comparison of mitosis and meiosis (step 6) MEIOSIS II One division of the nucleus and cytoplasm Result: Two genetically identical diploid cells Used for: Growth, tissue repair, asexual reproduction Two divisions of the nucleus and cytoplasm Result: Four genetically unique haploid cells Used for: Sexual reproduction

8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring Genetic variation in gametes results from crossing over independent orientation at metaphase I random fertilization. Teaching Tips • The possible number of combinations produced by independent orientation of human chromosomes at meiosis metaphase I is 223, , or 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! • Another way to represent the various combinations produced by independent orientation of chromosomes at meiosis metaphase I is to continue the shoe analogy. Imagine that you have two pairs of shoes. One pair is black; the other is white. You want to make a new pair of shoes by drawing one shoe from each original pair. Four possible pairs can be made. You can have (1) the left black and left white, (2) the right black and right white, (3) the left black and right white, or (4) the right black and left white. Actually using two pairs of shoes from your students can inject humor and create a concrete example that reduces confusion. For an additional bit of humor, ask the class if 46 students want to contribute their shoes as you try to demonstrate all 8,388,608 combinations! Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Two equally probable arrangements of chromosomes at Figure 8.15-3 Possibility A Possibility B Two equally probable arrangements of chromosomes at metaphase I Metaphase II Figure 8.15-3 Results of the independent orientation of chromosomes at metaphase I (step 3) Gametes Combination 1 Combination 2 Combination 3 Combination 4

8.17 Crossing over further increases genetic variability Genetic recombination is the production of new combinations of genes due to crossing over. Crossing over is an exchange of corresponding segments between nonsister chromatids of homologous chromosomes. Nonsister chromatids join at a chiasma (plural, chiasmata), the site of attachment and crossing over. Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids. Teaching Tips • If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes (although this analogy is quite limited). 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 (think of shoes with switched shoelaces). Critiquing this limited analogy may also help students to think through the process of crossing over. • In the shoe analogy, after exchanging shoelaces, we have “recombinant shoes”! • Challenge students to consider the number of unique humans that can be formed by the processes of the independent orientation of chromosomes, random fertilization, and crossing over. Without crossing over, we already calculated over 70 trillion possibilities. But as the text notes in Module 8.17, there are typically one to three crossover events for each human chromosome, and these can occur at many different places along the length of the chromosome. The potential number of combinations far exceeds any number that humans can comprehend, representing the truly unique nature of each human being (an important point that delights many students!). Active Lecture Tips • See the Activity Student Demonstration of Mitosis and Meiosis Using Chromosome Cut-Outs as Models on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Animation: Crossing Over © 2015 Pearson Education, Inc.

Sister chromatids Chiasma Tetrad Figure 8.17a-0 Figure 8.17a-0 Chiasmata, the sites of crossing over Tetrad

Figure 8.17b-0 How crossing over leads to genetic recombination Tetrad (pair of homologous chromosomes in synapsis) c e 1 Breakage of nonsister chromatids C E c e 2 Joining of nonsister chromatids C E Chiasma c e 3 Separation of homologous chromosomes at anaphase I C E C e c E c e Figure 8.17b-0 How crossing over leads to genetic recombination 4 Separation of chromatids at anaphase II and completion of meiosis C E Parental type of chromosome C e Recombinant chromosome c E Recombinant chromosome c e Parental type of chromosome Gametes of four genetic types

Alterations of Chromosome Number and Structure © 2015 Pearson Education, Inc.

8.18 Accidents during meiosis can alter chromosome number Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis. Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes. Student Misconceptions and Concerns • Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips • Students might be confused by the term nondisjunction. But simply put, it is an error in the sorting of chromosomes during mitosis or meiosis. • The Human Genome website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at www.genomics.energy.gov. © 2015 Pearson Education, Inc.

8.19 A karyotype is a photographic inventory of an individual’s chromosomes A karyotype is an ordered display of magnified images of an individual’s chromosomes arranged in pairs. Karyotypes are often produced from dividing cells arrested at metaphase of mitosis and allow for the observation of homologous chromosome pairs, chromosome number, and chromosome structure. Student Misconceptions and Concerns • Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips • The Human Genome website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at www.genomics.energy.gov. © 2015 Pearson Education, Inc.

Packed red and white blood cells Hypotonic solution Fixative Figure 8.19-1-3 Packed red and white blood cells Hypotonic solution Fixative Stain White blood cells Centrifuge Blood culture Fluid Figure 8.19-1-3 Preparation of a karyotype from a blood sample (part 1, step 3)

Figure 8.19-2 Figure 8.19-2 Preparation of a karyotype from a blood sample (part 2)

Centromere Sister chromatids Pair of homologous chromosomes Figure 8.19-3 Centromere Sister chromatids Pair of homologous chromosomes Figure 8.19-3 Preparation of a karyotype from a blood sample (part 3) Sex chromosomes

8.20 CONNECTION: An extra copy of chromosome 21 causes Down syndrome Trisomy 21 involves the inheritance of three copies of chromosome 21 and is the most common human chromosome abnormality. Student Misconceptions and Concerns • Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips • If you have several hundred students or more in your class, it is likely that at least one of your students has a sibling with Down syndrome. The authors note that overall, about one in every 700 babies are born with Down syndrome. • The National Down Syndrome Society has a website at www.ndss.org. It is a wonderful resource. • The Human Genome website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at www.genomics.energy.gov. Active Lecture Tips • See the Activity Applying the Concept of Non-Disjunction to Trisomy on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

8.20 CONNECTION: An extra copy of chromosome 21 causes Down syndrome A person with trisomy 21 has a condition called Down syndrome, which produces a characteristic set of symptoms, including characteristic facial features, short stature, heart defects, susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and varying degrees of developmental disabilities. The incidence increases with the age of the mother. Student Misconceptions and Concerns • Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips • If you have several hundred students or more in your class, it is likely that at least one of your students has a sibling with Down syndrome. The authors note that overall, about one in every 700 babies are born with Down syndrome. • The National Down Syndrome Society has a website at www.ndss.org. It is a wonderful resource. • The Human Genome website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at www.genomics.energy.gov. Active Lecture Tips • See the Activity Applying the Concept of Non-Disjunction to Trisomy on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

Infants with Down syndrome (per 1,000 births) Figure 8.20b 90 80 70 60 Infants with Down syndrome (per 1,000 births) 50 40 30 Figure 8.20b Maternal age and incidence of Down syndrome 20 10 20 25 30 35 40 45 Age of mother

8.21 CONNECTION: Abnormal numbers of sex chromosomes do not usually affect survival Sex chromosome abnormalities seem to upset the genetic balance less than an unusual number of autosomes. This may be because of the small size of the Y chromosome or X chromosome inactivation. Student Misconceptions and Concerns • Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips • Some syndromes related to human sexuality are not the result of abnormalities in sex chromosome number. Androgen insensitivity syndrome produces sterile males who possess mostly female sex characteristics. People with this condition are genetically male but have bodies that fail to respond to male sex hormones. The National Institutes of Health website “Genetics Home Reference” can provide additional details about this and most genetic disorders at http://ghr.nlm.nih.gov. • The Human Genome website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at www.genomics.energy.gov. © 2015 Pearson Education, Inc.

8.22 EVOLUTION CONNECTION: New species can arise from errors in cell division Errors in mitosis or meiosis may produce polyploid species, with more than two chromosome sets. The formation of polyploid species is widely observed in many plant species but less frequently found in animals. Student Misconceptions and Concerns • Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips • In general, flowering plants are more likely to form new species through polyploidy than animals, because unlike most animals, many flowering plants can fertilize themselves. • The gray tree frog, which is found over most of the eastern half of the United States, from Florida and Texas to Ontario and Maine, consists of two species: Hyla chrysoscelis, which is diploid, and Hyla versicolor, which is tetraploid. The two species cannot be distinguished, except by the number of chromosomes in their cells. The tetraploid species is thought to have been formed by an error in meiosis, similar to that frequently seen in plants. • The Human Genome website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at www.genomics.energy.gov. Tetraploid Grey Tree Frog © 2015 Pearson Education, Inc.

8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer Chromosome breakage can lead to four types of changes in chromosome structure. A deletion is the loss of a chromosome segment. A duplication is the repeat of a chromosome segment. An inversion is the reversal of a chromosome segment. A translocation is the attachment of a segment to a nonhomologous chromosome. A translocation may be reciprocal. Student Misconceptions and Concerns • Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips • Challenge students to create a simple sentence and then modify that sentence to represent (a) a deletion, (b) a duplication, and (c) an inversion as an analogy to these changes to a chromosome. • The Human Genome website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at www.genomics.energy.gov. © 2015 Pearson Education, Inc.

8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer Inversions are less likely than deletions or duplications to produce harmful effects, because in inversions all genes are still present in their normal number. Many deletions cause serious physical or mental problems. Translocations may or may not be harmful. Student Misconceptions and Concerns • Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips • Challenge students to create a simple sentence and then modify that sentence to represent (a) a deletion, (b) a duplication, and (c) an inversion as an analogy to these changes to a chromosome. • The Human Genome website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at www.genomics.energy.gov. © 2015 Pearson Education, Inc.

You should now be able to Compare the parent-offspring relationship in asexual and sexual reproduction. Explain why cell division is essential for prokaryotic and eukaryotic life. Explain how daughter prokaryotic chromosomes are separated from each other during binary fission. Compare the structure of prokaryotic and eukaryotic chromosomes. Describe the stages of the cell cycle. © 2015 Pearson Education, Inc.

You should now be able to List the phases of mitosis and describe the events characteristic of each phase. Compare cytokinesis in animal and plant cells. Explain how anchorage, cell density, and chemical growth factors control cell division. Explain how cancerous cells are different from healthy cells. Describe the functions of mitosis. Explain how chromosomes are paired. Distinguish between somatic cells and gametes and between diploid cells and haploid cells. © 2015 Pearson Education, Inc.

You should now be able to Explain why sexual reproduction requires meiosis. List the phases of meiosis I and meiosis II and describe the events characteristic of each phase. Compare mitosis and meiosis, noting similarities and differences. Explain how genetic variation is produced in sexually reproducing organisms. Explain how and why karyotyping is performed. Describe the causes and symptoms of Down syndrome. © 2015 Pearson Education, Inc.

You should now be able to Describe the consequences of abnormal numbers of sex chromosomes. Define nondisjunction, explain how it can occur, and describe what can result. Explain how new species form from errors in cell division. Describe the main types of chromosomal changes. Explain why cancer is not usually inherited. © 2015 Pearson Education, Inc.

Figure 8.0-0 Figure 8.0-0 Cellular Reproduction and Genetics

Genetically identical daughter cells M Cytokinesis G2 Mitosis Figure 8.UN01 S G1 (DNA synthesis) Genetically identical daughter cells M Cytokinesis G2 Mitosis Figure 8.UN01 Reviewing the concepts, 8.4 Cytokinesis (division of the cytoplasm) Mitosis (division of the nucleus)

n n 2n Haploid gametes (n = 23) Egg cell n n Sperm cell Meiosis Figure 8.UN02 Haploid gametes (n = 23) Egg cell n n n n Sperm cell Meiosis Fertilization Human life cycle Figure 8.UN02 Reviewing the concepts, 8.12 2n 2n Multicellular diploid adults (2n = 46) Diploid zygote (2n = 46) Mitosis

Figure 8.UN03 Figure 8.UN03 Connecting the concepts, question 1

Figure 8.UN04 Figure 8.UN04 Testing your knowledge, question 12