The chromosomes are structures in living cells that contain

Transcription

1 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure and DNA Replication 15. Eukaryotic Chromosomes, Mitosis, 47 EUKARYOTIC CHROMOSOMES, MITOSIS, AND MEIOSIS C HAPTER O UTLINE 15.1 Molecular Structure of Eukaryotic Chromosomes 15.2 Mitotic Cell Division 15.3 Meiosis and Sexual Reproduction 15.4 Variation in Chromosome Structure and Number A scanning electron micrograph of highly compacted chromosomes found in a dividing cell. The chromosomes are structures in living cells that contain the genetic material. Genes are physically located within the chromosomes. Biochemically, chromosomes are composed of a very long molecule of DNA, which is the genetic material, and proteins, which are bound to the DNA and provide it with an organized structure. The primary function of the genetic material in the chromosomes is to store the information needed to produce the characteristics of an organism. To fulfill their role at the molecular level, DNA sequences facilitate four important processes. These are (1) the synthesis of RNA and cellular proteins, (2) the replication of chromosomes, (3) the compaction of chromosomes so they can fit within living cells, and (4) the proper segregation of chromosomes between dividing cells. In this chapter, we will examine the last two of these topics. This chapter begins with a discussion of the structure of eukaryotic chromosomes at the molecular level. Then we will turn to the process of reproduction in eukaryotic species at the cellular level. In these discussions we will be concerned with two phenomena. First, we will consider how cells divide to produce new daughter cells. Second, we will examine sexual reproduction from a cellular and genetic perspective. We will pay close attention to the sorting of chromosomes during cell division. Lastly, we will examine variation in the structure and number of chromosomes. As you will learn, a variety of mechanisms that alter chromosome structure and number can have important consequences for the organisms that carry them Molecular Structure of Eukaryotic Chromosomes We now turn our attention to the ways that eukaryotic chromosomes are folded to fit in a living cell. A typical eukaryotic chromosome contains a single, linear, double-stranded DNA molecule that may be hundreds of millions of base pairs in length. If the DNA from a single set of human chromosomes were stretched from end to end, the length would be over 1 meter! By comparison, most eukaryotic cells are only m (micrometers) in diameter, and the cell nucleus is only about 2 4 m in diameter. Therefore, to fit inside the nucleus, the DNA in a eukaryotic cell must be folded and packaged by a staggering amount. Before biologists understood chromosome structure, they described the genetic material according to its appearance under the microscope. When a cell is dividing, the nuclear membrane is no longer present, and the chromosomes become very compact. Such chromosomes are readily stained with colored dyes. The term chromosome literally means colored body. This is the form of the genetic material that we are accustomed to seeing in photomicrographs. The term chromatin was first used to describe the genetic material that is found in the nucleus of nondividing cells. While in the nucleus, the genetic material is much less compact and appears to be in a twisted, spaghettilike configuration. Over the past couple of decades, as researchers have gained a more complete understanding of genetic material, the meaning of these two terms, namely chromosome and chromatin, have changed. The term chromosome is now used to describe a discrete unit of genetic material. For example, a human somatic cell contains 46 chromosomes. It would also be correct to say there are 46 chromosomes in the nucleus of a nondividing cell.

2 48 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure 15. Eukaryotic and DNA Replication Chromosomes, Mitosis, 300 UNIT III CHAPTER 15 By comparison, the term chromatin has taken on a biochemical meaning. Chromatin is now used to describe the DNA-protein complex that makes up eukaryotic chromosomes. The chromosomes found in the nucleus are composed of chromatin, as are the highly condensed chromosomes found in dividing cells. Chromosomes are very dynamic structures that alternate between tight and loose compaction states in response to changes in protein composition. In this section, we will focus our attention on two issues of chromosome structure. First, we will consider how chromosomes are compacted and organized within the cell nucleus. Then, we will examine the additional compaction that is necessary to produce the highly condensed chromosomes that occur during cell division. Nucleosome: 8 histone proteins 146 nucleotide base pairs of DNA H2B H4 H2B H4 H2A H3 DNA Linker region Amino terminal tail of histone protein DNA Wraps Around Histone Proteins to Form Nucleosomes The first way that DNA is compacted is by wrapping itself around a group of proteins called histones. As shown in Figure 15.1, a repeating structural unit of eukaryotic chromatin is the nucleosome, which is 11 nm (nanometers) in diameter and composed of double-stranded DNA wrapped around an octamer of histone proteins. Each octamer contains eight histone subunits. Different kinds of histone proteins form the histone octamer, two each of four kinds called H2A, H2B, H3, and H4. H2A and H2B are so named because they are similar in structure. Histone proteins are very basic proteins because they contain a large number of positively charged lysine and arginine amino acids. The negative charges that are found in the phosphate of DNA are attracted to the positive charges on histone proteins. The DNA lies on the surface of the histone octamer and makes 1.65 turns around it. The amount of DNA that is required to wrap around the histone octamer is 146 or 147 bp (base pairs). The amino terminal tail of each histone protein protrudes from the histone octamer. As discussed later, covalent modifications of these tails is one way to control the degree of chromatin compaction. H1 11 nm Figure 15.1 Structure of a nucleosome. A nucleosome is composed of double-stranded DNA wrapped around an octamer of histone proteins. A linker region connects two adjacent nucleosomes. Histone H1 is bound to the linker region, as are other proteins not shown in this figure. The nucleosomes are connected by linker regions of DNA that vary in length from 20 to 100 bp, depending on the species and cell type. A particular histone named histone H1 is bound to the linker region, as are other types of proteins. The overall structure of connected nucleosomes resembles beads on a string. This structure shortens the length of the DNA molecule about sevenfold. Evidence for the beads-on-a-string structure is described next. Noll Confirmed Kornberg s Beads-on-a-String Model by Digestion of the Linker Region The beads-on-a-string model of nucleosome structure was originally proposed by Roger Kornberg in Markus Noll decided to test Kornberg s model by digesting chromatin with DNase-I, an enzyme that cuts the DNA backbone. He reasoned that if the model was correct, the linker region of DNA would be more accessible to DNase-I than would the 146-bp region that is tightly bound to the histones. Therefore, he expected incubation with DNase-I to make cuts in the linker region and produce DNA pieces that would be approximately 200 bp in length, given that the DNA used had linker regions of about 50 bp. (Note: The size of the DNA fragments was expected to vary somewhat, since the linker region is not of constant length and the cut within the linker region may occur at different sites.) Figure 15.2 describes Noll s experimental protocol. He began with nuclei from rat liver cells and incubated them with low, medium, or high concentrations of DNase-I. At high concentrations, the enzyme should make a cut in every linker region, while at medium and low concentrations, DNase-I may occasionally miss cutting a linker region, which would produce larger DNA fragments in multiples of 200 bp. Following digestion, the DNA was extracted from the cell nuclei, and then analyzed by gel electrophoresis to determine the sizes of the DNA fragments. (Gel electrophoresis is described in Chapter 20.)

3 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure and DNA Replication 15. Eukaryotic Chromosomes, Mitosis, 49 EUKARYOTIC CHROMOSOMES, MITOSIS, AND MEIOSIS 301 Figure 15.2 Noll s DNase-I digestion experiment, which verified the beads-on-a-string model of DNA compaction. HYPOTHESIS DNA wraps around histone proteins in a regular, repeating pattern. STARTING MATERIAL Nuclei from rat liver cells. Experimental level Conceptual level 1 Incubate nuclei with low, medium, and high concentrations of DNase-I. The conceptual level shows a low DNase-I concentration. DNase-I Before digestion (beads on a string) 37 C 37 C 37 C After digestion (DNA is cut in linker region) Low Medium High Treat with detergent and add phenol. 2 Extract the DNA from the cell nucleus by dissolving the membranes with detergent and adding phenol. DNA goes into aqueous phase. Aqueous phase (contains DNA) Marker Low Medium Phenol phase (contains membranes and proteins) High DNA in aqueous solution (low DNase-I) 3 Load the DNA into a well of a gel and separate the DNA according to size. The marker lane contains a mixture of DNA fragments of known sizes. Gel 600 bp 400 bp Low 200 bp 4 Visualize the DNA fragments by staining the DNA with ethidium bromide, which binds to DNA and is fluorescent when excited by UV light. Solution with ethidium bromide Stain gel UV light Photograph gel

4 50 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure 15. Eukaryotic and DNA Replication Chromosomes, Mitosis, 302 UNIT III CHAPTER 15 5 THE DATA Results from the experiment: 600 bp 400 bp Low Medium High As shown in the data, at high DNase-I concentrations, the chromosomal DNA was digested into fragments of approximately 200 bp in length. This result is predicted by the beads-on-astring model. At lower DNase-I concentrations, longer pieces were observed that were in multiples of 200 bp (400, 600, and so on). These longer pieces are explained by occasional uncut linker regions. For example, a DNA piece might contain two nucleosomes and be 400 bp in length. Taken together, these results strongly supported the nucleosome model for chromatin structure. 200 bp DNase-I concentration: 30 units ml units ml units ml 1 Nucleosomes Compact to Form a 30-nm Fiber Nucleosome units are organized into a more compact structure that is 30 nm in diameter, known as the 30-nm fiber (Figure 15.3a). Histone H1 and other proteins are important in the formation of the 30-nm fiber, which shortens the nucleosome structure another sevenfold. The structure of the 30-nm fiber has proven difficult to determine because the conformation of the DNA may be substantially altered when extracted from living cells. A current model for the 30-nm fiber was proposed by Rachel Horowitz and Christopher Woodcock in the 1990s (Figure 15.3b). According to their model, linker regions in the 30- nm structure are variably bent and twisted, and little direct contact is observed between nucleosomes. The 30-nm fiber forms an asymmetric, three-dimensional zigzag of nucleosomes. At this level of compaction, the overall picture of chromatin that emerges is an irregular, fluctuating structure with stable nucleosome units connected by bendable linker regions. (a) Micrograph of a 30-nm fiber (b) Three-dimensional zigzag model 30 nm Chromatin Loops Are Anchored to the Nuclear Matrix Thus far, we have examined two mechanisms that compact eukaryotic DNA, the formation of nucleosomes and their arrangement into a 30-nm fiber. Taken together, these two events shorten the folded DNA about 49-fold. A third level of compaction involves interactions between the 30-nm fibers and a filamentous network of proteins in the nucleus called the nuclear matrix. This matrix consists of the nuclear lamina, which is a collection of protein fibers that line the inner nuclear membrane, and an internal nuclear matrix that is connected to the lamina (Figure 15.4a). The internal matrix is an intricate network of irregular protein fibers plus many other proteins that bind to these fibers. The nuclear matrix is involved in the compaction of the 30-nm fiber by participating in the formation of radial loop domains. These loops, often 25,000 to 200,000 base pairs in size, Figure 15.3 The 30-nm fiber. (a) A photomicrograph of the 30-nm fiber. (b) In this three-dimensional zigzag model, the linker DNA forms a bendable structure with little contact between adjacent nucleosomes. are anchored to the nuclear matrix (Figure 15.4b). In this way, the nuclear matrix organizes the chromosomes within the nucleus. Each chromosome in the cell nucleus is located in a discrete and nonoverlapping chromosome territory, which can be experimentally viewed in nondividing cells (refer back to Chapter 4, Figure 4.17). Figure 15.5 shows a model comparing human chromosomes in their fully compacted state, when a cell is preparing to divide, with chromosomes in nondividing cells. Each chromosome in nondividing cells occupies its own discrete region in the cell nucleus that usually does not overlap with the territory of adjacent chromosomes. In other words, different chromosomes are not substantially intertwined with each other, even when they are in a noncompacted condition.

5 Gene Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure and DNA Replication 15. Eukaryotic Chromosomes, Mitosis, 51 EUKARYOTIC CHROMOSOMES, MITOSIS, AND MEIOSIS 303 Internal nuclear matrix protein bound to a protein fiber Protein fiber of internal nuclear matrix Nuclear lamina Protein fiber 30-nm fiber Gene Nuclear pore Inner nuclear membrane Radial loop domain Gene Outer nuclear membrane Protein that attaches the base of a DNA loop to a protein fiber (a) Proteins that form the nuclear matrix (b) Radial loop domain bound to a protein fiber Figure 15.4 Structure of the nuclear matrix and its attachment to the 30-nm fiber. (a) This schematic drawing shows the arrangement of the matrix within a cell nucleus. The nuclear lamina is a collection of fibrous proteins that line the inner nuclear membrane. The internal nuclear matrix is also composed of protein fibers and many other proteins associated with them. (b) The radial loops are attached to the protein fibers of the nuclear matrix. (a) (b) Figure 15.5 A molecular model showing chromosome territories in the cell nucleus of humans. Each of the 23 pairs of human chromosomes is labeled with a different color. (a) Compacted chromosomes in a cell that is preparing to divide. (b) Chromosomes in the cell nucleus of a nondividing cell. Each of these chromosomes occupies its own distinct, nonoverlapping territory within the nucleus. The compaction level of chromosomes in the cell nucleus is not completely uniform. This variability can be seen with a light microscope and was first observed by the German cytologist E. Heitz in He used the term heterochromatin to describe the highly compacted regions of chromosomes. In general, these regions are transcriptionally inactive because of their tight conformation, which prevents transcription factors and RNA polymerase from gaining access to genes. By comparison, the less condensed regions, known as euchromatin, reflect areas that are capable of gene transcription. Euchromatin is the form of chromatin in which the 30-nm fiber forms radial loop domains. In heterochromatin, these radial loop domains are compacted even further. In nondividing cells, most chromosomal regions are euchromatic and some localized regions are heterochromatic.

6 52 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure 15. Eukaryotic and DNA Replication Chromosomes, Mitosis, 304 UNIT III CHAPTER 15 During Cell Division, Chromosomes Undergo Maximum Compaction When cells prepare to divide, the chromosomes become even more compacted or condensed. This aids in their proper sorting and movement during metaphase, which is a stage of cell division described in the next section. Figure 15.6 illustrates the levels of compaction that contribute to the formation of a metaphase chromosome. DNA in the nucleus is always compacted by forming nucleosomes and condensing into a 30-nm fiber (Figure 15.6a,b,c). In euchromatin, the 30-nm fibers are arranged in radial loop domains that are relatively loose, meaning that a fair amount of space is between the 30-nm fibers (Figure 15.6d). The average width of such loops is about 300 nm. By comparison, heterochromatin involves a much tighter packing of the loops, so little space is between the 30-nm fibers (Figure 15.6e). Heterochromatic regions tend to be wider, in the range of 700 nm. When cells prepare to divide, all of the euchromatin is converted to heterochromatin. Because the 30-nm fibers are much closer together in heterochromatin, the conversion of DNA double helix 2 nm (a) DNA double helix 11 nm 1 Wrapping of DNA around histone proteins. Histones Nucleosome (b) Nucleosomes ( beads on a string ) Histone H1 2 Formation of a threedimensional zigzag structure via histone H1 and other DNA-binding proteins. (c) 30-nm fiber 30 nm 300 nm 3 Anchoring of radial loop domains to the nuclear matrix. (d) Radial loop domains 4 Further compaction of radial loops to form heterochromatin. 700 nm (e) Heterochromatin 5 Formation of a scaffold from the nuclear matrix. All chromatin is heterochromatin. 1,400 nm (f) Metaphase chromosome Figure 15.6 The steps in eukaryotic chromosomal compaction leading to the metaphase chromosome.

7 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure and DNA Replication 15. Eukaryotic Chromosomes, Mitosis, 53 EUKARYOTIC CHROMOSOMES, MITOSIS, AND MEIOSIS 305 (a) Metaphase chromosome Scaffold DNA strand 280 nm (b) Metaphase chromosome treated with high salt to remove proteins Figure 15.7 The importance of histones and scaffolding proteins in the compaction of eukaryotic chromosomes. (a) Transmission electron micrograph of a metaphase chromosome. (b) This photomicrograph shows a metaphase chromosome following treatment with a high salt concentration to remove the histone proteins. The label on the left points to the scaffold that anchors the bases of the radial loops. The right label points to an elongated strand of DNA. Biological inquiry: After they have replicated and become compacted in preparation for cell division, chromosomes are often shaped like an X, as in part (a) of this figure. Which proteins are primarily responsible for this X shape? euchromatin to heterochromatin greatly shortens the chromosomes. In a metaphase chromosome, which contains two copies of the DNA (Figure 15.6f), the width averages about 1,400 nm, but the length of a metaphase chromosome is much shorter than the same chromosome in the cell nucleus during interphase. These highly condensed chromosomes undergo little gene transcription because it is difficult for transcription proteins to gain access to the compacted DNA. Therefore, most transcriptional activity ceases during cell division, which usually lasts for a relatively short time. In metaphase chromosomes, the highly compacted radial loops remain anchored to a scaffold, which is formed from proteins in the nuclear matrix (Figure 15.7). Experimentally, the scaffold proteins that hold the loops in place can be separated into an observable form. If a metaphase chromosome is treated with a high concentration of salt to remove histone proteins, the highly compact configuration is lost, but the bottoms of the elongated DNA loops remain attached to the scaffold. In the photomicrograph shown in Figure 15.7b, a label points to an elongated DNA strand emanating from the darkly stained scaffold. The scaffold retains the shape of the original metaphase chromosome even though the DNA strands have become greatly elongated. These remarkable results illustrate the importance of both nuclear matrix proteins (which form the scaffold) and histones (which are needed to compact the DNA) for the structure of chromosomes. The Histone Code Controls Chromatin Compaction In this section, we have learned that the genomes of eukaryotic species are greatly compacted to fit inside the cell nucleus. Even euchromatin, which is looser than heterochromatin, is still compacted to such a degree that it is difficult for transcription factors and RNA polymerase to access and transcribe genes. As described in Chapters 12 and 13, chromatin must be loosened up so that genes can be transcribed into RNA. As discussed earlier, each of the histone proteins consists of a globular domain and a flexible, charged amino terminus called an amino terminal tail. The DNA wraps around the globular domains, as depicted in Figure 15.1, and the amino terminal tails protrude from the chromatin. In recent years, researchers have discovered that particular amino acids in the amino terminal tails are subject to several types of covalent modifications, including acetylation, methylation, and phosphorylation. Over 50 different enzymes have been identified in mammals that selectively modify amino terminal tails. Figure 15.8 shows examples of sites in the tails of H2A, H2B, H3, and H4 that can be modified. These tail modifications can have two effects. First, they may directly influence interactions between nucleosomes. Second, histone modifications provide binding sites that are recognized by proteins. According to the histone code hypothesis, proposed by Brian Strahl and David Allis in 2000, the pattern of histone modification is recognized by particular proteins, much like a language or code. For example, one pattern might involve phosphorylation of the serine at the first amino acid in H2A and acetylation of the fifth and eighth amino acids in H4, which are lysines. A different pattern could involve acetylation of the fifth amino acid, a lysine, in H2B and methylation of the third amino acid in H4, which is an arginine. The pattern of covalent modifications of amino terminal tails provides binding sites for proteins that subsequently affect the degree of chromatin compaction. One pattern of histone modification may attract proteins that cause the chromatin to become even more compact. This would silence the transcription of genes in the region. Alternatively, a different combination of histone modifications may attract proteins, such as chromatin remodeling enzymes discussed in Chapter 13, that serve to loosen the chromatin and thereby promote gene transcription.

8 54 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure 15. Eukaryotic and DNA Replication Chromosomes, Mitosis, 306 UNIT III CHAPTER 15 P Ser m Arg m Lys 5 Amino terminal tail ac Lys 5 m ac P ac LysSer 10 Lys 10 Globular domains Lys 5 ac P Lys ac Ser 15 Lys H2A ac P ac Lys 10 Lys Ser m ac Arg Lys 15 ac 5 Lys m ac ac Arg Lys Lys H3 ac 20 Lys Lys 20 In this way, the histone code plays a key role in accessing the information within the genomes of eukaryotic species. Researchers are trying to unravel which patterns of histone modifications promote compaction, and which promote a loosening of chromatin structure. In other words, they are trying to decipher the histone code Mitotic Cell Division We now turn our attention to the mechanism of cell division and its relationship to chromosome structure and replication. During the process of mitotic cell division, a cell divides to produce two new cells that are genetically identical to the original cell. By convention, the original cell is usually called the mother cell, and the new cells are the two daughter cells. Mitotic cell division involves mitosis, which is the division of the nucleus into two nuclei, and cytokinesis, which is the division of one mother cell into two daughter cells. One purpose of mitotic cell division is asexual reproduction. 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 and maintenance of multicellularity. Organisms such as plants, animals, and most fungi are derived from a single cell 10 ac m H2B Figure 15.8 Examples of covalent modifications that occur to the amino terminal tails of histone proteins. The amino acids are numbered from the amino terminus. The modifications shown here are m for methylation, p for phosphorylation, and ac for acetylation. Many more modifications can occur to the amino terminal tails; the ones shown here represent common examples. H4 that subsequently undergoes repeated cellular divisions to become a multicellular organism. Humans, for example, begin as a single fertilized egg and repeated cellular divisions produce an adult with several trillion cells. As you might imagine, the precise transmission of chromosomes is critical during every cell division so that all cells of the body receive the correct amount of genetic material. In this section, we will explore how the process of mitotic cell division requires the duplication, organization, and sorting of chromosomes. We will also examine how a single cell is separated into two distinct cells by cytokinesis. But first, we need to consider some general features of chromosomes in eukaryotic species. Eukaryotic Chromosomes Are Inherited in Sets To understand the chromosomal composition of cells and the behavior of chromosomes during cell division, scientists observe chromosomes with the use of microscopes. Cytogenetics is the field of genetics that involves the microscopic examination of chromosomes. As discussed earlier in this chapter, when a cell prepares to divide the chromosomes become more tightly compacted, which shortens them and thereby increases their diameter. A consequence of this shortening is that distinctive shapes and numbers of chromosomes become visible with a light microscope. Figure 15.9 shows the general procedure for preparing and viewing chromosomes from a eukaryotic cell. In this example, the cells are obtained from a sample of human blood. In particular, the chromosomes within lymphocytes (a type of white blood cell) are examined. A sample of the blood cells is obtained and treated with drugs that stimulate the cells to divide. The actively dividing cells are centrifuged to concentrate them and then mixed with a hypotonic solution that makes the cells swell. The expansion in cell structure causes the chromosomes to spread out from each other, making it easier to see each individual chromosome. Next, the cells are concentrated by a second centrifugation and treated with a fixative, which chemically freezes them so that the chromosomes will no longer move around. The cells are then exposed to a chemical dye that binds to the chromosomes and stains them. As we will learn later, this gives chromosomes a distinctive banding pattern that greatly enhances their contrast and ability to be uniquely identified. The cells are then placed on a slide and viewed with a light microscope. In a cytogenetics laboratory, the microscopes are equipped with an electronic camera to photograph the chromosomes. On a computer screen, the chromosomes can be organized in a standard way, usually from largest to smallest. A photographic representation of the chromosomes, as in the photo in step 5 of Figure 15.9, is called a karyotype. A karyotype reveals how many chromosomes are found within an actively dividing cell. 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. By convention, the chromosomes are

9 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure and DNA Replication 15. Eukaryotic Chromosomes, Mitosis, 55 EUKARYOTIC CHROMOSOMES, MITOSIS, AND MEIOSIS A sample of blood is collected and treated with drugs that stimulate cell division. The sample is then subjected to centrifugation. Blood cells Supernatant Pellet 4 The slide is viewed by a light microscope equipped with a camera; the sample is seen on a computer screen. The chromosomes can be photographed and arranged electronically on the screen. 2 The supernatant is discarded, and the cell pellet is suspended in a hypotonic solution. This causes the cells to swell. Hypotonic solution 5 3 The sample is subjected to centrifugation a second time to concentrate the cells. The cells are suspended in a fixative, stained, and placed on a slide. Fix Stain Blood cells For a diploid human cell, 2 complete sets of chromosomes from a single cell constitute a karyotype of that cell. Figure 15.9 The procedure for making a karyotype. Biological inquiry: Researchers usually treat cells with drugs that stimulate them to divide prior to the procedure for making a karyotype. Why would this be useful? 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 (Figure 15.9). This numbering system does not apply to the sex chromosomes, which determine the sex of the individual. Sex chromosomes are designated with the letters X and Y in humans. A second feature of many eukaryotic species is that an individual has two sets of chromosomes. Again, if we consider humans as an example, the karyotype shown in Figure 15.9 contains two sets of chromosomes, with 23 different chromosomes in each set. Therefore, this human cell contains a total of 46 chromosomes. A person s cells have 46 chromosomes each 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, so diploid organisms are referred to as 2n. For example, humans are 2n, where n 23. Most human cells are diploid. An exception involves gametes, namely sperm and egg cells. Gametes are haploid or 1n, which means they contain one set of chromosomes. When a species is diploid, the members of a pair of chromosomes are called homologues. As you can see in Figure 15.9, a cell has two copies of chromosome 1, two copies of chromosome 2, and so forth. Within each pair, the chromosome on the left is a homologue to the one on the right and vice versa. In the case of animals, one of each of these pairs comes from an organism s mother, and one comes from the father; these are referred to as maternal and paternal chromosomes, respectively. Homologous chromosomes are very similar to each other. Each of the two chromosomes in a homologous pair is nearly identical in size and contains a similar composition of genetic material. A particular gene found on one copy of a chromosome is also found on the homologue. Because one homologue is received from each parent, the two homologues may vary with regard to the way that a gene affects an organism s traits.

10 56 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure 15. Eukaryotic and DNA Replication Chromosomes, Mitosis, 308 UNIT III CHAPTER 15 As an example, let s consider an eye color gene in humans. One chromosome might carry the form of an eye color gene that confers brown eyes, while the gene on the homologue could confer blue eyes. This topic will be considered in Chapter 16. The DNA sequences on homologous chromosomes are very similar. In most cases, the sequence of bases on one homologue would differ by less than 1% from the sequence on the other homologue. For example, the DNA sequence of chromosome 1 that you inherited from your mother would be greater 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 an eye color gene 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 pairs of sex chromosomes (for example, X and Y). These chromosomes differ in size and genetic composition. Certain genes that are 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. In Preparation for Cell Division, Eukaryotic Chromosomes Are Replicated and Compacted to Produce Pairs Called Sister Chromatids Now that we understand that chromosomes are found in sets, and that many eukaryotic species are diploid, we will now turn our attention to how those chromosomes are replicated and sorted during cell division. Let s begin with the process of chromosome replication. In Chapter 11, we examined the molecular process of DNA replication. Figure describes the process at the chromosomal level. Prior to DNA replication, the DNA of each eukaryotic chromosome consists of a linear DNA 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 created. These copies, along with associated proteins, lie side 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. As shown in the inset to Figure 15.10, the two sister chromatids are tightly associated at a region called the centromere. The centromere serves as an attachment site for a group of proteins that form the kinetochore, which is necessary for sorting each chromosome. With regard to the cell cycle, which is described in Chapter 9, Figure provides an overview that relates chromosome replication and cell division. In the G 1 phase, the original cell had 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. In G 1, the chromosomes are not highly compacted. During the S, or synthesis, phase, these chromosomes replicate to yield 12 chromatids (that is, 6 pairs of sister chromatids). At the start of mito- 1 2 Each chromosome replicates prior to mitosis. At the start of mitosis, the chromosomes become compact. (a) Chromosome replication and compaction Sister chromatids Kinetochore proteins Centromere (a region of DNA beneath kinetochore proteins) One chromatid One chromatid Pair of sister chromatids (b) Schematic drawing of a metaphase chromosome Figure Replication and compaction of chromosomes into pairs of sister chromatids. (a) Chromosomal replication producing 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 kinetochore proteins that bind to each other and to the centromeres on each chromatid. sis, the chromatids become highly compacted, and during the process of cell division they are divided equally into two daughter cells. The term M phase refers to the sequential events of mitosis and cytokinesis. After cell division is completed, these two daughter cells each contain six chromosomes (three pairs of homologues). The Transmission of Chromosomes Requires a Sorting Process Known as Mitosis Mitosis is the sorting process that ensures that each daughter cell will obtain the correct number and types of chromosomes. Mitosis was first observed microscopically in the 1870s by a German biologist named Walter Flemming, who coined the term mitosis (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. We now know that the mitotic spindle apparatus (also known simply as the mitotic spindle) is responsible for orga-

11 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure and DNA Replication 15. Eukaryotic Chromosomes, Mitosis, 57 EUKARYOTIC CHROMOSOMES, MITOSIS, AND MEIOSIS Chromosome replication 3 produces 6 pairs of sister chromatids. Replication is completed. Cell prepares to divide. S Interphase 1 Prior to cell division, a mother cell has 6 chromosomes, 2 sets of 3 each. G 1 Cytokinesis Telophase Anaphase M Mitosis Metaphase Prometaphase Prophase G 2 4 Replicated chromosomes condense in preparation for mitosis. Two daughter cells form, each containing 6 chromosomes. 5 Chromosomes separate during mitosis and 2 cells are formed during cytokinesis. Figure The eukaryotic cell cycle and cell division. Dividing cells progress through a series of stages. G 1, S, and G 2 are collectively known as interphase, and M phase includes mitosis and cytokinesis. This diagram shows the progression of a cell through the cell cycle to produce two daughter cells. Note: The width of the phases shown in this figure is not meant to reflect their actual length. As discussed in Chapter 9, G 1 is typically the longest phase of the cell cycle, while M phase is relatively short. nizing and sorting the chromosomes during mitosis. The structure of the mitotic spindle in animal cells is shown in Figure The mitotic spindle is formed from two structures called the centrosomes. Because they are a site for organizing microtubules, centrosomes are also referred to as microtubule organizing centers (MTOCs). A single centrosome duplicates during interphase. After they separate from each other during mitosis, each centrosome defines a pole of the spindle apparatus, one within each of the future daughter cells. In animal cells, a conspicuous structure found in the centrosome is a pair of centrioles. However, centrioles are not found in many other eukaryotic species, such as plants, and are not required for spindle formation. The spindle is formed from protein fibers called microtubules that are rooted in the centrosomes. 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. The astral microtubules, which extend away from the chromosomes, are important for positioning the spindle apparatus within the cell. The polar microtubules project into the region between the two poles. Polar microtubules that overlap with each other play a role in the separation of the two poles. Finally, the kinetochore microtubules are attached to kinetochores, which are bound to the centromere of each chromosome. Now that we understand the structure of the mitotic spindle, we can examine the sequence of events that occurs during mitosis. Figure depicts the process of mitosis in an animal cell, though the process is quite similar in a plant cell. In the simplified diagrams shown along the bottom of this figure, the original mother cell contains six chromosomes, as in Figure

12 58 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure 15. Eukaryotic and DNA Replication Chromosomes, Mitosis, 310 UNIT III CHAPTER 15 Centrosome with centriole pair Pole Astral microtubules Figure The structure of the mitotic spindle. The mitotic spindle is formed by the centrosomes from 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. Kinetochore Polar microtubules Pole Kinetochore microtubules Sister chromatids One set of chromosomes is again depicted in red, while the homologous set is blue; remember that these represent maternal and paternal chromosomes. Mitosis is subdivided into phases known as prophase, prometaphase, metaphase, anaphase, and telophase. Prior to mitosis, the cells are in interphase, a phase of the cell cycle during which the chromosomes are decondensed and found in the nucleus (Figure 15.13a). At the start of mitosis, in prophase, the chromosomes have already replicated to (a) Interphase (b) Prophase (c) Prometaphase Chromosomes Nuclear membrane Sister chromatids Spindle pole Mitotic spindle Two centrosomes, each with centriole pairs 1 Chromosomes have already replicated during interphase. 2 Sister chromatids condense and spindle starts to form. Nuclear membrane begins to dissociate into vesicles. 3 Kinetochore microtubule Nuclear membrane has completely dissociated into vesicles and the spindle is fully formed. Sister chromatids attach to spindle via kinetochore microtubules. Figure The process of mitosis in an animal cell. The top panels illustrate the cells of a newt progressing through mitosis. The bottom panels are schematic drawings that emphasize the sorting and separation of the chromosomes in which the original diploid cell had 6 chromosomes (3 in each set). At the start of mitosis, these have already replicated into 12 chromatids. The final result is 2 daughter cells each containing 6 chromosomes.

13 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure and DNA Replication 15. Eukaryotic Chromosomes, Mitosis, 59 EUKARYOTIC CHROMOSOMES, MITOSIS, AND MEIOSIS 311 produce 12 chromatids, joined as six pairs of sister chromatids (Figure 15.13b). As prophase proceeds, the nuclear membrane begins to dissociate into small vesicles. At the same time, the chromatids condense into highly compacted structures that are readily visible by light microscopy. The mitotic spindle is completely formed during prometaphase (Figure 15.13c). As mitosis progresses, the centrosomes move apart and demarcate the two poles. Once the nuclear membrane has dissociated, the spindle fibers can interact with the sister chromatids. Initially, kinetochore microtubules are rapidly formed and can be seen under a microscope growing out from the two poles. As it grows, if a kinetochore microtubule happens to make contact with a kinetochore, it is said to be captured and remains firmly attached to the kinetochore. This seemingly random process is how sister chromatids become attached to kinetochore microtubules. Alternatively, if a kinetochore microtubule does not collide with a kinetochore, the microtubule will eventually depolymerize and retract to the centrosome. 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. Eventually, the pairs of sister chromatids are aligned along a plane halfway between the poles called the metaphase plate. The pairs of sister chromatids have become organized into a single row within this plane. When this alignment is complete, the cell is in metaphase of mitosis (Figure 15.13d). The chromatids can then be equally distributed into two daughter cells. (d) Metaphase (e) Anaphase (f) Telophase and cytokinesis Metaphase plate Individual chromosomes Cleavage furrow Polar microtubule 4 Sister chromatids align along the metaphase plate. 5 Sister chromotids separate and individual chromosomes move toward poles as kinetochore microtubles shorten. Polar microtubles lengthen and push poles apart. 6 Chromosomes decondense and nuclear membranes re-form. Cleavage furrow separates the 2 cells.

14 60 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure 15. Eukaryotic and DNA Replication Chromosomes, Mitosis, 312 UNIT III CHAPTER m Cleavage furrow G 1 Cytokinesis S G 2 Mitosis and cytokinesis ultimately produce two daughter cells having the same number of chromosomes as the mother cell. Barring rare mutations, the two daughter cells are genetically identical to each other and to the mother cell from which they were derived. Thus, the critical consequence of this sorting process is to ensure genetic consistency from one cell to the next. The development of multicellularity relies on the repeated process of mitosis and cytokinesis. For diploid organisms that are multicellular, most of the somatic cells are diploid and genetically identical to each other. Next, we will consider how diploid cells can divide to produce haploid cells. (a) Cleavage of an animal cell (b) Formation of a cell plate in a plant cell 10 m Cell plate Figure Micrographs showing cytokinesis in animal and plant cells. The next step in the sorting process occurs during anaphase (Figure 15.13e). At this stage, the connections between the pairs of sister chromatids are broken. 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. During telophase, the chromosomes have reached their respective poles and decondense. The nuclear membranes now re-form to produce two separate nuclei. In Figure 15.13f, two nuclei are being produced that contain six chromosomes each. In most cases, mitosis is quickly followed by cytokinesis, in which the two nuclei are segregated into separate daughter cells. While the stages 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 15.14a). In plants, the two daughter cells are separated by the formation of a cell plate (Figure 15.14b), which forms a cell wall between the two daughter cells Meiosis and Sexual Reproduction We now turn our attention to sexual reproduction. As discussed earlier, a diploid cell contains two homologous sets of chromosomes, while 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, one from each of the 23 pairs. Sexual reproduction requires a fertilization event in which two haploid gametes unite to create a diploid cell called a zygote. In the case of many multicellular species, the zygote then grows and divides by mitosis into a multicellular organism with many diploid cells. 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 following this process. For this to occur, 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 half the total number of chromosomes, but not just any 23 chromosomes will do. A gamete must receive one chromosome from each of the 23 pairs. For this to happen, two rounds of divisions are necessary, termed meiosis I and meiosis II (Figure 15.15). 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. In this section, we will examine the cellular events of meiosis that reduce the chromosome number from diploid to haploid. In addition, we will briefly consider how this process plays a role in the life cycles of fungi, plants, and animals. The First Meiotic Division, Meiosis I, Separates Homologous Chromosomes Like mitosis, meiosis begins after a cell has progressed through the G 1, S, and G 2 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

15 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure and DNA Replication 15. Eukaryotic Chromosomes, Mitosis, 61 EUKARYOTIC CHROMOSOMES, MITOSIS, AND MEIOSIS 313 G 1 phase prior to meiosis Homologous pair of chromosomes prior to chromosomal replication Meiosis I 1 Chromosomes replicate and then condense. Diploid cell with replicated and condensed chromosomes 1 Homologous chromosomes condense. Sister chromatids Haploid cells with pairs of sister chromatids 2 Homologous chromosomes separate. 2 Synaptonemal complex forming Synapsis begins. Meiosis II 3 Sister chromatids separate. 3 Bivalent Bivalents form. Chiasma Four haploid cells with individual chromosomes 4 Crossing over occurs. Figure 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 4 haploid cells. with each other, lying side by side to form a bivalent, also called a tetrad (Figure 15.16). The process of forming a bivalent is termed synapsis. In most eukaryotic species, a synaptonemal complex is formed between homologous chromosomes. 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 each other correctly. At present, the precise role of the synaptonemal complex is not clearly understood. 5 Chiasma becomes visible as chromosome arms separate during late prophase. Figure 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, homologues exchange segments of chromosomes.

16 62 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure 15. Eukaryotic and DNA Replication Chromosomes, Mitosis, 314 UNIT III CHAPTER 15 The second event that occurs at the beginning of meiosis, but not usually during mitosis, is crossing over, which involves a physical exchange between chromosome pieces of the bivalent (Figure 15.16). As discussed in Chapter 17, crossing over may increase the genetic variation of a 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 it physically resembles the Greek letter chi,. The number of crossovers is carefully controlled by cells and depends on the size of the chromosome and the species. The range of crossovers for eukaryotic chromosomes is typically 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, while chromosomes in certain plant species may undergo 20 or more crossovers. Figure The stages of meiosis in an animal cell. Meiosis I (a) Prophase I (b) Prometaphase I (c) Metaphase I Bivalent Fragments of nuclear membrane Metaphase plate Sister chromatids Spindle forming Centrosome Bivalent 1 Homologous chromosomes synapse to form bivalents, and crossing over occurs. Chromosomes condense and the nuclear membrane begins to vesiculate. 2 Nuclear membrane completely vesiculates, and bivalents become attached to kinetochore microtubules. 3 Bivalents align along the metaphase plate. Meiosis II (f) Prophase II (g) Prometaphase II (h) Metaphase II 6 Sister chromatids condense and 7 Nuclear membrane completely the spindle starts to form. Nuclear vesiculates. Sister chromatids attach to membrane begins to vesiculate. spindle via kinetochore microtubules. 8 Sister chromatids align along the metaphase plate.

17 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure and DNA Replication 15. Eukaryotic Chromosomes, Mitosis, 63 EUKARYOTIC CHROMOSOMES, MITOSIS, AND MEIOSIS 315 Now that we have an understanding of bivalent formation and crossing over, we are ready to consider the phases of meiosis (Figure 15.17). These simplified diagrams depict a diploid cell (2n) that contains a total of six chromosomes (as in our look at mitosis in Figure 15.13). Prior to meiosis, the chromosomes are replicated in S phase to produce pairs of sister chromatids. This single replication event is then followed by the two sequential cell divisions called meiosis I and II. Like mitosis, each of these is subdivided into prophase, prometaphase, metaphase, anaphase, and telophase. (d) Anaphase I 4 Homologous chromosomes separate 5 and move toward opposite poles. (i) Anaphase II (e) Telophase I and cytokinesis Cleavage furrow Nuclear membranes re-form and the chromosomes decondense. The 2 cells are separated by a cleavage furrow. (j) Telophase II and cytokinesis Four haploid cells 9 Sister chromatids separate and individual chromosomes move toward poles as kinetochore microtubules shorten. Polar microtubules lengthen and push poles apart. 10 Chromosomes decondense and nuclear membranes re-form. Cleavage furrow separates the 2 cells into 4 cells. The sorting that occurs during meiosis I separates homologues from each other (Figure 15.17). In prophase I, the replicated chromosomes condense and the homologous chromosomes form bivalents as the nuclear membrane starts to vesiculate (fragment into small vesicles). In prometaphase I, the spindle apparatus is complete, and the chromatids are attached to kinetochore microtubules. At metaphase I, the bivalents are organized along the metaphase plate. However, notice how this pattern of alignment is strikingly different from that observed during mitosis (see Figure 15.13d). 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 (blue and red) homologues. In Figure 15.17c, one of the red homologues is to the left of the metaphase plate and the other two are to the right, while two of the blue homologues are to the left of the metaphase plate and other one is to the right. In other cells, homologues could 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). Because eukaryotic species typically have many chromosomes per set, homologues can be randomly aligned along the metaphase plate in a variety of ways. For example, consider that humans have 23 chromosomes per set. The possible number of different, random alignments equals 2 n, where n equals the number of chromosomes per set. Thus, in humans, this equals 2 23, or over 8 million possibilities. Because the homologues 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, it is very unlikely that any two human gametes will have the same combination of homologous chromosomes. The segregation of homologues occurs during anaphase I (Figure 15.17d). 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. Finally, at telophase I, the sister chromatids have reached their

18 64 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure 15. Eukaryotic and DNA Replication Chromosomes, Mitosis, 316 UNIT III CHAPTER 15 Table 15.1 A Comparison of Mitosis, Meiosis I, II Event Mitosis Meiosis I Meiosis II Synapsis during prophase: No Yes, bivalents are formed No Crossing over during prophase: Rarely Commonly Rarely Attachment to poles at A pair of sister chromatids is A pair of sister chromatids is A pair of sister chromatids prometaphase: attached to both poles. attached to just one pole. is attached to both poles. Alignment along the Sister chromatids Bivalents Sister chromatids metaphase plate: Type of separation at Sister chromatids separate. Bivalents separate. Sister chromatids separate. anaphase: A single chromatid, now A pair of sister chromatids A single chromatid, now called a chromosome, moves to each pole. called a chromosome, moves moves to each pole. to each pole. respective poles, and they then decondense. The nuclear membranes now re-form to produce two separate nuclei. If we consider the end result of meiosis I, we see that two cells are produced, each with three pairs of sister chromatids; it is a reduction division. The original diploid cell had its chromosomes in homologous pairs, while the two cells produced at the end of meiosis I are considered haploid they do not have pairs of homologous chromosomes. The Second Meiotic Division, Meiosis II, Separates Sister Chromatids Meiosis I is followed by cytokinesis and then meiosis II (see Figure 15.17). An S phase 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 15.13). 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, unlike anaphase I in which bivalents are separated. Changes in a Few Key Steps in Meiosis and Mitosis Account for the Different Outcomes of These Two Processes If we compare the outcome of meiosis with that of mitosis, the results are quite different. Mitosis produces two diploid daughter cells that are genetically identical. In our previous example shown in Figure 15.13, the starting cell had six chromosomes (three homologous pairs of chromosomes) and both daughter cells had copies of the same six chromosomes. By comparison, meiosis reduces the number of sets of chromosomes. In the example shown in Figure 15.17, the starting cell also had six chromosomes while the four daughter cells had only three chromosomes. But the daughter cells did not contain a random mix of three chromosomes. Each haploid daughter cell contained one complete set of chromosomes, while the original diploid mother cell had two complete sets. Table 15.1 emphasizes the differences between certain key steps in meiosis and mitosis that account for the different outcomes of these two processes. During prophase of meiosis I, the homologues synapse 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. This affects their alignment during metaphase. 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 homologues separate, while the sister chromatids remain together. In contrast, sister chromatid separation occurs during anaphase of mitosis and meiosis II. Taken together, the steps of meiosis result in a process in which two sequential cell divisions create four haploid cells, while the steps of mitosis create two diploid cells. Mitosis, Meiosis, and Fertilization Allow Sexually Reproducing Species to Produce Haploid and Diploid Cells at Different Times in Their Life Cycles Let s now turn our attention to the relationship between mitosis, meiosis, and sexual reproduction in animals, plants, fungi, and protists. 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 involves an alternation between haploid cells or organisms and diploid cells or organisms (Figure 15.18).

19 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure and DNA Replication 15. Eukaryotic Chromosomes, Mitosis, 65 EUKARYOTIC CHROMOSOMES, MITOSIS, AND MEIOSIS Meiosis occurs 2 in cells within testes or ovaries to produce Sperm (1n) gametes (1n). Egg (1n) Diploid adult (2n) Somatic cells are diploid (2n). (a) Animal life cycle (mouse) diploid dominant Key Diploid (2n) Haploid (1n) 5 During fertilization, sperm and egg unite to create a zygote. Zygote (2n) Diploid plant sporophyte (2n) Young leaf (fiddlehead) Repeated mitotic cell divisions produce a diploid multicellular organism. Figure A comparison of three types of sexual life cycles. Biological inquiry: What is the main reason for meiosis in animals? What is the reason for mitosis in animals? 3 4 Repeated mitotic cell divisions produce a diploid multicellular organism. Repeated mitotic cell divisions produce a haploid multicellular organism. 4 Sori Multicellular haploid (1n) organism 1 Somatic cells are haploid (1n). Spore (1n) Somatic cells are diploid (2n). Diploid Sperm (1n) zygote (2n) Egg (1n) Two gametes unite during fertilization to create a diploid zygote. Reproductive cells (1n) Diploid zygote (2n) (b) Fungal life cycle (bread mold) haploid dominant 1 Certain cells in sporophyte undergo meiosis to produce haploid spores. Certain haploid cells act as reproductive cells. Haploid spores (1n) (c) Plant life cycle (fern) alternation of generations Haploid plant gametophyte (1n) Haploid reproductive cells unite to form a dipoid zygote (2n). Meiosis of the zygote produces 4 haploid spores. Repeated mitotic cell divisions produce a haploid multicellular organism. Somatic cells are haploid (1n). Certain cells within the gametophyte differentiate into gametes. 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 15.18a). By comparison, many fungi and some protists are just the opposite, they are haploid-dominant species (Figure 15.18b). In fungi, such as bread mold, the multicellular organism is haploid (1n). Haploid fungal cells are most commonly produced by mitosis. During sexual reproduction, haploid cells unite to create a diploid zygote, which then immediately proceeds through meiosis to create four haploid cells called spores. Each spore goes through mitotic cellular divisions to create a multicellular haploid organism. Plants and some algae have life cycles that are intermediate between the extreme cases of diploid or haploid dominance. Such species exhibit an alternation of generations (Figure 15.18c). The species alternate between multicellular diploid organisms, called sporophytes, and multicellular haploid 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 create a sporophyte. Among different plant species, the relative sizes of the haploid and diploid organisms vary greatly. In bryophytes, such as mosses, the haploid gametophyte is a visible multicellular organism, while the diploid sporophyte is tiny and survives within the haploid organism. In other plants, such as ferns, as shown in Figure 15.18c, both the diploid sporophyte and haploid gametophyte can grow independently. The sporophyte is considerably larger and is the organism that we commonly think of as a fern.

20 66 Brooker Widmaier Graham Stiling: III. Nucleic Acid Structure 15. Eukaryotic and DNA Replication Chromosomes, Mitosis, 318 UNIT III CHAPTER 15 In seed-bearing plants, such as roses and oak trees, the diploid sporophyte is the large multicellular plant, while 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 Variation in Chromosome Structure and Number In the three previous sections of this chapter, we have examined two important features of chromosomes. First, we considered their structure, and second, we explored two sorting processes that determine the chromosome number following cell division. In this section, we will examine how the structures and numbers of chromosomes can vary between different species and within the same species. The study of chromosomal variation is important for several reasons. Geneticists have discovered that variations in chromosome structure and number can have major effects on the characteristics of an organism. For example, 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 force in the evolution of new species. This topic is considered in Chapter 25. Chromosome variation can be viewed in two ways. On relatively rare occasions, the structure or number of chromosomes can be changed so that an individual is different from most other members of the same species. This is generally viewed as an abnormality. Alternatively, some types of variation are normal. For example, the structure and number of chromosomes among different species tend to show wide variation. In this section, we will examine both abnormal and normal types of variation. Let s begin with natural (normal) 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 species. Chromosome composition within a given species tends to remain relatively constant. In most cases, normal individuals of the same species will have the same number and types of chromosomes. For example, as mentioned previously, the normal chromosome composition of human cells is two sets of 23 chromosomes for a total of 46. Other diploid species may 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 chromosomal composition are observed. The chromosomes of a given species can 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 that are revealed when the chromosomes are treated with stains. Based on centromere location, each chromosome is classified as metacentric (near the middle), submetacentric (off center), acrocentric (near one end), or telocentric (at the end) (Figure 15.19a). 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), while the long arm is designated with the letter q. In the case of telocentric chromosomes, the short arm may be nearly nonexistent. When preparing a karyotype, the chromosomes are aligned with the short arms on top and the long arms on the bottom. 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 banding (Figure 15.19b). 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. Also, 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 normal chromosomes can be modified by mutations. These chromosomal mutations are categorized as deficiencies, duplications, inversions, and translocations (Figure 15.20). Deficiencies and duplications are changes in the total amount of genetic material in a single chromosome. When a deficiency occurs, a segment of chromosomal material is missing. In other words, the affected chromosome is deficient in a significant amount of genetic material. The term deletion is also used to describe a missing region of a chromosome. In contrast, a duplication occurs when a section of a chromosome occurs two or more times in a row. The consequences of a deficiency or duplication depend on their size and whether they include genes or portions of genes that are vital to the development of the organism. When deficiencies or duplications have an effect, they are usually detrimental. Larger changes in the amount of genetic material