Chapter 16 Introduction PDF

Chapter 16 Introduction 12/12/24, 12:14 AM Page 331 CHAPTER 16 The Eukaryotic Cell Cycle, Mitosis, and Meiosis about:srcdoc Page 1 of 4 Chapter 16 Introduction 12/12/24...

Chapter 16 Introduction 12/12/24, 12:14 AM Page 331 CHAPTER 16 The Eukaryotic Cell Cycle, Mitosis, and Meiosis about:srcdoc Page 1 of 4 Chapter 16 Introduction 12/12/24, 12:14 AM A scanning electron micrograph of human chromosomes. These highly compacted chromosomes were found in a dividing cell. Biophoto Associates/Science Source about:srcdoc Page 2 of 4 Chapter 16 Introduction 12/12/24, 12:14 AM CHAPTER OUTLINE 16.1 The Eukaryotic Cell Cycle 16.2 Mitotic Cell Division 16.3 Meiosis 16.4 Sexual Reproduction 16.5 Variation in Chromosome Structure and Number Summary of Key Concepts Assessing Your Knowledge and Skills Over 10,000,000,000,000! Researchers estimate that the adult human body contains somewhere between 10 trillion and 50 trillion cells. It is an almost incomprehensible number. Even more amazing is the accuracy of the process that produces these cells. After a human sperm and egg unite, the fertilized egg goes through a long series of cell divisions to produce an adult with over 10 trillion cells. Let’s suppose you randomly removed a cell from your arm and compared it with a cell from your foot. If you about:srcdoc Page 3 of 4 Chapter 16 Introduction 12/12/24, 12:14 AM examined the chromosomes in both cells under the microscope, they would look identical. The DNA sequences along those chromosomes would also be the same, barring rare mutations. Similar comparisons could be made among the trillions of cells in your body. When you consider how many cell divisions are needed to produce an adult human, the precision of cell division is truly remarkable. What accounts for this high level of accuracy? As we will examine in this chapter, cell d ivision, the reproduction of cells, is a highly regulated process that distributes and monitors the integrity of the genetic material. The eukaryotic cell cycle is a series of phases needed for cell division. The cells of eukaryotic species follow one of two different sorting processes so that new daughter cells receive the correct number and types of chromosomes. The first sorting process we will explore, called mitosis, ensures that two daughter cells receive the same amount of genetic material as the mother cell that produced them. The second sorting process we will consider, called meiosis, is needed for sexual reproduction. In meiosis, cells that have two sets of chromosomes produce daughter cells with a single set of chromosomes. Lastly, we will look at variation in the structure and number of chromosomes. As you will see, certain mechanisms that alter chromosome structure and number have important consequences for the organisms that carry them. about:srcdoc Page 4 of 4 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM 16.1 The Eukaryotic Cell Cycle about:srcdoc Page 1 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM Learning Outcomes: 1. CoreSKILL » Describe the features of chromosomes, and explain how sets of chromosomes are examined microscopically. 2. Outline the phases of the eukaryotic cell cycle. 3. Explain how cyclins and cdks work together to advance a cell through the eukaryotic cell cycle. Life is a continuum in which new living cells are formed by the division of pre-existing cells. The Latin axiom Omnis cellula e cellula, meaning “Every cell originates from another cell,” was first proposed in 1858 by Rudolf Virchow, a German biologist. From an evolutionary perspective, cell division has a very ancient origin. All living organisms, from unicellular bacteria to multicellular plants and animals, have been produced by a series of repeated rounds of cell growth and division extending back to the beginnings of life nearly 4 billion years ago. A cell cycle is a series of events that leads to cell division. In all species, it is a highly regulated process, to ensure that cell division occurs at the appropriate time. As discussed in about:srcdoc Page 2 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM Chapter 19, bacterial cells produce more cells via binary fission. The cell cycle in eukaryotes is more complex, in part, because eukaryotic cells have sets of chromosomes that need to be sorted properly. In this section, we will examine the phases of the eukaryotic cell cycle and see how the cell cycle is controlled by proteins that carefully monitor the division process to ensure its accuracy. But first, we need to consider some general features of chromosomes in eukaryotic species. Page 332 Chromosomes Are Inherited in Sets and Occur in Homologous Pairs To understand the chromosomal composition of cells and the behavior of chromosomes during cell division, scientists use microscopes to observe cells and chromosomes. Cytogenetics is the field of genetics that involves the microscopic examination of chromosomes. When a cell prepares to divide, the chromosomes become more tightly compacted, a process that decreases their apparent length and increases their diameter. A consequence of this compaction is that distinctive shapes and numbers of chromosomes become visible under a light microscope. about:srcdoc Page 3 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM Microscopic Examination of Chromosomes Figure 16.1 shows the general procedure for preparing and viewing chromosomes from a eukaryotic cell. In this example, the cells were obtained from a sample of human blood. Specifically, the chromosomes within leukocytes (white blood cells) were examined. A sample of the blood cells was treated with drugs that stimulated them to divide. The actively dividing cells were centrifuged to concentrate them into a pellet, which was then mixed with a hypotonic solution that caused the cells to swell. The expansion of the cells caused the chromosomes to spread out from each other, making it easier to see each individual chromosome. Figure 16.1 The procedure for making a karyotype. In this example, the chromosomes were treated with Giemsa stain, and the resulting bands are called G bands. (4): Véronique Burger/Science Source; (5): Courtesy of the Genomic Centre for Cancer Research and Diagnosis, CancerCare Manitoba, University of Manitoba, Winnipeg, Manitoba, Canada Concept Concept Check: Check: Researchers usually treat cells with drugs that stimulate them to divide before beginning the procedure for making a karyotype. Why is this treatment about:srcdoc Page 4 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM useful? Answer Next, the cells were concentrated by a second centrifugation and treated with a fixative, which chemically fixed them in place so the chromosomes could no longer move around. The cells were then exposed to a chemical dye, such as Giemsa stain, that binds to the chromosomes and stains them. This gives chromosomes a distinctive banding pattern that greatly enhances their contrast and ability to be uniquely identified; in this case, the bands are called G bands. Page 333 The cells were then placed on a slide and viewed with a light microscope. In a cytogenetics laboratory, microscopes are equipped with an electronic camera to photograph the chromosomes. On a computer screen, the images of the chromosomes are organized in a standard way, usually from largest to smallest. A photographic representation of chromosomes, such as the photo in step 5 of Figure 16.1, is called a karyotype. A karyotype reveals the number, size, and form of chromosomes found within an actively dividing cell. It should also be noted that the chromosomes viewed in actively dividing cells have already replicated. The two copies are still joined to each other and are referred to as a pair of sister chromatids (see inset to Figure 16.1). Sets of Chromosomes about:srcdoc Page 5 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM Sets of Chromosomes What type of information is learned from a karyotype? By studying the karyotypes of many species, scientists have discovered that eukaryotic chromosomes occur in sets. Each set is composed of several different types of chromosomes. For example, one set of human chromosomes contains 23 different types of chromosomes (see Figure 16.1). By convention, the chromosomes are numbered according to size, with the largest chromosomes having the smallest numbers. For example, human chromosomes 1, 2, and 3 are relatively large, whereas 21 and 22 are the two smallest. This numbering system does not apply to the sex chromosomes, which determine the sex of the individual. Sex chromosomes in humans are designated with the letters X and Y; females are XX and males are XY. The chromosomes that are not sex chromosomes are called autosomes. Humans have 22 different types of autosomes. A second feature of many eukaryotic species is that most cells contain two sets of chromosomes. The karyotype shown in Figure 16.1 contains two sets of chromosomes, with 23 different chromosomes in each set. Therefore, this human cell contains a total of 46 chromosomes. Each cell has two sets because the individual inherited one set from the father and one set from the mother. When the cells of an organism carry two sets of chromosomes, that organism is said to be diploid. Geneticists use the letter n to represent a set of chromosomes. Diploid organisms are referred to as 2n, because they have two sets of chromosomes. For example, humans are 2n, where n = 23. Most human cells are diploid. An exception is the gametes, the sperm and egg cells. Gametes are haploid, or 1n, which means they contain one set of chromosomes. about:srcdoc Page 6 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM Homologous Pairs of Chromosomes When an organism is diploid, the members of a pair of chromosomes are called homologs (see inset to Figure 16.1). The term homology refers to any similarity that is due to common ancestry. Pairs of homologous chromosomes are evolutionarily derived from the same chromosome. However, homologous chromosomes are not usually identical because over many generations they have accumulated some genetic changes that make them distinct. How similar are homologous chromosomes to each other? Each of the two chromosomes in a homologous pair is nearly identical in size and contains a very similar composition of genetic material. A particular gene found on one copy of a chromosome is usually found on the homolog. However, because one homolog is received from each parent, the two homologs may vary in the way that a gene affects an organism’s traits. As an example, let’s consider a gene in humans called Herc2, which plays a major role in determining eye color. The Herc2 gene is found on chromosome 15. One copy of chromosome 15 might carry the form of this eye-color gene that confers brown eyes, whereas the gene on the homolog might confer blue eyes. The topic of how genes affect an organism’s traits will be considered in Chapter 17. The DNA sequences on homologous chromosomes are very similar. In most cases, the sequence of bases on one homolog differs by less than 1% from the sequence on the other homolog. For example, the DNA sequence of chromosome 1 that you inherited from your mother is likely to be more than 99% identical to the DNA sequence of chromosome 1 that you inherited from your father. Nevertheless, keep in mind that the sequences are not identical. The slight differences in DNA sequence provide important variation in gene function. Again, if we use the eye-color gene Herc2 as an example, a minor difference in DNA sequence distinguishes two forms of the gene, brown versus blue. The striking similarity between homologous chromosomes does not apply to the sex chromosomes (for example, X and Y). These chromosomes differ in size and genetic composition. Certain genes found on the X chromosome are not found on the Y chromosome, and vice versa. The X and Y chromosomes are not considered homologous chromosomes, although they do have short regions of homology. about:srcdoc Page 7 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM The Cell Cycle Is a Series of Phases That Lead to Cell Division Eukaryotic cells that are destined to divide advance through the cell cycle, a series of changes that involves growth, replication, and division, and ultimately produces new cells. Figure 16.2 provides an overview of the cell cycle. In this diagram, the mother cell has three pairs of chromosomes, for a total of six individual chromosomes. Such a cell is diploid (2n) and contains three chromosomes per set (n = 3). The paternal set is shown in blue, and the homologous maternal set is shown in red. about:srcdoc Page 8 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM Figure 16.2 The eukaryotic cell cycle. Dividing cells advance through a series of phases denoted G1, S, G2, and M. This diagram shows the advancement of a cell through the cell cycle to produce two daughter cells. The original diploid cell had three pairs of chromosomes, for a total of six individual chromosomes. During S phase, these replicate to yield 12 chromatids. After mitosis is complete, the two daughter cells each contain six individual chromosomes. The width of the phases shown in this figure is not meant to reflect their actual length. G1 is typically the longest phase of the cell cycle, whereas M phase is relatively short. Concept Concept Check: Check: Which phases make up interphase? Answer How the Cell Cycle Works The phases of the cell cycle are G1 (first gap), S (synthesis of DNA, the genetic material), G2 (second gap), and M phase (mitosis and cytokinesis). The G1 and G2 phases were originally described as gap phases to indicate the periods between DNA synthesis and mitosis. In actively about:srcdoc Page 9 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM dividing cells, the G1, S, and G2 phases are collectively known as interphase. During interphase, the cell grows and copies its chromosomes in preparation for cell division. Alternatively, a cell may exit the cell cycle and remain for long periods of time in a phase called G0 (G zero). The G0 phase is an alternative to proceeding through G1. A cell in the G0 phase has postponed division or, in the case of terminally differentiated cells (such as muscle cells in an adult animal), will never divide again. G0 is a nondividing phase. G 1 Phase The G1 phase is a period in a cell’s life when it may become committed to divide. Depending on the environmental conditions and the presence of signaling molecules, a cell in the G1 phase may accumulate molecular changes that cause it to advance through the rest of the cell cycle. Cell growth typically occurs during the G1 phase. Page 334 about:srcdoc Page 10 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM S Phase During the S phase, each chromosome is replicated to form a pair of sister chromatids (see Figure 16.1). When S phase is completed, a cell has twice as many chromatids as the number of chromosomes in the G1 phase. For example, a human cell in G1 phase has 46 distinct chromosomes, whereas the same cell in G2 phase will have 46 pairs of sister chromatids, for a total of 92 chromatids. G 2 Phase During the G2 phase, a cell synthesizes the proteins necessary for chromosome sorting and cell division. Some cell growth may occur. about:srcdoc Page 11 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM M Phase The first part of M phase is mitosis. The purpose of mitosis is to divide one cell nucleus into two nuclei, distributing the duplicated chromosomes so that each daughter cell receives the same complement of chromosomes. As noted previously, a human cell in G2 phase has 92 chromatids, which are found in 46 pairs. During mitosis, these pairs of chromatids are separated and sorted so that each daughter cell receives 46 chromosomes. In most cases, mitosis is followed by cytokinesis, which is the division of the cytoplasm to produce two distinct daughter cells. Cell Cycle Length The length of the cell cycle varies considerably among different cell types, ranging from several minutes in quickly growing embryos to several months in slow-growing adult cells. For fast- dividing mammalian cells in adults, such as skin cells, the length of the cycle is often in the range of 10 to 24 hours. The various phases within the cell cycle also vary in length. G1 is often the longest and also the most variable phase, and M is usually the shortest. For a cell that divides in 24 hours, the following lengths of time for the various phases are typical: Page 335 G1 phase: 11 hours S phase: 8 hours G2 phase: 4 hours M phase: 1 hour What factors determine whether or not a cell will divide? First, cell division is controlled by external factors, such as environmental conditions and signaling molecules. The effects of growth factors on cell division are discussed in Chapter 9 (refer back to Figure 9.10). Second, internal factors affect cell division. These include cell cycle control molecules and checkpoints, as we will discuss next. about:srcdoc Page 12 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM Cyclins and Cyclin-Dependent Kinases Advance a Cell Through the Cell Cycle In eukaryotes, the cell cycle is a highly regulated process to ensure that cells divide at the appropriate time. As discussed in Chapter 15, this regulation is also necessary to minimize the occurrence of mutations, which could have harmful effects and potentially lead to cancer. Different categories of proteins are needed so that cell division occurs properly. Cyclins and Cyclin-Dependent Kinases Proteins called cyclins and cyclin-dependent kinases (cdks) are responsible for advancing a cell through the phases of the cell cycle. Cyclins are so named because their amounts are cyclic— their levels rise and fall during the cell cycle. To be active, the cyclin-dependent kinases controlling the cell cycle must bind to (are dependent on) cyclins. The numbers of different types of cyclins and cdks vary from species to species. Figure 16.3 provides a simplified description of how cyclins and cdks work together to advance a cell through the cell cycle. 1. During G1, the amount of a particular cyclin termed G1 cyclin increases in response to sufficient nutrients and growth factors. 2. G1 cyclin binds to a cdk to form an activated G1 cyclin/cdk complex. Once activated, cdk functions as a protein kinase that phosphorylates other proteins needed to advance the cell to the next phase in the cell cycle. For example, certain proteins involved with DNA synthesis are phosphorylated and activated, thereby allowing the cell to replicate its DNA in S phase. 3. After the cell passes into the S phase, G1 cyclin is degraded. Similar events advance the cell through other phases of the cell cycle. 4. A different cyclin, called mitotic cyclin, accumulates late in G2. It binds to a cdk to form an activated mitotic cyclin/cdk complex. This complex phosphorylates proteins that are needed to advance the cell into M phase. After M phase is completed, mitotic cyclin is degraded. about:srcdoc Page 13 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM Figure 16.3 Checkpoints in the cell cycle. This is a general diagram of the eukaryotic cell cycle. Advancement through the cell cycle requires the formation of activated cyclin/cdk complexes. Cells make different types of cyclin proteins, which are typically degraded after the cell has advanced to the next phase. Also, a few different types of cdks are made, depending on the species and cell type. The formation of activated cyclin/cdk complexes is regulated by checkpoint proteins. about:srcdoc Page 14 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM Core Skill: Connections Look back at Figure 15.14. How do checkpoint proteins prevent cancer? Answer  Click the arrowheads to expand.  Figure 15.14 Additional eBook Question Control of the Cell Cycle Checkpoints Advancement through the cell cycle is a process that is highly regulated to ensure that the chromosomes are intact and that the conditions are appropriate for a cell to divide. Three critical regulatory points called checkpoints are found in the cell cycle of eukaryotic cells (see Figure 16.3). At these checkpoints, a variety of proteins, referred to as checkpoint proteins, control whether a cell will advance past a given checkpoint. Some checkpoint proteins function about:srcdoc Page 15 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM as sensors to determine if a cell is in the proper condition to divide. If these proteins sense that conditions are unfavorable, other checkpoint proteins will be activated that will prevent the cell from advancing through the cell cycle. These other checkpoint proteins often exert their effects by inhibiting cyclin-dependent kinases. Page 336 At the G1 checkpoint, also called the restriction point, the checkpoint proteins determine if conditions are favorable for cell division. In addition, G1 checkpoint proteins sense if the DNA has incurred damage. Although DNA damage is a relatively rare event, it is occasionally detected? If so, the checkpoint proteins prevent the formation of active cyclin/cdk complexes, thereby stopping the advancement of the cell cycle. A second checkpoint exists in G2. At this checkpoint, proteins also check the DNA for damage and ensure that all of the DNA has been replicated. In addition, the G2 checkpoint proteins monitor the levels of the proteins that are needed to advance through M phase. A third checkpoint, called the metaphase checkpoint, has proteins that monitor the integrity of the spindle apparatus. As we will see later in Section 16.2, the spindle apparatus is involved in chromosome sorting. Metaphase is a step in mitosis during which all of the chromosomes should be attached to the spindle apparatus. If a chromosome is not correctly attached, the metaphase checkpoint proteins will stop the cell cycle. This checkpoint prevents cells from incorrectly sorting their chromosomes during division. Checkpoint proteins delay the cell cycle until problems are fixed or prevent cell division when problems cannot be fixed. A primary aim of checkpoint proteins is to prevent the division of a cell that has incurred DNA damage or harbors abnormalities in chromosome number. As discussed in Chapter 15, when the functions of checkpoint genes are lost due to mutation, the likelihood increases that undesirable genetic changes will occur that can cause additional mutations and cancerous growth. about:srcdoc Page 16 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM Core Skill: Process of Science Feature Investigation | Masui and Markert’s Study of Oocyte Maturation Led to the Identification of Cyclins and Cyclin- Dependent Kinases During the 1960s and 1970s, researchers were intensely searching for the factors that promote cell division. In 1971, Japanese zoologist Yoshio Masui and American biologist Clement Markert developed a way to test whether a substance causes a cell to advance from one phase of the cell cycle to the next. They chose to study frog oocytes—cells that mature into egg cells. At the time of their work, researchers had already determined that frog oocytes naturally become dormant in the G2 phase of the cell cycle for up to 8 months ( Figure 16.4). During mating season, female frogs produce a hormone called progesterone. After progesterone enters an oocyte and binds to intracellular receptors, the oocyte advances from G2 to the beginning of M phase, where the chromosomes condense and become visible under the microscope. This phenomenon is called maturation. When a sperm fertilizes the egg, M phase is completed, and the zygote about:srcdoc Page 17 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM continues to undergo cellular divisions. Figure 16.4 Oocyte maturation in certain species of frogs. Because progesterone is a signaling molecule, Masui and Markert speculated that this hormone affects the functions and/or amounts of proteins that trigger the oocyte to advance through the cell cycle. To test this hypothesis, they developed the procedure described in Figure 16.5, using the oocytes of the leopard frog (Rana pipiens). They began by exposing oocytes to progesterone in vitro, and then they incubated these oocytes for 2 hours or 12 hours. As a control, they also used oocytes that had not been exposed to progesterone. These three types of cells were called the donor oocytes. Page 337 about:srcdoc Page 18 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM HYPOTHESIS Progesterone induces the synthesis of factor(s) that advance(s) frog oocytes through the cell cycle from G2 to M phase. KEY MATERIALS Oocytes from Rana pipiens. Experimental level Conceptual level 1. Expose oocytes to progesterone, then incubate for 2 or 12 hours. As a control, also use oocytes that have not been exposed to progesterone. All 3 types are donor oocytes. 2. Using a micropipette, transfer some cytosol from the 3 types of donor oocytes to recipient oocytes that have not been exposed to progesterone. 3. Incubate for several hours, and observe the recipient oocytes under the microscope to determine if the recipient oocytes advance to M phase. Advancement to M phase can be determined by the condensation of the chromosomes. 4. THE DATA about:srcdoc Page 19 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM Recipient oocytes proceeded to M Donor oocytes phase? Control, no progesterone exposure No Progesterone exposure, incubation for 2 No hours Progesterone exposure, incubation for Yes 12 hours 5. CONCLUSION Exposure of oocytes to progesterone for 12 hours results in the synthesis of factor(s) that advance(s) frog oocytes through the cell cycle from G2 to M phase. 6. SOURCE Masui, Y., and Markert, C. L. 1971. Cytoplasmic Control of Nuclear Behavior During Meiotic Maturation of Frog Oocytes. Journal of Experimental Zoology 177: 129−145. Figure 16.5 The experimental approach of Masui and Markert to identify cyclin and cyclin- dependent kinase (cdk). about:srcdoc Page 20 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM Next, Masui and Markert used a micropipette to transfer a small amount of cytosol from the three types of donor oocytes to recipient oocytes that had not been exposed to progesterone. As seen in the data, the recipient oocytes that had been injected with cytosol from the control donor oocytes or from oocytes that had been incubated with progesterone for only 2 hours did not advance to M phase. However, cytosol from donor oocytes that had been incubated with progesterone for 12 hours caused the recipient oocytes to advance to M phase. Masui and Markert concluded that a cytosolic factor, which required more than 2 hours to be synthesized after progesterone treatment, had been transferred to the recipient oocytes and induced maturation. The factor that caused the oocytes to advance (or mature) from G2 to M phase was originally called the maturation-promoting factor (MPF). After MPF was discovered in frogs, it was found in all eukaryotic species that researchers studied. MPF is important in the division of all types of cells, not just oocytes. It took another 17 years before Manfred Lohka, Marianne Hayes, and James Maller were able to purify the components that make up MPF. This was a difficult undertaking because these components are found in very small amounts in the cytosol and are easily degraded during purification procedures. We now know that MPF is a complex made of a mitotic cyclin and a cyclin-dependent kinase (cdk), as shown in Figure 16.3. Experimental Questions about:srcdoc Page 21 of 22 16.1 The Eukaryotic Cell Cycle 12/12/24, 12:15 AM Experimental Questions 1. At the time of Masui and Markert’s study, summarized in Figure 16.5, what was known about the effects of progesterone on oocytes? Answer 2. CoreSKILL » What hypothesis did Masui and Markert propose to explain the function of progesterone? Describe the procedure used to test the hypothesis. Answer 3. CoreSKILL » How did the researchers explain the difference between the results with 2-hour-exposed donor oocytes and those with 12-hour- exposed donor oocytes? Answer about:srcdoc Page 22 of 22 16.2 Mitotic Cell Division 12/12/24, 12:16 AM Page 338 16.2 Mitotic Cell Division about:srcdoc Page 1 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM Learning Outcomes: 1. Explain how the replication of eukaryotic chromosomes produces sister chromatids. 2. Describe the structure and function of the spindle apparatus. 3. Outline the key events that occur during the phases of mitosis. We now turn our attention to a mechanism of cell division and its relationship to chromosome replication and sorting. During the process of mitotic cell division, a cell divides to produce two new cells (the daughter cells) that are genetically identical to the original cell (the mother cell). Mitotic cell division involves mitosis—the division of one nucleus into two nuclei—followed by cytokinesis—in which the mother cell divides into two daughter cells. Why is mitotic cell division important? One reason is asexual reproduction, a process in which genetically identical offspring are produced from a single parent. Certain unicellular eukaryotic organisms, such as baker’s yeast (Saccharomyces cerevisiae) and the amoeba, increase their numbers in this manner. A second important reason for mitotic cell division is the production about:srcdoc Page 2 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM and maintenance of multicellularity. Organisms such as plants, animals, and most fungi are derived from a single cell that subsequently undergoes repeated cell divisions to become a multicellular organism. In this section, we will explore the process of mitotic cell division, which requires the replication, organization, and sorting of chromosomes. We will also examine how a single cell is separated into two daughter cells by cytokinesis. In Preparation for Cell Division, Eukaryotic Chromosomes Are Replicated and Compacted to Produce Pairs Called Sister Chromatids We now turn our attention to how chromosomes are replicated and sorted during cell division. In Chapter 11, we examined the molecular process of DNA replication. Figure 16.6 describes the process at the chromosomal level. Prior to DNA replication, the DNA of each eukaryotic chromosome consists of a linear double helix that is found in the nucleus and is not highly compacted. When the DNA is replicated, two identical copies of the original double helix are produced. As discussed earlier, these copies, along with associated proteins, lie side- about:srcdoc Page 3 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM by-side and are termed sister chromatids. When a cell prepares to divide, the sister chromatids become highly compacted and readily visible under the microscope. Figure 16.6 Replication and compaction of chromosomes into pairs of sister chromatids. (a) Chromosomal replication produces a pair of sister chromatids. While the chromosomes are elongated, they are replicated to produce two copies that are connected and lie parallel to each other. This is a pair of sister chromatids. Later, when the cell is preparing to divide, the sister chromatids condense into more compact structures that are easily seen with a light microscope. (b) A schematic drawing of a metaphase chromosome. This structure has two chromatids that lie side-by-side. The two chromatids are held together by cohesin proteins (not shown in this drawing). The kinetochore is a group of proteins that are attached to the centromere and play a key role during chromosome sorting. about:srcdoc Page 4 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM Core Concept: Information The process of mitosis ensures that each daughter cell receives a complete copy of the genetic material.  Click the arrowhead to expand.  Additional eBook Question As shown in Figure 16.6b, the two sister chromatids are tightly associated at a region called the centromere. A protein called cohesin holds the sister chromatids together. In addition, the centromere serves as an attachment site for a group of proteins that form the kinetochore, a structure necessary for sorting the chromosomes. Page 339 about:srcdoc Page 5 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM The Spindle Apparatus Organizes and Sorts Chromosomes During Cell Division What structure is responsible for organizing and sorting the chromosomes during cell division? The answer is the spindle apparatus ( Figure 16.7). It is composed of microtubules—protein fibers that are components of the cytoskeleton (refer back to Table 4.1). In animal cells, microtubule growth and organization start at two centrosomes, structures that are also referred to as microtubule-organizing centers (MTOCs). A single centrosome duplicates during interphase. When the cell enters mitosis, each centrosome defines a pole of the spindle apparatus, one within each of the future daughter cells. The centrosome in animal cells has a pair of centrioles. Each one is composed of nine sets of triplet microtubules. However, centrioles are not found in many other eukaryotic species, such as plants, and are not required for spindle formation. about:srcdoc Page 6 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM Figure 16.7 The structure of the spindle apparatus. The spindle apparatus in animal cells is formed by the centrosomes, which produce three types of microtubules. The astral microtubules emanate away from the region between the poles. The polar microtubules project into the region between the two poles. The kinetochore microtubules are attached to the kinetochores of sister chromatids. Note: For simplicity, this diagram shows only one pair of homologous chromosomes. Eukaryotic species typically have multiple chromosomes per set. Each centrosome organizes the construction of the microtubules by rapidly polymerizing tubulin proteins. The three types of spindle microtubules are termed astral, polar, and kinetochore microtubules (see Figure 16.7). about:srcdoc Page 7 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM The Transmission of Chromosomes Requires a Sorting Process Known as Mitosis Mitosis is the sorting process for dividing one cell nucleus into two nuclei ( Figure 16.8). The duplicated chromosomes are distributed so that each daughter cell receives one copy of each chromosome. Mitosis was first observed microscopically in the 1870s by a German biologist, Walther Flemming, who coined the term (from the Greek mitos, meaning thread). He studied the large, transparent skin cells of salamander larvae as they were dividing and noticed that chromosomes are constructed of “threads” that are doubled in appearance along their length. These double threads divided and moved apart, one going to each of the two daughter nuclei. By this mechanism, Flemming pointed out, the two daughter cells receive an identical group of threads, the same as the number of threads in the mother cell. Mitosis and Cytokinesis about:srcdoc Page 8 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM Figure 16.8 The process of mitosis in an animal cell. The top panels are fluorescence micrographs of a newt cell advancing through mitosis. The drawings below emphasize the sorting of the chromosomes, in which the diploid mother cell had six chromosomes (three in each set). At the start of mitosis, these have already replicated into 12 chromatids. The final result is two daughter cells, each containing six chromosomes. a–f: Photographs by Dr. Conly L. Rieder, East Greenbush, New York, 12061 Concept Concept Check: Check: What are the functions of the three types of microtubules? Answer Mitosis Figure 16.8 depicts the process of mitosis in an animal cell, though the process is quite similar in a plant cell. Mitosis occurs as a continuum of phases known as prophase, prometaphase, metaphase, anaphase, and telophase. In the simplified diagrams along the bottom of Figure 16.8, the original cell contains six chromosomes. One set of chromosomes is depicted in red, and the homologous set is blue. The different colors are intended to distinguish maternal and paternal chromosomes. about:srcdoc Page 9 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM Interphase Prior to mitosis, the cells are in interphase, which consists of the G1, S, and G2 phases of the cell cycle. The chromosomes have replicated in S phase and are decondensed and found in the nucleus ( Figure 16.8a). The nucleolus, which is the site where the components of ribosomes assemble into ribosomal subunits, is visible during interphase. Prophase At the start of mitosis, in prophase, the chromosomes have already replicated to produce 12 chromatids, joined as six pairs of sister chromatids that have condensed into highly compacted structures readily visible by light microscopy ( Figure 16.8b). As prophase proceeds, the nuclear envelope begins to dissociate into small vesicles. The nucleolus is no longer visible. about:srcdoc Page 10 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM Prometaphase During prometaphase, the nuclear envelope completely fragments into small vesicles, and the spindle apparatus is fully formed ( Figure 16.8c). As prometaphase advances, the centrosomes move apart and demarcate the two poles. Once the nuclear envelope has dissociated, the spindle fibers can interact with the sister chromatids. How do the sister chromatids become attached to the spindle apparatus? Initially, microtubules are rapidly formed and can be seen under a microscope growing out from the two poles. As it grows, if a microtubule happens to make contact with a kinetochore, it is said to be captured and remains firmly attached to the kinetochore. Alternatively, if a microtubule does not collide with a kinetochore, the microtubule eventually depolymerizes and retracts to the centrosome. This random process is how sister chromatids become attached to kinetochore microtubules. As the end of prometaphase nears, the two kinetochores on each pair of sister chromatids are attached to kinetochore microtubules from opposite poles. As these events are occurring, the sister chromatids are seen under the microscope to undergo jerky movements as they are tugged, back and forth, between the two poles by the kinetochore microtubules. Metaphase about:srcdoc Page 11 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM Metaphase Eventually, the pairs of sister chromatids are aligned in a single row along the metaphase plate, a plane halfway between the poles of the spindle apparatus. When this alignment is complete, the cell is in metaphase of mitosis ( Figure 16.8d). The chromatids can then be equally distributed into two daughter cells. Page 340 Anaphase During anaphase, the connections between the pairs of sister chromatids are broken ( Figure 16.8e). Each chromatid, now an individual chromosome, is linked to only one of the two poles by one or more kinetochore microtubules. As anaphase proceeds, the kinetochore microtubules shorten, pulling the chromosomes toward the pole to which they are attached. In addition, the two poles move farther away from each other. This occurs because the overlapping polar microtubules lengthen and push against each other, thereby pushing the poles farther apart. about:srcdoc Page 12 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM Telophase During telophase, the chromosomes have reached their respective poles and decondense. The nuclear envelope now re-forms to produce two separate nuclei. In Figure 16.8f, two nuclei that contain six chromosomes each are being produced. Cytokinesis In most cases, mitosis is quickly followed by cytokinesis, in which the two nuclei are segregated into separate daughter cells. Whereas the phases of mitosis are similar between plant and animal cells, the process of cytokinesis is quite different. In animal cells, cytokinesis involves the formation of a cleavage furrow, which constricts like a drawstring to separate the cells ( Figure 16.9a). In plants, vesicles from the Golgi apparatus move along microtubules to the center of the cell and coalesce to form a cell plate ( Figure 16.9b), which then forms a cell wall between the two daughter cells. Cell Division about:srcdoc Page 13 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM Figure 16.9 Micrographs showing cytokinesis in animal and plant cells. a: Don W. Fawcett/Science Source; b: Kent Wood/Science Source Cytokinesis Cytoskeletal filaments, which are discussed in Chapter 4, play key roles in the process of cytokinesis. Microtubules are important for the proper positioning of the cleavage plane in animal cells and in the formation of a cell plate in plants. In animals, actin is involved in the formation of the cleavage furrow. Page 341 What are the results of mitosis and cytokinesis? These processes ultimately produce two daughter cells with the same number of chromosomes as the mother cell. Barring rare about:srcdoc Page 14 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM mutations, the two daughter cells are genetically identical to each other and to the mother cell from which they were derived. The critical consequence of this sorting process is ensuring genetic consistency from one cell to the next. The development of multicellularity relies on the repeated process of mitosis and cytokinesis. Core Concept: Evolution Mitosis in Eukaryotes Evolved from Binary Fission That Occurs in Prokaryotic Cells The process of mitosis allows eukaryotic cells to properly sort their chromosomes during cell division. By comparing cell division among prokaryotic cells, simple eukaryotes, and more complex eukaryotes, biologists have pieced together a progression of how mitosis may have evolved. Binary Fission in Bacterial Cells As described in Chapter 19 (look ahead to Figure 19.14), bacterial cells divide by a relatively simple process known as binary fission ( Figure 16.10a). After chromosome replication, each copy of the bacterial about:srcdoc Page 15 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM chromosome becomes anchored to the plasma membrane. Proteins called FtsZ form a ring at the site where the mother cell will divide into two daughter cells. about:srcdoc Page 16 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM Figure 16.10 A comparison of cell division among bacteria, simple eukaryotes, and more complex eukaryotes. Dinoflagellates In protists known as dinoflagellates, nuclear division is much simpler than in animal and plant cells ( Figure 16.10b). After chromosome replication, the chromosomes become attached to the nuclear envelope. The nuclear envelope does not break apart. Microtubules, which are described in Chapter 4 (see Table 4.1), are formed in the cytosol and pass through tunnels in the nuclear envelope. The nucleus then divides by a process that resembles binary fission. Page 342 Diatoms and Some Yeasts In diatoms (a type of protist) and some yeasts (a type of fungus), microtubules form within the cell nucleus ( Figure 16.10c). Kinetochore microtubules attach to chromosomes and facilitate their sorting, and other microtubules promote the separation of the nucleus into two separate nuclei. As in dinoflagellates, the nuclear envelope does not break apart during this process. Complex Eukaryotes As we have seen, mitosis in complex eukaryotes, such as animals and plants, involves the breaking apart of the nuclear envelope and the formation of the spindle apparatus ( Figure 16.10d). After the chromosomes are sorted, the nuclear envelope then re-forms. Interestingly, the FtsZ proteins found in bacteria are evolutionarily related to tubulin, which is the main component of eukaryotic microtubules. Filaments composed of FtsZ proteins assemble into a structure called a Z-ring ( Figure 16.10a). In eukaryotic cells, about:srcdoc Page 17 of 18 16.2 Mitotic Cell Division 12/12/24, 12:16 AM the homologous protein, tubulin, forms linear microtubules and has acquired additional roles in cell division. These include the division of the cell nucleus and the sorting of chromosomes. The relationship of tubulin to FtsZ is an example of descent with modification. about:srcdoc Page 18 of 18 16.3 Meiosis 12/12/24, 12:16 AM Page 343 16.3 Meiosis about:srcdoc Page 1 of 17 16.3 Meiosis 12/12/24, 12:16 AM Learning Outcomes: 1. Describe the processes of synapsis and crossing over. 2. Outline the key events that occur during the phases of meiosis. 3. Compare and contrast mitosis and meiosis, focusing on key steps that account for the different outcomes of these two processes. As discussed earlier, a diploid cell contains two homologous sets of chromosomes, whereas a haploid cell contains a single set. For example, a diploid human cell contains 46 chromosomes, but a human gamete—sperm or egg cell—is a haploid cell that contains only 23 chromosomes. Meiosis is the process by which haploid cells are produced from a cell that was originally diploid. The term meiosis, which means "to make smaller," refers to the fewer chromosomes found in cells that have undergone this process. For haploid cells to be produced, the chromosomes must be correctly sorted and distributed in a way that reduces the chromosome number to half its original diploid value. In the case of human gametes, for example, each gamete must receive one chromosome from each of the 23 about:srcdoc Page 2 of 17 16.3 Meiosis 12/12/24, 12:16 AM pairs. For this to happen, two rounds of divisions are necessary, termed meiosis I and meiosis II ( Figure 16.11). When a cell begins meiosis, it contains chromosomes that are found in homologous pairs. When meiosis is completed, a single diploid cell with homologous pairs of chromosomes has produced four haploid cells. Figure 16.11 How the process of meiosis reduces chromosome number. This simplified diagram emphasizes the reduction in chromosome number as a diploid cell divides by meiosis to produce four haploid cells. about:srcdoc Page 3 of 17 16.3 Meiosis 12/12/24, 12:16 AM How Meiosis Works In this section, we will examine the cellular events of meiosis that reduce the chromosome number from diploid to haploid. In the following section, we will consider how this process plays a role in the sexual reproduction of animals, plants, fungi, and protists. Bivalent Formation and Crossing Over Occur at the Beginning of Meiosis Like mitosis, meiosis begins after a cell has advanced through the G1, S, and G2 phases of the cell cycle. However, two key events occur at the beginning of meiosis that do not occur in mitosis. First, homologous pairs of sister chromatids associate with each other, lying side by side to form a bivalent, also called a tetrad ( Figure 16.12). The process of forming a bivalent is termed synapsis. In most eukaryotic species, a protein structure called the synaptonemal complex connects homologous chromosomes during a portion of meiosis. However, the synaptonemal complex is not required for the pairing of homologous chromosomes because some species of fungi completely lack such a complex, yet their chromosomes associate with about:srcdoc Page 4 of 17 16.3 Meiosis 12/12/24, 12:16 AM each other correctly. Figure 16.12 Formation of a bivalent and crossing over during meiosis I. At the beginning of meiosis, homologous chromosomes pair with each other to form a bivalent, usually with a synaptonemal complex between them. Crossing over then occurs between homologous chromatids within the bivalent. During this process, homologs exchange segments of chromosomes. The second event that occurs at the beginning of meiosis, but not usually during mitosis, is cros sing over, which involves a physical exchange between chromosome segments of the bivalent ( Figure 16.12). As discussed in Chapter 18, crossing over increases the genetic variation of sexually reproducing species. After crossing over occurs, the arms of the chromosomes tend to separate but remain adhered at a crossover site. This connection is called a chiasma (plural, chiasmata), because the connected chromosomal arms resemble the Greek letter chi, χ. The number of crossovers is carefully controlled by cells and depends on the size of the chromosome and the species. The range of this number for eukaryotic chromosomes is typically from one or two to a couple dozen. During the formation of sperm in humans, for example, an average chromosome undergoes slightly more than two crossovers, whereas chromosomes in certain plant species may undergo 20 or more. about:srcdoc Page 5 of 17 16.3 Meiosis 12/12/24, 12:16 AM Meiosis with Crossing Over Meiosis I Separates Homologous Chromosomes Now that you have an understanding of bivalent formation and crossing over, we are ready to consider the phases of meiosis ( Figure 16.13). These simplified diagrams depict a diploid cell (2n) with three chromosomes per set for a total of six chromosomes (as in the diagram of mitosis in Figure 16.8). Prior to meiosis, the chromosomes are replicated in S phase to produce pairs of sister chromatids. This single replication event is then followed by sequential divisions called meiosis I and II. Like mitosis, each of these processes is a continuous series of stages called prophase, prometaphase, metaphase, anaphase, and telophase. The sorting that occurs during meiosis I separates homologous chromosomes from each other ( Figure 16.13a–e). Meiosis I about:srcdoc Page 6 of 17 16.3 Meiosis 12/12/24, 12:16 AM Figure 16.13 The phases of meiosis in an animal cell. Stages of Meiosis about:srcdoc Page 7 of 17 16.3 Meiosis 12/12/24, 12:16 AM Core Skill: Modeling The goal of this modeling challenge is to predict the outcome of meiosis if one pair of chromosomes does not separate properly during meiosis II. Modeling Challenge: In Figure 16.13, the starting cell in meiosis I has 3 homologous pairs of chromosomes that differ in length: short, medium, and long. Let’s suppose that during meiosis I, the segregation of the long chromosomes into the two daughter cells occurs abnormally, and both of the long chromosomes go into the same daughter cell. Draw a model showing the chromosomal composition of the four daughter cells at the end of meiosis II. For each pair of homologs, draw one red and the other blue, as in the figure. You do not need to include crossovers in your model. about:srcdoc Page 8 of 17 16.3 Meiosis 12/12/24, 12:16 AM Page 344 Prophase I During prophase I, the replicated chromosomes condense, the homologous chromosomes form bivalents, and crossing over occurs. The nuclear envelope then starts to fragment into small vesicles. about:srcdoc Page 9 of 17 16.3 Meiosis 12/12/24, 12:16 AM Prometaphase I In prometaphase I, the nuclear envelope is completely broken apart into vesicles, and the spindle apparaus is entirely formed. The sister chromatids become attached to kinetochore microtubules. However, a key difference exists between mitosis and meiosis I. In mitosis, each pair of sister chromatids is attached to both poles (see Figure 16.8c). In meiosis I, each pair of sister chromatids is attached to just one pole via kinetochore microtubules ( Figure 16.13b). Metaphase I about:srcdoc Page 10 of 17 16.3 Meiosis 12/12/24, 12:16 AM Metaphase I At metaphase I, the bivalents are organized along the metaphase plate. Notice how this pattern of alignment is strikingly different from that observed during mitosis (see Figure 16.8d). In particular, the sister chromatids are aligned in a double row rather than a single row (as in mitosis). Furthermore, the arrangement of sister chromatids within this double row is random with regard to the (red and blue) homologs. (Remember that these different colors indicate maternal and paternal chromosomes.) In Figure 16.13c, one of the red homologs is to the left of the metaphase plate, and the other two are to the right, whereas two of the blue homologs are to the left of the metaphase plate and the other one is to the right. In other cells, homologs can be arranged differently along the metaphase plate (for example, three blues to the left and none to the right, or none to the left and three to the right). Random Orientation of Chromosomes During Meiosis Because eukaryotic species typically have many chromosomes per set, maternal and paternal homologs can be randomly aligned along the metaphase plate in a variety of ways. The possible number of different, random alignments equals 2n, where n equals the number of chromosomes per set. The reason why the random alignments equals 2n is because each chromosome is found in a homologous pair and each member of the pair can align on either side of the metaphase plate. It is a matter of chance which daughter cell of meiosis I will get the maternal chromosome of a homologous pair, and which will get the paternal chromosome. In humans, who have 23 chromosomes per set, 2n equals 223, or over 8 million possibilities. Because the homologs are genetically similar but not identical, we see from this calculation that the random alignment of homologous chromosomes provides a mechanism to promote a vast amount of genetic diversity among the resulting haploid cells. When meiosis is complete, any two human gametes are extremely unlikely to have the same combination of homologous chromosomes. about:srcdoc Page 11 of 17 16.3 Meiosis 12/12/24, 12:16 AM Anaphase I The segregation of homologs occurs during anaphase I ( Figure 16.13d). The connections between bivalents break, but not the connections that hold sister chromatids together. Each joined pair of chromatids migrates to one pole, and the homologous pair of chromatids moves to the opposite pole, both pulled by kinetochore microtubules. Telophase I At telophase I, the sister chromatids have reached their respective poles and then decondense. The nuclear envelope now re-forms to produce two separate nuclei. If we consider the end result of meiosis I, we see that two nuclei are produced, each with half the number of sister chromatids as the starting cell in prophase; this is called a reduction division. The original diploid cell had two sets of chromosomes (three per set); its chromosomes were in homologous pairs. In contrast, the two cells produced as a result of meiosis I and cytokinesis are considered haploid—they do not have pairs of homologous chromosomes. Page 345 about:srcdoc Page 12 of 17 16.3 Meiosis 12/12/24, 12:16 AM Meiosis II Separates Sister Chromatids Meiosis I is followed by cytokinesis and then meiosis II (see Figure 16.13f–j). DNA replication does not occur between meiosis I and meiosis II. The sorting events of meiosis II are similar to those of mitosis, but the starting point is different. For a diploid cell with six chromosomes, mitosis begins with 12 chromatids that are joined as six pairs of sister chromatids (see Figure 16.8). By comparison, the two cells that begin meiosis II each have six chromatids that are joined as three pairs of sister chromatids. Otherwise, the steps that occur during prophase, prometaphase, metaphase, anaphase, and telophase of meiosis II are analogous to a mitotic division. Sister chromatids are separated during anaphase II. Meiosis II Mitosis and Meiosis Differ in a Few Key Steps How are the outcomes of mitosis and meiosis different? Mitosis produces two diploid daughter cells that are genetically identical. In our example shown in Figure 16.8, the starting cell had six chromosomes (three homologous pairs of chromosomes), and both daughter cells received copies of the same six chromosomes. By comparison, meiosis reduces the number of sets of about:srcdoc Page 13 of 17 16.3 Meiosis 12/12/24, 12:16 AM chromosomes. In the example shown in Figure 16.13, the starting cell also had six chromosomes, whereas the resulting four daughter cells have only three chromosomes. However, the daughter cells do not contain a random mix of three chromosomes. Each haploid daughter cell contains one complete set of chromosomes, whereas the original diploid mother cell had two complete sets. How do we explain the different outcomes of mitosis and meiosis? Table 16.1 emphasizes the differences between certain key steps in mitosis and meiosis that account for the different outcomes of these two processes. Unique Features of Meiosis DNA replication occurs prior to mitosis and meiosis I, but not between meiosis I and II. During prophase of meiosis I, the homologs bind to each other to form bivalents. This explains why crossing over occurs commonly during meiosis but rarely during mitosis. During prometaphase of mitosis and meiosis II, pairs of sister chromatids are attached to both poles. In contrast, during meiosis I, each pair of sister chromatids (within a bivalent) is attached to a single pole. Bivalents align along the metaphase plate during metaphase of meiosis I, whereas sister chromatids align along the metaphase plate during metaphase of mitosis and meiosis II. At anaphase of meiosis I, the homologous chromosomes separate, but the sister chromatids remain together. In contrast, sister chromatid separation occurs during anaphase of mitosis and meiosis II. Taken together, the steps of mitosis produce two diploid cells that are genetically identical, whereas the steps of meiosis involve two sequential cell divisions that produce four haploid cells that may not be genetically identical. Page 346 about:srcdoc Page 14 of 17 16.3 Meiosis 12/12/24, 12:16 AM Table 16.1 A Comparison of Mitosis, Meiosis I, and Meiosis II Event Mitosis Meiosis I Meiosis II DNA Occurs prior to mitosis Occurs prior to Does not occur replication: meiosis I between meiosis I and II Synapsis No Yes, bivalents are No during formed. prophase: Crossing over Rarely Commonly Rarely during prophase: Attachment to A pair of sister A pair of sister A pair of sister poles at chromatids is attached chromatids is chromatids is attached prometaphase: to kinetochore attached to to kinetochore microtubules from both kinetochore microtubules from both poles. microtubules from poles. just one pole. Alignment Sister chromatids align. Bivalents align. Sister chromatids align. along the metaphase plate: Type of Sister chromatids Homologous Sister chromatids separation at separate. A single chromosomes separate. A single anaphase: chromatid, now called separate. A pair of chromatid, now called a chromosome, moves sister chromatids a chromosome, moves to each pole. moves to each pole. to each pole. End result Two daughter cells that — Four daughter cells when the are diploid that are haploid mother cell is about:srcdoc Page 15 of 17 16.3 Meiosis 12/12/24, 12:16 AM diploid: Comparison of Meiosis and Mitosis Page 347 THE QUESTION A diploid cell has 12 chromosomes, or 6 pairs. In the following diagram, in what phase of mitosis, meiosis I, or meiosis II, is the cell? T OPIC What What topic topic in in biology biology does does this this question question address? address? The topic is cell division. More specifically, the question is asking you to be able to look at a drawing of a dividing cell and discern which phase of cell division the cell is in. I NFORMATION What What information information do do you you know know based based onon the the question question and and your your understanding understanding of of the the topic? topic? In the question, you are given a diagram of a about:srcdoc Page 16 of 17 16.3 Meiosis 12/12/24, 12:16 AM cell at a particular phase of the cell cycle. This cell is derived from a mother cell with 6 pairs of chromosomes. From your understanding of the topic, you may remember the various phases of mitosis, meiosis I, and meiosis II, which are described in Figures 16.8 and 16.13. If so, you may initially realize that the cell is in metaphase. P ROBLEM-SOLVING S TRATEGY Sort Sort out out the the steps steps in in aa complicated complicated process. process. To solve this problem, you may need to describe the steps, starting with a mother cell that has 6 pairs of chromosomes. Keep in mind that a mother cell with 6 pairs of chromosomes has 12 chromosomes during G1, which then replicate to form 12 pairs of sister chromatids during S phase. Therefore, at the beginning of M phase, this mother cell will have 12 pairs of sister chromatids. During mitosis, the 12 pairs of sister chromatids will align at metaphase. During meiosis I, 6 bivalents will align along the metaphase plate in the mother cell. During meiosis II, 6 pairs of sister chromatids will align along the metaphase plate in the two cells. ANSWER The cell is in metaphase of meiosis II. You can tell because the chromosomes are lined up in a single row along the metaphase plate, and the cell has only 6 pairs of sister chromatids. If it were mitosis, the cell would have 12 pairs of sister chromatids. If it were in meiosis I, bivalents would be aligned along the metaphase plate. about:srcdoc Page 17 of 17 16.4 Sexual Reproduction 12/12/24, 12:17 AM Page 348 16.4 Sexual Reproduction about:srcdoc Page 1 of 7 16.4 Sexual Reproduction 12/12/24, 12:17 AM Learning Outcome: 1. Describe an advantage of sexual reproduction. 2. Distinguish among the life cycles of diploid-dominant species, haploid- dominant species, and species that exhibit an alternation of generations. Sexual reproduction is a process in which two haploid gametes unite in a fertilization event to form a diploid cell called a zygote. For multicellular species such as animals and plants, the zygote then grows and divides by mitotic cell divisions into a multicellular organism with many diploid cells. By contrast, asexual reproduction occurs when offspring are produced from a single parent without the fusion of gametes. The offspring are clones of the parent organism. In this section, we will consider the advantages and disadvantages of sexual reproduction and see how this process occurs among animals, fungi, and plants. Sexual Reproduction Provides a Mechanism for about:srcdoc Page 2 of 7 16.4 Sexual Reproduction 12/12/24, 12:17 AM Greater Genetic Diversity in Offspring Compared with asexual reproduction, sexual reproduction has some disadvantages. For example, two types of gametes (sperm and eggs) must be made, often in large numbers, and this requires energy. Furthermore, specialized body parts are often needed for the production of offspring. Also, in animals, the two sexes must be able to find each other, and they expend energy during courtship. Because the vast majority of eukaryotic species reproduce sexually, evolutionary biologists assume that sexual reproduction must carry some advantage(s) that is (are) acted upon by natural selection. One key feature of sexual reproduction is that it allows for greater genetic variation in offspring. As an example, let's suppose that one member of a deer species (a male) is able to survive at higher temperatures compared to other members, but is unable to digest a grass that is found in its environment. Another member of the same species (a female) cannot survive at higher temperatures, but can digest the grass. These differences in traits are due to genetic variation between the male and female. If these two individuals mated, the following combinations of traits are possible in the resulting offspring: can survive at higher temperatures and cannot digest the grass cannot survive at higher temperatures and can digest the grass can survive at higher temperatures and can digest the grass cannot survive at higher temperatures and cannot digest the grass The third and fourth categories of offspring display a combination of traits that is different from the traits of either parent. This is due to genetic recombination—the exchange of genetic material via sexual reproduction, which leads to the production of offspring with combinations of traits that differ from those found in either parent. One prevalent hypothesis about the advantage of sexual reproduction is that it allows for a more rapid adaptation to environmental changes than does asexual reproduction. As a result of sexual reproduction, some offspring may carry a combination of traits that promote survival and reproduction and will be favored by natural selection. In our previous example, if the about:srcdoc Page 3 of 7 16.4 Sexual Reproduction 12/12/24, 12:17 AM environment became hotter and the grass became a more prevalent food source, sexual reproduction could produce offspring that are better adapted to this environmental change. Such offspring would be more likely to survive and reproduce, and thus pass these traits to future generations. Life Cycles Among Different Species Vary with Regard to Their Haploid and Diploid Phases For any given species, the sequence of events that produces another generation of organisms is known as a life cycle. For sexually reproducing organisms, this usually involves an alternation between haploid cells or organisms and diploid cells or organisms ( Figure 16.14). Page 349 about:srcdoc Page 4 of 7 16.4 Sexual Reproduction 12/12/24, 12:17 AM Figure 16.14 A comparison of three types of life cycles for sexually reproducing organisms. Concept Concept Check: Check: What is the main purpose of meiosis in animals? What is the main purpose of mitosis in animals? Answer about:srcdoc Page 5 of 7 16.4 Sexual Reproduction 12/12/24, 12:17 AM Diploid-Dominant Species Most species of animals are diploid, and their haploid gametes are considered to be a specialized type of cell. For this reason, animals are viewed as diploid-dominant species ( Figure 16.14a). Certain diploid cells in the testes or ovaries undergo meiosis to produce haploid sperm or eggs, respectively. During fertilization, sperm and egg unite to form a diploid zygote, which then undergoes repeated mitotic cell divisions to produce a diploid multicellular organism. Haploid-Dominant Species By comparison, most fungi and some protists are just the opposite; they are haploid-dominant sp ecies ( Figure 16.14b). In fungi, the multicellular organism is haploid (1n). For example, bread mold, which is the fuzzy stuff that grows on stale bread, is the haploid phase of the organism. Haploid fungal cells are most commonly produced by mitosis. During sexual reproduction, certain haploid cells within the organism unite to form a diploid zygote, which then immediately proceeds through meiosis to produce four haploid cells called spores. Each spore goes through mitotic cellular divisions to produce a haploid multicellular organism. Page 350 about:srcdoc Page 6 of 7 16.4 Sexual Reproduction 12/12/24, 12:17 AM Alternation of Generations Plants and some protists have life cycles that are intermediate between diploid and haploid dominance. Such species exhibit an alternation of generations ( Figure 16.14c). The species alternate between diploid multicellular organisms called sporophytes, and haploid multicellular organisms called gametophytes. Meiosis in certain cells within the sporophyte produces haploid spores, which divide by mitosis to produce the gametophyte. Particular cells within the gametophyte differentiate into haploid gametes. Fertilization occurs between two gametes, producing a diploid zygote that then undergoes repeated mitotic cell divisions to produce a sporophyte. Among different plant species, the relative sizes of the haploid and diploid organisms vary greatly. In mosses, the haploid gametophyte is a visible, multicellular organism, whereas the diploid sporophyte is smaller and remains attached to the haploid organism. In other plants, such as ferns ( Figure 16.14c), both the diploid sporophyte and the haploid gametophyte grow independently. The sporophyte is considerably larger and is the organism with leaves that we commonly think of as a fern. In seed-bearing plants, such as roses and oak trees, the diploid sporophyte is the large multicellular plant, whereas the gametophyte is composed of only a few cells and is formed within the sporophyte. When comparing animals, plants, and fungi, it’s interesting to consider how gametes are made. Animals produce gametes by meiosis. In contrast, plants and fungi produce reproductive cells by mitosis. The gametophyte of plants is a haploid multicellular organism that is created by mitotic cellular divisions of a haploid spore. Within the multicellular gametophyte, certain cells become specialized as gametes. about:srcdoc Page 7 of 7 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM 16.5 Variation in Chromosome Structure and Number about:srcdoc Page 1 of 23 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM Learning Outcomes: 1. Describe how chromosomes can vary in size, centromere location, and number. 2. Identify the four ways that the structure of a chromosome can be changed via mutation. 3. Compare and contrast changes in the number of sets of chromosomes and changes in the number of individual chromosomes. 4. Give examples of how changes in chromosome number affect the characteristics of animals and plants. In the previous sections of this chapter, we examined two important features of chromosomes: First, we considered how chromosomes occur in sets, and second, we explored two sorting processes that determine the number of sets of chromosomes following cell division. In this section, we will examine how the structures and numbers of chromosomes may vary between different species and within the same species. about:srcdoc Page 2 of 23 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM Why is the study of chromosomal variation important? First, geneticists have discovered that variations in chromosome structure and number can have major effects on the characteristics of an organism. We now know that several human genetic diseases are caused by such changes. In addition, changes in chromosome structure and number have been an important factor in the evolution of new species, which is a topic we will consider in Chapter 24. Chromosome variation can be viewed in two ways. Among different species, the structure and number of chromosomes tend to vary greatly. There is also considerable variety in the size and shape of the chromosomes of a given species. On relatively rare occasions, however, the structure or number of chromosomes changes so that an individual is different from most other members of the same species. This is generally viewed as an abnormality. In this section, we will examine both normal and abnormal types of chromosome variation. Natural Variation Exists in Chromosome Structure and Number Before we begin to examine chromosome variation, we need to have a reference point for a normal set of chromosomes. To determine what the normal chromosomes of a species look like, a cytogeneticist microscopically examines the chromosomes from several members of the about:srcdoc Page 3 of 23 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM species. Chromosome composition within a given species tends to remain relatively constant. In most cases, individuals of the same species have the same number and types of chromosomes. For example, as mentioned previously, the usual chromosome composition of human cells is two sets of 23 chromosomes, for a total of 46. Other diploid species have different numbers of chromosomes. The dog has 78 chromosomes (39 per set), the fruit fly has 8 chromosomes (4 per set), and the tomato has 24 chromosomes (12 per set). When comparing distantly related species, such as humans and fruit flies, major differences in chromosome composition are observed. The chromosomes of a given species also vary considerably in size and shape. Cytogeneticists have various ways to classify and identify chromosomes in their metaphase form. The three most commonly used features are size, location of the centromere, and banding patterns, which are revealed when the chromosomes are treated with stains. Based on centromere location, each metaphase chromosome is classified as metacentric (near the middle), submetacentric (off center), acrocentric (near one end), or telocentric (at the end) ( Figure 16.15). Figure 16.15 A comparison of centromeric locations among metaphase chromosomes. Because the centromere is not exactly in the center of a chromosome, each chromosome has a short arm and a long arm. The short arm is designated with the letter p (for the French petite), and the long arm is designated with the letter q. In the case of telocentric chromosomes, the short arm may be nearly nonexistent. When a karyotype is prepared (see Figure 16.1), the chromosomes are aligned with the short arms on top and the long arms on the bottom. Page 351 about:srcdoc Page 4 of 23 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM Because different chromosomes often have similar sizes and centromeric locations, cytogeneticists must use additional methods to accurately identify each type of chromosome within a karyotype. For detailed identification, chromosomes are treated with stains to produce characteristic banding patterns. Cytogeneticists use several different staining procedures to identify specific chromosomes. An example is Giemsa stain, which produces G bands (see Figure 16.1). The alternating pattern of G bands is unique for each type of chromosome. The banding pattern of eukaryotic chromosomes is useful in two ways. First, individual chromosomes can be distinguished from each other, even if they have similar sizes and centromeric locations. As described next, banding patterns are used to detect changes in chromosome structure that occur as a result of mutation. Mutations Can Alter Chromosome Structure Let’s now consider how the structures of chromosomes can be modified by a mutation, a heritable change in the genetic material. Chromosomal mutations, which involve the breaking and rejoining of chromosomes, are categorized as deletions, duplications, inversions, and translocations ( Figure 16.16). about:srcdoc Page 5 of 23 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM about:srcdoc Page 6 of 23 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM Figure 16.16 Types of changes in chromosome structure. The letters alongside the chromosomes are placed there as frames of reference. Concept Concept Check: Check: Which types of changes in chromosome structure shown here do not affect the total amount of genetic material? Answer Changes in the Total Amount of Genetic Material about:srcdoc Page 7 of 23 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM Changes in the Total Amount of Genetic Material Deletions and duplications are changes in the total amount of genetic material in a single chromosome. When a deletion occurs, a segment of chromosomal material is removed ( Figure 16.16a). The affected chromosome becomes deficient in a significant amount of genetic material. In a duplication, a section of a chromosome occurs two or more times ( Figure 16.16b). What are the consequences of a deletion or duplication? The possible effects depend on the size of the segment affected and whether it includes genes or portions of genes that are vital to the development of the organism. When a deletion or duplication has an effect, it is usually detrimental. Larger changes in the amount of genetic material tend to be more harmful because more genes are missing or duplicated. Chromosomal Rearrangements about:srcdoc Page 8 of 23 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM Chromosomal Rearrangements Inversions and translocations are chromosomal rearrangements. An inversion is a change in the direction of the genetic material along a single chromosome. When a segment of one chromosome has been inverted, the order of G bands is opposite that of a normal chromosome ( Figure 16.16c). A translocation occurs when one segment of a chromosome becomes attached to a different chromosome. In a simple translocation, a single piece of a chromosome is attached to another chromosome ( Figure 16.16d). In a reciprocal translocation, two different types of chromosomes exchange pieces by an abnormal crossing-over event, thereby producing two abnormal chromosomes carrying translocations ( Figure 16.16e). Changes in Chromosome Structure The Conseqence of Inversion Page 352 Variation Occurs in the Number of Chromosome about:srcdoc Page 9 of 23 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM Variation Occurs in the Number of Chromosome Sets and the Number of Individual Chromosomes Variations in chromosome number can be categorized in two ways: variation in the number of sets of chromosomes and variation in the number of particular chromosomes within a set. The suffix -ploid or -ploidy refers to a complete set of chromosomes. Organisms that are euploid (the prefix eu- means true) have chromosomes that occur in one or more complete sets. For example, in a species that is diploid, a euploid organism would have two sets of chromosomes in its somatic cells. In Drosophila melanogaster, for example, a normal individual has eight chromosomes. The species is diploid, having two sets of four chromosomes each ( Figure 16.17a). about:srcdoc Page 10 of 23 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM Figure 16.17 Types of variation in chromosome number. (a) The normal diploid number of chromosomes in a female Drosophila. The X chromosome is also called chromosome 1. Examples of chromosomes of (b) polyploid flies and (c) aneuploid flies. about:srcdoc Page 11 of 23 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM Variation in the Number of Sets of Chromosomes Organisms can vary in the number of sets of chromosomes they have. For example, on rare occasions, an abnormal fruit fly can be produced with 12 chromosomes, that is, having three sets of 4 chromosomes each ( Figure 16.17b). Organisms with three or more sets of chromosomes are called polyploid. A triploid organism is referred to as 3n, a tetraploid organism as 4n, and so forth. All such organisms are euploid because they have complete sets of chromosomes. Aneuploidy A second way that chromosome number can vary is called aneuploidy. This is an alteration in the number of a particular chromosome, so the total number of chromosomes is not an exact multiple of a set. For example, an abnormal fruit fly could have nine chromosomes instead of eight because it has three copies of chromosome 2 instead of the normal two copies ( Figure 16.17c). Instead of being perfectly diploid, a trisomic animal is 2n + 1. Such an animal is said to have trisomy 2. By comparison, a fruit fly could be lacking a single chromosome, such as chromosome 3, and have a total of seven chromosomes (2n – 1). This animal is said to be monosomic and is described as having monosomy 3. about:srcdoc Page 12 of 23 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM Variations in chromosome number are fairly widespread and have a significant effect on the characteristics of plants and animals. For these reasons, researchers want to understand the mechanisms that cause these variations. In some cases, a change in chromosome number is the result of the abnormal sorting of chromosomes during cell division. The term nondisjunction refers to an event in which the chromosomes do not separate properly during cell division. Nondisjunction can occur during meiosis I or meiosis II and produces haploid cells that have too many or too few chromosomes. Figure 16.18 illustrates the consequences of nondisjunction during meiosis I. In this case, one pair of homologs moved into the cell on the left instead of separating from each other. This results in the production of aneuploid cells, with either too many or too few chromosomes. If such a cell becomes a gamete that fuses with another gamete during fertilization, the zygote and the resulting organism will have an abnormal number of chromosomes in all of its cells. about:srcdoc Page 13 of 23 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM Figure 16.18 Nondisjunction during meiosis I. For simplicity, this cell shows only three pairs of homologous chromosomes. One of the three pairs did not disjoin (separate) properly, and both homologs have moved into the cell on the left. The resulting haploid cells shown at the bottom are all aneuploid, resulting in gametes with four chromosomes and two chromosomes, instead of about:srcdoc Page 14 of 23 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM three. Changes in Chromosome Number Have Important Consequences How do changes in chromosome number affect the characteristics of animals and plants? Let’s consider a few examples. about:srcdoc Page 15 of 23 16.5 Variation in Chromosome Structure and Number 12/12/24, 12:17 AM Changes in Chromosome Number in Animals In many cases, animals do not tolerate deviations from diploidy well. For example, polyploidy in mammals is generally a lethal condition. However, a few cases of naturally occurring variations from diploidy do occur in animals. Male bees, which are produced from unfertilized eggs, contain a single set of chromosomes and are therefore haploid organisms. By comparison, fertilized eggs become female bees, which are diploid. A few examples of vertebrate polyploid animals have been discovered. Interestingly, on rare occasions, animals tha