AP Bio Textbook - Meiosis and Sexual Life Cycles PDF

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This document is about the process of meiosis and sexual life cycles in organisms. It describes the concepts of offspring inheriting genes from parents, and how fertilization and meiosis contribute to genetic variation.

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Superset Meiosis and Sexual Life Cycles 13 Figure 13.1 What accounts for family resemblance? KEy ConCEptS Variations on a theme...

Superset Meiosis and Sexual Life Cycles 13 Figure 13.1 What accounts for family resemblance? KEy ConCEptS Variations on a theme We all know that offspring resemble their parents more than they do unrelated indi- 13.1 Offspring acquire genes from parents by inheriting chromosomes viduals. If you examine the family members shown in Figure 13.1, you can pick out some similar features among them. The transmission of traits from one generation 13.2 Fertilization and meiosis alternate to the next is called inheritance, or heredity (from the Latin heres, heir). However, in sexual life cycles sons and daughters are not identical copies of either parent or of their siblings. Along 13.3 Meiosis reduces the number with inherited similarity, there is also variation. What are the biological mecha- of chromosome sets from nisms leading to the “family resemblance” evident among the family members in diploid to haploid the photo? A detailed answer to this question eluded biologists until the advance of 13.4 Genetic variation produced in genetics in the 20th century. sexual life cycles contributes Genetics is the scientific study of heredity and inherited variation. In this unit, to evolution you’ll learn about genetics at multiple levels, from organisms to cells to molecules. We begin by examining how chromosomes pass from parents to offspring in sexu- A sperm fertilizing an egg. ally reproducing organisms. The processes of meiosis (a special type of cell division) and fertilization (the fusion of sperm and egg, as seen in the small photo) maintain a species’ chromosome count during the sexual life cycle. We will describe the cellu- lar mechanics of meiosis and explain how this process differs from mitosis. Finally, we will consider how both meiosis and fertilization contribute to genetic variation, such as that seen in Figure 13.1. When you see this blue icon, log in to MasteringBiology Get Ready for This chapter and go to the Study Area for digital resources. 254 ConCEpt 13.1 comparison of Asexual and Sexual Reproduction Offspring acquire genes from Only organisms that reproduce asexually have offspring parents by inheriting chromosomes that are exact genetic copies of themselves. In asexual reproduction, a single individual (like a yeast cell or an Family friends may tell you that you have your mother’s nose amoeba; see Figure 12.2a) is the sole parent and passes copies or your father’s eyes. Of course, parents do not, in any literal of all its genes to its offspring without the fusion of gametes. sense, give their children a nose, eyes, hair, or any other traits. For example, single-celled eukaryotic organisms can reproduce What, then, is actually inherited? asexually by mitotic cell division, in which DNA is copied and allocated equally to two daughter cells. The genomes of the off- Inheritance of Genes spring are virtually exact copies of the parent’s genome. Some Parents endow their offspring with coded information in multicellular organisms are also capable of reproducing asexu- the form of hereditary units called genes. The genes we ally (Figure 13.2). Because the cells of the offspring arise via inherit from our mothers and fathers are our genetic link to mitosis in the parent, the offspring is usually genetically identi- our parents, and they account for family resemblances such cal to its parent. An individual that reproduces asexually gives as shared eye color or freckles. Our genes program specific rise to a clone, a group of genetically identical individuals. traits that emerge as we develop from fertilized eggs into Genetic differences occasionally arise in asexually reproducing adults. organisms as a result of changes in the DNA called mutations, The genetic program is written in the language of DNA, which we will discuss in Concept 17.5. the polymer of four different nucleotides you learned about In sexual reproduction, two parents give rise to offspring in Concepts 1.1 and 5.5. Inherited information is passed on that have unique combinations of genes inherited from the two in the form of each gene’s specific sequence of DNA nucleo- parents. In contrast to a clone, offspring of sexual reproduction tides, much as printed information is communicated in the vary genetically from their siblings and both parents: They are form of meaningful sequences of letters. In both cases, the variations on a common theme of family resemblance, not language is symbolic. Just as your brain translates the word exact replicas. Genetic variation like that shown in Figure 13.1 apple into a mental image of the fruit, cells translate genes is an important consequence of sexual reproduction. What into freckles and other features. Most genes program cells mechanisms generate this genetic variation? The key is the to synthesize specific enzymes and other proteins, whose behavior of chromosomes during the sexual life cycle. cumulative action produces an organism’s inherited traits. The programming of these traits in the form of DNA is one Figure 13.2 Asexual reproduction in two multicellular of the unifying themes of biology. organisms. (a) This relatively simple animal, a hydra, reproduces The transmission of hereditary traits has its molecular by budding. The bud, a localized mass of mitotically dividing cells, develops into a small hydra, which detaches from the parent (LM). basis in the replication of DNA, which produces copies of (b) All the trees in this circle of redwoods arose asexually from genes that can be passed from parents to offspring. In animals a single parent tree, whose stump is in the center of the circle. and plants, reproductive cells called gametes are the vehicles that transmit genes from one generation to the next. During 0.5 mm fertilization, male and female gametes (sperm and eggs) unite, passing on genes of both parents to their offspring. Except for small amounts of DNA in mitochondria and chloroplasts, the DNA of a eukaryotic cell is packaged into chromosomes within the nucleus. Every species has a charac- teristic number of chromosomes. For example, humans have 46 chromosomes in their somatic cells—all cells of the body except the gametes and their precursors. Each chromosome consists of a single long DNA molecule, elaborately coiled in Parent association with various proteins. One chromosome includes Bud several hundred to a few thousand genes, each of which is a precise sequence of nucleotides along the DNA molecule. A gene’s specific location along the length of a chromosome is called the gene’s locus (plural, loci; from the Latin, mean- ing “place”). Our genetic endowment (our genome) consists (a) Hydra (b) Redwoods of the genes and other DNA that make up the chromosomes Video: Hydra Budding we inherited from our parents. Animation: Asexual Reproduction CHAPTeR 13 Meiosis and Sexual Life Cycles 255 ConCEpt ChECK 13.1 Figure 13.3 1. MAKE ConnECtionS Using what you know of gene Research Method Preparing a Karyotype expression in a cell, explain what causes the traits of parents (such as hair color) to show up in their offspring. Application A karyotype is a display of condensed chromosomes (See Concept 5.5.) arranged in pairs. Karyotyping can be used to screen for defective 2. How does an asexually reproducing eukaryotic organism chromosomes or abnormal numbers of chromosomes associated with produce offspring that are genetically identical to each certain congenital disorders, such as Down syndrome. other and to their parents? 3. WhAt iF? A horticulturalist breeds orchids, trying to obtain a plant with a unique combination of desirable traits. After many years, she finally succeeds. To produce more plants like this one, should she crossbreed it with another plant or clone it? Why? For suggested answers, see Appendix A. ConCEpt 13.2 Fertilization and meiosis alternate in sexual life cycles A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism, from conception to technique Karyotypes are prepared from isolated somatic cells, production of its own offspring. In this section, we use humans which are treated with a drug to stimulate mitosis and then grown as an example to track the behavior of chromosomes through in culture for several days. Cells arrested when the chromosomes the sexual life cycle. We begin by considering the chromo- are most highly condensed—at metaphase—are stained and then viewed with a microscope equipped with a digital camera. An some count in human somatic cells and gametes. We will image of the chromosomes is displayed on a computer monitor, then explore how the behavior of chromosomes relates to the and digital software is used to arrange them in pairs according to human life cycle and other types of sexual life cycles. their appearance. Pair of homologous Sets of chromosomes in Human cells duplicated chromosomes In humans, each somatic cell has 46 chromosomes. During mitosis, the chromosomes become condensed enough to be Centromeres 5 μm visible under a light microscope. At this point, they can be distinguished from one another by their size, the position of their centromeres, and the pattern of colored bands produced by certain chromatin-binding stains. Careful examination of a micrograph of the 46 human Sister chromosomes from a single cell in mitosis reveals that there chromatids are two chromosomes of each of 23 types. This becomes clear Metaphase when images of the chromosomes are arranged in pairs, starting chromosome with the longest chromosomes. The resulting ordered display is called a karyotype (Figure 13.3). The two chromosomes of a pair have the same length, centromere position, and staining pattern: These are called homologous chromosomes (or homologs). Both chromosomes of each pair carry genes con- trolling the same inherited characters. For example, if a gene for eye color is situated at a particular locus on a certain chro- mosome, then its homologous chromosome (its homolog) will Results This karyotype shows the chromosomes from a human also have a version of the eye-color gene at the equivalent locus. male (as seen by the presence of the XY chromosome pair), colored The two chromosomes referred to as X and Y are an impor- to emphasize their chromosome banding patterns. The size of the tant exception to the general pattern of homologous chro- chromosome, position of the centromere, and pattern of stained bands help identify specific chromosomes. Although difficult to mosomes in human somatic cells. Typically, human females discern in the karyotype, each metaphase chromosome consists of have a homologous pair of X chromosomes (XX), while males two closely attached sister chromatids (see the diagram of the first have one X and one Y chromosome (XY; see Figure 13.3). pair of homologous duplicated chromosomes). Only small parts of the X and Y are homologous. Most of the 256 UNIT THRee Genetics genes carried on the X chromosome do not have counter- 22 autosomes plus a single sex chromosome. An unfertilized parts on the tiny Y, and the Y chromosome has genes lacking egg contains an X chromosome; a sperm contains either an on the X. Due to their role in sex determination, the X and X or a Y chromosome. Y chromosomes are called sex chromosomes. The other Each sexually reproducing species has a characteristic dip- chromosomes are called autosomes. loid and haploid number. For example, the fruit fly Drosophila The occurrence of pairs of homologous chromosomes in melanogaster has a diploid number (2n) of 8 and a haploid each human somatic cell is a consequence of our sexual origins. number (n) of 4, while for dogs, 2n is 78 and n is 39. The chro- We inherit one chromosome of a pair from each parent. Thus, mosome number generally does not correlate with the size or the 46 chromosomes in our somatic cells are actually two sets complexity of a species’ genome; it simply reflects how many of 23 chromosomes—a maternal set (from our mother) and a linear pieces of DNA make up the genome, which is a function paternal set (from our father). The number of chromosomes in of the evolutionary history of that species (see Concept 21.5). a single set is represented by n. Any cell with two chromosome Now let’s consider chromosome behavior during sexual life sets is called a diploid cell and has a diploid number of chro- cycles. We’ll use the human life cycle as an example. mosomes, abbreviated 2n. For humans, the diploid number is 46 (2n = 46), the number of chromosomes in our somatic cells. Behavior of chromosome Sets In a cell in which DNA synthesis has occurred, all the chromo- in the Human Life cycle somes are duplicated, and therefore each consists of two identi- The human life cycle begins when a haploid sperm from the cal sister chromatids, associated closely at the centromere and father fuses with a haploid egg from the mother (Figure 13.5). along the arms. (Even though the chromosomes are duplicated, we still say the cell is diploid, or 2n. This because it has only Figure 13.5 the human life cycle. In each generation, the two sets of information regardless of the number of chroma- number of chromosome sets is halved during meiosis but doubles at tids, which are merely copies of the information in one set.) fertilization. For humans, the number of chromosomes in a haploid cell Figure 13.4 helps clarify the various terms that we use to is 23, consisting of one set (n = 23); the number of chromosomes in the diploid zygote and all somatic cells arising from it is 46, consisting describe duplicated chromosomes in a diploid cell. of two sets (2n = 46). Unlike somatic cells, gametes contain a single set of chro- Key Haploid gametes (n = 23) mosomes. Such cells are called haploid cells, and each has a haploid number of chromosomes (n). For humans, the Haploid (n) Egg (n) haploid number is 23 (n = 23). The set of 23 consists of the Diploid (2n) Figure 13.4 Describing chromosomes. A cell from an organism with a diploid number of 6 (2n = 6) is depicted here following chromosome duplication and condensation. Each of the six duplicated chromosomes consists of two sister chromatids associated closely along Sperm (n) their lengths. Each homologous pair is composed of one chromosome from the maternal set (red) and one from the paternal set (blue). Each set is made up of three chromosomes in this example (long, medium, MEIOSIS FERTILIZATION and short). Together, one maternal and one paternal chromatid in a pair of homologous chromosomes are called nonsister chromatids. Key Maternal set of chromosomes (n = 3) Ovary Testis 2n = 6 Paternal set of chromosomes (n = 3) Diploid zygote (2n = 46) Sister chromatids of one duplicated chromosome Centromere Mitosis and development Multicellular diploid Two nonsister Pair of homologous adults (2n = 46) chromatids in chromosomes a homologous pair (one from each set) This figure introduces a color code that will be used for other life cycles later in this book. The aqua arrows identify haploid stages ViSUAL SKiLLS How many sets of chromosomes are present in this diagram? How many pairs of homologous chromosomes are present? of a life cycle, and the tan arrows identify diploid stages. BioFlix® Animation: chromosomes Animation: The Human Life cycle CHAPTeR 13 Meiosis and Sexual Life Cycles 257 This union of gametes, culminating in fusion of their nuclei, Fertilization and meiosis alternate in sexual life cycles, main- is called fertilization. The resulting fertilized egg, or zygote, taining a constant number of chromosomes in each species is diploid because it contains two haploid sets of chromosomes from one generation to the next. bearing genes representing the maternal and paternal family lines. As a human develops into a sexually mature adult, mitosis The Variety of Sexual Life cycles of the zygote and its descendant cells generates all the somatic Although the alternation of meiosis and fertilization is com- cells of the body. Both chromosome sets in the zygote and all the mon to all organisms that reproduce sexually, the timing of genes they carry are passed with precision to the somatic cells. these two events in the life cycle varies, depending on the spe- The only cells of the human body not produced by mito- cies. These variations can be grouped into three main types of sis are the gametes, which develop from specialized cells life cycles. In the type that occurs in humans and most other called germ cells in the gonads—ovaries in females and testes animals, gametes are the only haploid cells (Figure 13.6a). in males (see Figure 13.5). Imagine what would happen if Meiosis occurs in germ cells during the production of gam- human gametes were made by mitosis: They would be diploid etes, which undergo no further cell division prior to fertiliza- like the somatic cells. At the next round of fertilization, when tion. After fertilization, the diploid zygote divides by mitosis, two gametes fused, the normal chromosome number of 46 producing a multicellular organism that is diploid. would double to 92, and each subsequent generation would Plants and some species of algae exhibit a second type of life double the number of chromosomes yet again. This does not cycle called alternation of generations (Figure 13.6b). This happen, however, because in sexually reproducing organ- type includes both diploid and haploid stages that are multicel- isms, gamete formation involves a type of cell division called lular. The multicellular diploid stage is called the sporophyte. meiosis. This type of cell division reduces the number of sets Meiosis in the sporophyte produces haploid cells called spores. of chromosomes from two to one in the gametes, counter- Unlike a gamete, a haploid spore doesn’t fuse with another cell balancing the doubling that occurs at fertilization. As a result but divides mitotically, generating a multicellular haploid stage of meiosis, each human sperm and egg is haploid (n = 23). called the gametophyte. Cells of the gametophyte give rise to Fertilization restores the diploid condition by combining two gametes by mitosis. Fusion of two haploid gametes at fertiliza- sets of chromosomes, and the human life cycle is repeated, tion results in a diploid zygote, which develops into the next generation after generation (see Figure 13.5). sporophyte generation. Therefore, in this type of life cycle, the In general, the steps of the human life cycle are typical of sporophyte generation produces a gametophyte as its offspring, many sexually reproducing animals. Indeed, the processes and the gametophyte generation produces the next sporophyte of fertilization and meiosis are also the hallmarks of sexual generation (see Figure 13.6b). The term alternation of generations reproduction in plants, fungi, and protists just as in animals. fits well as a name for this type of life cycle. Figure 13.6 three types of sexual life cycles. The common feature of all three cycles is the alternation of meiosis and fertilization, key events that contribute to genetic variation among offspring. The cycles differ in the timing of these two key events. (Small circles are cells; large circles are organisms.) Key Haploid (n) Haploid multi- Haploid unicellular or Diploid (2n) cellular organism multicellular organism (gametophyte) n Gametes n n Mitosis n Mitosis Mitosis n Mitosis n n n n n MEIOSIS FERTILIZATION Spores n n Gametes Gametes n MEIOSIS FERTILIZATION Zygote MEIOSIS FERTILIZATION 2n 2n 2n 2n Diploid Zygote 2n Diploid multicellular Mitosis multicellular Mitosis organism organism Zygote (sporophyte) (a) Animals (b) Plants and some algae (c) Most fungi and some protists ViSUAL SKiLLS For each type of life cycle, indicate whether haploid cells undergo mitosis, and if they do, describe the cells that are formed. 258 UNIT THRee Genetics A third type of life cycle occurs in most fungi and some Figure 13.7 overview of meiosis: how meiosis reduces protists, including some algae (Figure 13.6c). After gametes chromosome number. After the chromosomes duplicate in interphase, the diploid cell divides twice, yielding four haploid daughter cells. This fuse and form a diploid zygote, meiosis occurs without a mul- overview tracks just one pair of homologous chromosomes, which for ticellular diploid offspring developing. Meiosis produces not the sake of simplicity are drawn in the condensed state throughout. gametes but haploid cells that then divide by mitosis and give rise to either unicellular descendants or a haploid multicellu- Interphase lar adult organism. Subsequently, the haploid organism car- ries out further mitoses, producing the cells that develop into Pair of homologous gametes. The only diploid stage found in these species is the chromosomes in diploid parent cell single-celled zygote. Note that either haploid or diploid cells can divide by mitosis, depending on the type of life cycle. Only diploid cells, however, can undergo meiosis because haploid cells Chromosomes have only a single set of chromosomes that cannot be further Pair of duplicated duplicate homologous chromosomes reduced. Though the three types of sexual life cycles differ in the timing of meiosis and fertilization, they share a funda- mental result: genetic variation among offspring. Sister ConCEpt ChECK 13.2 chromatids Diploid cell with duplicated 1. MAKE ConnECtionS In Figure 13.4, how many DNA chromosomes molecules (double helices) are present (see Figure 12.5)? What is the haploid number of this cell? Is a set of chromosomes haploid or diploid? Meiosis I 2. ViSUAL SKiLLS In the karyotype shown in Figure 13.3, how many pairs of chromosomes are present? How many sets? 1 3. WhAt iF? A certain eukaryote lives as a unicellular Homologous organism, but during environmental stress, it produces chromosomes gametes. The gametes fuse, and the resulting zygote separate undergoes meiosis, generating new single cells. What type of organism could this be? Haploid cells with For suggested answers, see Appendix A. duplicated chromosomes Meiosis II ConCEpt 13.3 2 Sister chromatids separate Meiosis reduces the number of chromosome sets from diploid to haploid Several steps of meiosis closely resemble corresponding steps Haploid cells with unduplicated chromosomes in mitosis. Meiosis, like mitosis, is preceded by the duplica- tion of chromosomes. However, this single duplication is DRAW it Redraw the cells in this figure using a simple double helix followed by not one but two consecutive cell divisions, called to represent each DNA molecule. meiosis I and meiosis II. These two divisions result in four Animation: Overview of Meiosis daughter cells (rather than the two daughter cells of mitosis), each with only half as many chromosomes as the parent cell—one set, rather than two. chromosome (see Figure 13.4). In contrast, the two chromo- somes of a homologous pair are individual chromosomes that The Stages of Meiosis were inherited from each parent. Homologs appear alike in The overview of meiosis in Figure 13.7 shows, for a single the microscope, but they may have different versions of genes pair of homologous chromosomes in a diploid cell, that both at corresponding loci; each version is called an allele of that members of the pair are duplicated and the copies sorted into gene (see Figure 14.4). Homologs are not associated with each four haploid daughter cells. Recall that sister chromatids are other in any obvious way except during meiosis. two copies of one chromosome, closely associated all along Figure 13.8 describes in detail the stages of the two divi- their lengths; this association is called sister chromatid cohesion. sions of meiosis for an animal cell whose diploid number is 6. Together, the sister chromatids make up one duplicated Study this figure thoroughly before going on. CHAPTeR 13 Meiosis and Sexual Life Cycles 259 Figure 13.8 exploring Meiosis in an Animal cell MEIOSIS I: Separates homologous chromosomes Telophase I Prophase I Metaphase I Anaphase I and Cytokinesis Sister chromatids Centrosome remain attached (with centriole pair) Chiasmata Kinetochore Sister (at centromere) chromatids Kinetochore Spindle microtubules microtubules Metaphase plate Cleavage Fragments furrow of nuclear Homologous Pair of envelope chromosomes homologous Centromere separate chromosomes Duplicated homologous Chromosomes line up The two homologous Two haploid cells chromosomes (red and blue) by homologous pairs. chromosomes of each form; each chromosome pair up and exchange segments; pair separate. still consists of two 2n = 6 in this example. sister chromatids. Telophase I and Prophase I Metaphase I Anaphase I Cytokinesis Centrosome movement, spindle Pairs of homologous Breakdown of proteins that are When telophase I begins, formation, and nuclear envelope chromosomes are now responsible for sister chromatid each half of the cell has breakdown occur as in mitosis. arranged at the metaphase cohesion along chromatid a complete haploid set of Chromosomes condense progres- plate, with one chromo- arms allows homologs to duplicated chromosomes. sively throughout prophase I. some of each pair facing separate. Each chromosome is each pole. composed of two sister During early prophase I, before the The homologs move toward chromatids; one or both stage shown above, each chromo- Both chromatids of one opposite poles, guided by the chromatids include regions of some pairs with its homolog, homolog are attached to spindle apparatus. nonsister chromatid DNA. aligned gene by gene, and kinetochore microtubules crossing over occurs: The DNA from one pole; the chroma- Sister chromatid cohesion Cytokinesis (division of molecules of nonsister chromatids tids of the other homolog persists at the centromere, the cytoplasm) usually are broken (by proteins) and are are attached to microtubules causing chromatids to move as occurs simultaneously with rejoined to each other. from the opposite pole. a unit toward the same pole. telophase I, forming two haploid daughter cells. At the stage shown above, each homologous pair has one or more In animal cells like these, a X-shaped regions called chiasmata cleavage furrow forms. (In (singular, chiasma), where plant cells, a cell plate forms.) crossovers have occurred. In some species, chromo- Later in prophase I, microtubules from somes decondense and one pole or the other attach to the nuclear envelopes form. kinetochores, one at the centromere of each homolog. (The two kineto- No chromosome duplication chores on the sister chromatids of a occurs between meiosis I and homolog are linked together by pro- meiosis II. teins and act as a single kinetochore.) Microtubules move the homologous Video: Meiosis I in Sperm Formation pairs toward the metaphase plate (see the metaphase I diagram). 260 UNIT THRee Genetics MEIOSIS II: Separates sister chromatids Telophase II Prophase II Metaphase II Anaphase II and Cytokinesis During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Sister chromatids separate Haploid daughter cells forming Telophase II and Prophase II Metaphase II Anaphase II Cytokinesis A spindle apparatus forms. The chromosomes are Breakdown of proteins Nuclei form, the chromo- positioned at the metaphase holding the sister chromatids somes begin decondensing, In late prophase II (not shown plate as in mitosis. together at the centromere and cytokinesis occurs. here), chromosomes, each still allows the chromatids to composed of two chromatids Because of crossing over in separate. The chromatids The meiotic division of one associated at the centromere, meiosis I, the two sister move toward opposite poles parent cell produces four are moved by microtubules chromatids of each chromo- as individual chromosomes. daughter cells, each with a toward the metaphase II plate. some are not genetically haploid set of (unduplicated) identical. chromosomes. The kinetochores of sister The four daughter cells are chromatids are attached to genetically distinct from one microtubules extending from another and from the parent opposite poles. cell. MAKE ConnECtionS Look at Figure 12.7 and imagine the two daughter cells undergoing another round of mitosis, yielding four cells. Compare the number of chromosomes in each of those four cells, after mitosis, with the number in each cell in Figure 13.8, after meiosis. What BioFlix® Animation: Meiosis is it about the process of meiosis that accounts for this difference, even Animation: Meiosis though meiosis also includes two cell divisions? CHAPTeR 13 Meiosis and Sexual Life Cycles 261 crossing Over and Synapsis During Figure 13.9 Crossing over and synapsis in prophase I: a closer look. Prophase I Pair of homologous Prophase I of meiosis is a very busy time. The prophase I cell chromosomes: shown in Figure 13.8 is at a point fairly late in prophase I, DNA breaks Centromere DNA breaks Paternal when pairing of homologous chromosomes, crossing over, Cohesins sister and chromosome condensation have already taken place. chromatids The sequence of events leading up to that point is shown Maternal in more detail in Figure 13.9. sister chromatids After interphase, the chromosomes have been duplicated 1 After interphase, the chromosomes have been dupli- and the sister chromatids are held together by proteins called cated, and sister chromatids are held together by proteins cohesins. 1 Early in prophase I, the two members of a homol- called cohesins (purple). Each pair of homologs associate ogous pair associate loosely along their length. Each gene along their length. The DNA molecules of two nonsister chromatids are broken at precisely corresponding points. on one homolog is aligned precisely with the corresponding The chromatin of the chromosomes starts to condense. allele of that gene on the other homolog. The DNA of two Synaptonemal nonsister chromatids—one maternal and one paternal—is complex forming broken by specific proteins at precisely matching points. 2 Next, the formation of a zipper-like structure called the synaptonemal complex holds one homolog tightly to the other. 3 During this association, called synapsis, the 2 A zipper-like protein complex, the synaptonemal complex (green), DNA breaks are closed up so that each broken end is joined to begins to form, attaching one homolog to the other. the corresponding segment of the nonsister chromatid. Thus, The chromatin continues to condense. a paternal chromatid is joined to a piece of maternal Sister chromatids Synaptonemal chromatid beyond the crossover point, and vice versa. complex 4 These points of crossing over become visible as chiasmata Sister chromatids (singular, chiasma) after the synaptonemal complex disas- sembles and the homologs move slightly apart from each other. The homologs remain attached because sister chromatids are still held together by sister chromatid cohesion, even though some of the DNA may no longer be attached to its original chro- Crossovers mosome. At least one crossover per chromosome must occur 3 The synaptonemal complex is fully formed; the two in order for the homologous pair to stay together as it moves to homologs are said to be in synapsis. During synapsis, the metaphase I plate, for reasons that will be explained shortly. the DNA breaks are closed up when each broken end is joined to the corresponding segment of the nonsister chromatid, producing crossovers. A comparison of Mitosis and Meiosis Figure 13.10 summarizes the key differences between meio- Chiasmata sis and mitosis in diploid cells. Basically, meiosis reduces the number of chromosome sets from two (diploid) to one (haploid), whereas mitosis conserves the number of chromo- some sets. Therefore, meiosis produces cells that differ genet- ically from their parent cell and from each other, whereas mitosis produces daughter cells that are genetically identical 4 After the synaptonemal complex disassembles, the homologs to their parent cell and to each other. move slightly apart from each other but remain attached because of sister chromatid cohesion, even though some of Three events unique to meiosis occur during meiosis I: the DNA may no longer be attached to its original chromo- some. The points of attachment where crossovers have 1. Synapsis and crossing over. During prophase I, occurred show up as chiasmata. The chromosomes continue duplicated homologs pair up and crossing over occurs, to condense as they move toward the metaphase plate. as described previously and in Figure 13.9. Synapsis and crossing over do not occur during prophase of mitosis. 3. Separation of homologs. At anaphase I of meiosis, the 2. Alignment of homologous pairs at the metaphase duplicated chromosomes of each homologous pair move plate. At metaphase I of meiosis, pairs of homologs are toward opposite poles, but the sister chromatids of each positioned at the metaphase plate, rather than individual duplicated chromosome remain attached. In anaphase chromosomes, as in metaphase of mitosis. of mitosis, by contrast, sister chromatids separate. 262 UNIT THRee Genetics of gametes differing greatly in their combinations of the chro- mosomes we inherited from our two parents. Figure 13.11 sug- gests that each chromosome in a gamete is exclusively maternal or paternal in origin. In fact, this is not the case, because cross- ing over produces recombinant chromosomes, individual chromosomes that carry genes (DNA) from two different par- ents (Figure 13.12). In meiosis in humans, an average of one to three crossover events occurs per chromosome pair, depend- ing on the size of the chromosomes and the position of their centromeres. As you learned in Figure 13.9, crossing over produces chro- mosomes with new combinations of maternal and paternal alleles. At metaphase II, chromosomes that contain one or more recombinant chromatids can be oriented in two alterna- tive, nonequivalent ways with respect to other chromosomes because their sister chromatids are no longer identical (see Figure 13.12). The different possible arrangements of non- identical sister chromatids during meiosis II further increase the number of genetic types of daughter cells that can result from meiosis. You’ll learn more about crossing over in Chapter 15. The important point for now is that crossing over, by combining DNA inherited from two parents into a single chromosome, is an important source of genetic variation in sexual life cycles. Random Fertilization The random nature of fertilization adds to the genetic variation arising from meiosis. In humans, each male and female gamete represents one of about 8.4 million (223) possible chromosome combinations due to independent assortment. The fusion of a male gamete with a female gamete during fertilization will pro- duce a zygote with any of about 70 trillion (223 * 223) diploid combinations. If we factor in the variation brought about by crossing over, the number of possibilities is truly astronomical. It may sound trite, but you really are unique. matched during meiosis. New and different combinations of alleles may work better than those that previously prevailed. In a stable environment, though, sexual reproduction seems as if it would be less advantageous than asexual repro- duction, which ensures perpetuation of successful combina- tions of alleles. Furthermore, sexual reproduction is more expensive energetically than asexual reproduction. In spite of these apparent disadvantages, sexual reproduction is almost universal among animals. Why is this? The ability of sexual reproduction to generate genetic diversity is the most commonly proposed explanation for the evolutionary persistence of this process. However, consider the unusual case of the bdelloid rotifer (Figure 13.13). It appears that this group may not have reproduced sexually for more than 50 million years of their evolutionary history, a model that has been supported by recent analysis of the genetic sequences in its genome. Does this mean that genetic diversity is not advantageous in this species? It turns out that bdelloid rotifers are an exception to the “rule” that sex alone generates 266 UNIT THRee Genetics

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