《生物学》课程参考书籍文献（英文版，Chapter 11 - Chapter 19）Part IV Reproduction and Heredity、Part V Molecular Genetics

2.3 The sequence of events during meiosis involves two nuclear divisions Prophase I complex In prophase I of meiosis, the DNA coils tighter, and indi- vidual chromosomes first become visible under the light microscope as a matrix of fine threads. Because the dnA Chromosome has already replicated before the onset of meiosis, each of these threads actually consists of two sister chromatids joined at their centromeres In prophase I, homologous chromosomes become closely associated in synapsis, ex- change segments by crossing over, and then separate An overview Prophase I is traditionally divided into five sequential eptotene kinesis eptotene. Chromosomes condense tightl FIGURE 12.7 Zygotene. A lattice of protein is laid down Structure of the synaptonemal complex. A portion of the the homologous chromosomes in the process of synaptonemal complex of the ascomycete Neotiella rutilans, a cup sis, forming a structure called a synaptonemal Pachytene. Pachytene begins when synapsis is com- zipper. Within the synaptonemal complex, the dna du- plete Gust after the synaptonemal complex forms; figure plexes unwind at certain sites, and single strands of 12.8), and lasts for days. This complex, about 100 nm DNA form base-pairs with complementary strands or across, holds the two replicated chromosomes in precise the otber homologue. The synaptonemal complex thus register, keeping each gene directly across from its part- provides the structural framework that enables crossing ner on the homologous chromosome, like the teeth of a over between the homologous chromosomes. As yo Interphase Leptotene Zygotene Pachyten Diplotene followed by diakinesis Crossing over can occur Chromatids Formation isassembly Chromatid 2 of the complex Chromatid 3 LRN chromatids Chromatid 4 FIGURE 12.8 Time course of prophase I. The five stages of prophase I represent stages in the formation and subsequent disassembly of the 230 Part IV Reproduction and Heredity

Prophase I In prophase I of meiosis, the DNA coils tighter, and indi￾vidual chromosomes first become visible under the light microscope as a matrix of fine threads. Because the DNA has already replicated before the onset of meiosis, each of these threads actually consists of two sister chromatids joined at their centromeres. In prophase I, homologous chromosomes become closely associated in synapsis, ex￾change segments by crossing over, and then separate. An Overview Prophase I is traditionally divided into five sequential stages: leptotene, zygotene, pachytene, diplotene, and dia￾kinesis. Leptotene. Chromosomes condense tightly. Zygotene. A lattice of protein is laid down between the homologous chromosomes in the process of synap￾sis, forming a structure called a synaptonemal complex (figure 12.7). Pachytene. Pachytene begins when synapsis is com￾plete (just after the synaptonemal complex forms; figure 12.8), and lasts for days. This complex, about 100 nm across, holds the two replicated chromosomes in precise register, keeping each gene directly across from its part￾ner on the homologous chromosome, like the teeth of a zipper. Within the synaptonemal complex, the DNA du￾plexes unwind at certain sites, and single strands of DNA form base-pairs with complementary strands on the other homologue. The synaptonemal complex thus provides the structural framework that enables crossing over between the homologous chromosomes. As you 230 Part IV Reproduction and Heredity 12.3 The sequence of events during meiosis involves two nuclear divisions. Chromosome homologues Synaptonemal complex FIGURE 12.7 Structure of the synaptonemal complex. A portion of the synaptonemal complex of the ascomycete Neotiella rutilans, a cup fungus. Interphase Leptotene Zygotene Pachytene Diplotene followed by diakinesis Chromatid 1 Chromatid 2 Chromatid 3 Chromatid 4 Disassembly of the synaptonemal complex Formation of the synaptonemal complex Chromatid 1 Chromatid 2 Chromatid 3 Chromatid 4 Paternal sister chromatids Maternal sister chromatids Time Crossing over can occur between homologous chromosomes FIGURE 12.8 Time course of prophase I. The five stages of prophase I represent stages in the formation and subsequent disassembly of the synaptonemal complex, the protein lattice that holds homologous chromosomes together during synapsis

will see, this has a key impact on how the homologues separate later In meiosis Diplotene. At the beginning of diplotene, the protein lattice of the synaptonemal complex disassem- bles. Diplotene is a period of in- tense cell growth. During this pe- od the chromosomes decondense and become very active in tran- Diakinesis. At the beginning of diakinesis. the transition into metaphase, transcription ceases and the chromosomes recondense FIGURE 12.9 Synapsis Chiasmata. This micrograph shows two distinct crossovers, or chiasmata. During prophase, the ends of the chromatids attach to the nuclear envelope at specific sites The sites the homologues attach to are adjacent, so that the members of each homologous pair of chromosomes are brought close together. They then line up side by side, al parently guided by heterochromatin sequences, in the rocess called snaps Over Within the synaptonemal complex, recombination is thought to be carried out during pachytene by very large protein assemblies called recombination nodules. A nod- FIGURE 12 ule's diameter is about 90 nm, spanning the central element The results of crossing over. During crossing over, nonsister of the synaptonemal complex Spaced along the synaptone- mal complex, these recombination nodules act as large multienzyme"recombination machines, "each nodule bringing about a recombination event. The details of the ossing over process are not well understood, but involve a complex series of events in which DNA segments are ex Chiasma formation changed between nonsister or sister chromatids. In hu- Evidence of crossing over can often be seen under the light mans, an average of two or three such crossover events microscope as an X-shaped structure known as a chiasma occur per chromosome pair (Greek, " cross"; plural, chiasmata; figure 12.9). The pres- When crossing over is complete, the synaptonemal com- ence of a chiasma indicates that two chromatids(one from plex breaks down, and the homologous chromosomes ar each homologue) have exchanged parts(figure 12. 10). Like released from the nuclear envelope and begin to move away small rings moving down two strands of rope, the chias- from each other. At this point, there are four chromatids mata move to the end of the chromosome arm as the ho for each type of chromosome(two homologous chromo- mologous chromosomes separate somes, each of which consists of two sister chromatids) The four chromatids do not separate completely, however, Synapsis is the close pairing of homologous because they are held together in two ways: (1)the two sis- chromosomes that takes place early in prophase I of er chromatids of each homologue, recently created by meiosis. Crossing over occurs between the paired DNA DNA replication, are held near by their common cen- tromeres;and(2)the paired homologues are held together known as chiasmata. The two homologues are locked nts where crossing over o d within the together by these exchanges and they do not disengage tonemal complex readil Chapter 12 Sexual Reproduction and Meiosis 231

will see, this has a key impact on how the homologues separate later in meiosis. Diplotene. At the beginning of diplotene, the protein lattice of the synaptonemal complex disassem￾bles. Diplotene is a period of in￾tense cell growth. During this pe￾riod the chromosomes decondense and become very active in tran￾scription. Diakinesis. At the beginning of diakinesis, the transition into metaphase, transcription ceases and the chromosomes recondense. Synapsis During prophase, the ends of the chromatids attach to the nuclear envelope at specific sites. The sites the homologues attach to are adjacent, so that the members of each homologous pair of chromosomes are brought close together. They then line up side by side, ap￾parently guided by heterochromatin sequences, in the process called synapsis. Crossing Over Within the synaptonemal complex, recombination is thought to be carried out during pachytene by very large protein assemblies called recombination nodules. A nod￾ule’s diameter is about 90 nm, spanning the central element of the synaptonemal complex. Spaced along the synaptone￾mal complex, these recombination nodules act as large multienzyme “recombination machines,” each nodule bringing about a recombination event. The details of the crossing over process are not well understood, but involve a complex series of events in which DNA segments are ex￾changed between nonsister or sister chromatids. In hu￾mans, an average of two or three such crossover events occur per chromosome pair. When crossing over is complete, the synaptonemal com￾plex breaks down, and the homologous chromosomes are released from the nuclear envelope and begin to move away from each other. At this point, there are four chromatids for each type of chromosome (two homologous chromo￾somes, each of which consists of two sister chromatids). The four chromatids do not separate completely, however, because they are held together in two ways: (1) the two sis￾ter chromatids of each homologue, recently created by DNA replication, are held near by their common cen￾tromeres; and (2) the paired homologues are held together at the points where crossing over occurred within the synaptonemal complex. Chiasma Formation Evidence of crossing over can often be seen under the light microscope as an X-shaped structure known as a chiasma (Greek, “cross”; plural, chiasmata; figure 12.9). The pres￾ence of a chiasma indicates that two chromatids (one from each homologue) have exchanged parts (figure 12.10). Like small rings moving down two strands of rope, the chias￾mata move to the end of the chromosome arm as the ho￾mologous chromosomes separate. Synapsis is the close pairing of homologous chromosomes that takes place early in prophase I of meiosis. Crossing over occurs between the paired DNA strands, creating the chromosomal configurations known as chiasmata. The two homologues are locked together by these exchanges and they do not disengage readily. Chapter 12 Sexual Reproduction and Meiosis 231 FIGURE 12.9 Chiasmata. This micrograph shows two distinct crossovers, or chiasmata. FIGURE 12.10 The results of crossing over. During crossing over, nonsister (shown above) or sister chromatids may exchange segments

Meiosis i Chiasmata hold Mitosis inetochores of sister The kinetochores of eparate: microtubules Metaphase unction as one kinetochores on Microtubules can opposite sides of the attach to only one side of each centromere Microtubules pull the homologous chromosomes Microtubules pull sister apart, but sister chromatids apal chromatids ar eld together Anaphase FIGURE 12.11 Chiasmata created by crossing over have a key impact on how chromosomes align in metaphase I In the first meiotic division, the chiasmata hold one sister chromatid to the other sister chromatid; consequently, the spindle microtubules can bind to only one side of each centromere, and the homologous chromosomes are drawn to opposite poles. In mitosis, microtubules attach to botb sides of each centromere;when the microtubules shorten, the sister chromatids are split and drawn to opposite poles Metaphase I By metaphase I, the second stage of meiosis I, the nuclear envelope has dispersed and the microtubules form a spin dle, just as in mitosis. During diakinesis of prophase I, the chiasmata move down the paired chromosomes from their original points of crossing over, eventually reachin D the ends of the chromosomes. At this point, they are called terminal chiasmata. Terminal chiasmata hold the homologous chromosomes together in metaphase I,so that only one side of each centromere faces outward from the complex; the other side is turned inward toward the ue(figure 12. 11). Consequently, spindle microtubules are able to attach to kinetochore proteins only on the outside of each centromere, and the cen D tromeres of the two homologues attach to microtubules fror des. this ment is in marked contrast to the attachment in mitosis, when kinetochores on both sides of a centromere bind to microtubules Each joined pair of homologues then lines up on the metaphase plate. The orientation of each pair on the spin lle axis is random: either the maternal or the paternal ho- FIGURE 12.12 mologue may orient toward a given pole(figure 12.12). Random orientation of chromosomes on the metaphase ure 12.13 illustrates the alignment of chromosomes dur plate. The number of possible chromosome orientations equals ing metaphase I. 2 raised to the power of the number of chromosome pairs. In this hypothetical cell with three chromosome pairs, eight(2 Chiasmata play an important role in aligning the possible orientations exist, four of them illustrated here. Each chromosomes on the metaphase plate orientation produces gametes with different combinations of 232 Part IV Reproduction and Heredity

Metaphase I By metaphase I, the second stage of meiosis I, the nuclear envelope has dispersed and the microtubules form a spin￾dle, just as in mitosis. During diakinesis of prophase I, the chiasmata move down the paired chromosomes from their original points of crossing over, eventually reaching the ends of the chromosomes. At this point, they are called terminal chiasmata. Terminal chiasmata hold the homologous chromosomes together in metaphase I, so that only one side of each centromere faces outward from the complex; the other side is turned inward toward the other homologue (figure 12.11). Consequently, spindle microtubules are able to attach to kinetochore proteins only on the outside of each centromere, and the cen￾tromeres of the two homologues attach to microtubules originating from opposite poles. This one-sided attach￾ment is in marked contrast to the attachment in mitosis, when kinetochores on both sides of a centromere bind to microtubules. Each joined pair of homologues then lines up on the metaphase plate. The orientation of each pair on the spin￾dle axis is random: either the maternal or the paternal ho￾mologue may orient toward a given pole (figure 12.12). Figure 12.13 illustrates the alignment of chromosomes dur￾ing metaphase I. Chiasmata play an important role in aligning the chromosomes on the metaphase plate. 232 Part IV Reproduction and Heredity Metaphase I Anaphase I Meiosis I Chiasmata Mitosis Metaphase Anaphase Kinetochores of sister chromatids remain separate; microtubules attach to both kinetochores on opposite sides of the centromere. Microtubules pull sister chromatids apart. Chiasmata hold homologues together. The kinetochores of sister chromatids fuse and function as one. Microtubules can attach to only one side of each centromere. Microtubules pull the homologous chromosomes apart, but sister chromatids are held together. FIGURE 12.11 Chiasmata created by crossing over have a key impact on how chromosomes align in metaphase I. In the first meiotic division, the chiasmata hold one sister chromatid to the other sister chromatid; consequently, the spindle microtubules can bind to only one side of each centromere, and the homologous chromosomes are drawn to opposite poles. In mitosis, microtubules attach to both sides of each centromere; when the microtubules shorten, the sister chromatids are split and drawn to opposite poles. FIGURE 12.12 Random orientation of chromosomes on the metaphase plate. The number of possible chromosome orientations equals 2 raised to the power of the number of chromosome pairs. In this hypothetical cell with three chromosome pairs, eight (23) possible orientations exist, four of them illustrated here. Each orientation produces gametes with different combinations of parental chromosomes

FIGURE 12.13 The stages of meiosis in a ly. Note the arrangement 改s .'v 小 phase Metaphase Meiosis I l业 Anaphase interphase 小个 L Telophase 歌 Chapter 12 Sexual Reproduction and Meiosis 233

Chapter 12 Sexual Reproduction and Meiosis 233 Prophase II Anaphase II Metaphase II Interphase Prophase I Meiosis I Meiosis II Metaphase I Anaphase I Telophase I Telophase II FIGURE 12.13 The stages of meiosis in a lily. Note the arrangement of chromosomes in metaphase I

Completing Meiosis After the long duration of prophase and metaphase, which together make up 90% or more of the time meiosis I takes meiosis I rapidly concludes. Anaphase I and telophase proceed quickly, followed-without an intervening period of DNA synthesis-by the second meiotic div Anaphase I In anaphase I, the microtubules of the spindle fibers begin to shorten. As they shorten, they break the chias mata and pull the centromeres toward the poles, drag- ing the chromosomes along with them. Because the mi crotubules are attached to kinetochores on only one side of each centromere. the individual centromeres are not pulled apart to form two daughter centromeres, as they ire in mitosis. Instead the entire centromere moves to one pole, taking both sister chromatids with it. When the spindle fibers have fully contracted, each pole has a com- plete haploid set of chromosomes consisting of one mem ber of each homologous pair. Because of the random ori entation of homologous chromosomes on the metaphase plate, a pole may receive either the maternal or the pater nal homologue from each chromosome pair. As a result, FIGURE 12 14 the genes on different chromosomes assort indepen- After meiosis I, sister chromatids are not identical. So-called dently; that is, meiosis I results in the independent harlequin"chromosomes, each containing one fluorescent dNA strand, illustrate the reciprocal exchange of genetic materia sortment of maternal and paternal chromosomes into during meiosis I between sister chromatids e Telophase I By the beginning of telophase I, the chromosomes hav Anaphase Il. The spindle fibers contract, splitting the segregated into two clusters, one at each pole of the cell centromeres and moving the sister chromatids to oppo- Now the nuclear membrane re-forms around each daugh site poles. ter nucleus. Because each chromosome within a daughter Telophase IL. Finally, the nuclear envelope re-forms nucleus replicated before meiosis I began, each now con- around the four sets of daughter chromosomes tains two sister chromatids attached by a common cen The final result of this division is four cells containin tromere. Importantly, the sister chromatids are no longer iden haploid sets of chromosomes(figure 12. 15). No two are tical, because of the crossing over that occurred in prophase alike, because of the crossing over in prophase I Nuclear I(figure 12. 14). Cytokinesis may or may not occur after envelopes then form around each haploid set of chromo telophase I. The second meiotic division, meiosis II,occurs somes. The cells that contain these haploid nuclei may de- after an interval of variable length velop directly into gametes, as they do in animals. Alterna- tively, they may themselves divide mitotically, as they do in The second meiotic division plants, fungi, and many protists, eventually producing greater numbers of gametes or, as in the case of some After a typically brief interphase, in which no DNA synthe- plants and insects, adult individuals of varying ploidy is occurs, the second meiotic division begins Meiosis II resembles a normal mitotic division. Prophase During meiosis I, homologous chromosomes move Il, metaphase Il, anaphase Il, and telophase II follow in toward or in anaphase I, and individual chromosomes cluster at the two poles in telophase I.At the end of meiosis IL, each of the four haploid cells Prophase Il. At the two poles of the cell the clusters contains one copy of every chromosome in the set, of chromosomes enter a brief prophase Il, each nuclear rather than two. Because of crossing over no two cells envelope breaking down as a new spindle forms are the same. These haploid cells may develop directly Metaphase IL. In metaphase Il, spindle fibers bind to into gametes, as in animals, or they may divide by both sides of the centromeres mitosis, as in plants, fungi, and many protists 234 Part IV Reproduction and Heredity

Completing Meiosis After the long duration of prophase and metaphase, which together make up 90% or more of the time meiosis I takes, meiosis I rapidly concludes. Anaphase I and telophase I proceed quickly, followed—without an intervening period of DNA synthesis—by the second meiotic division. Anaphase I In anaphase I, the microtubules of the spindle fibers begin to shorten. As they shorten, they break the chias￾mata and pull the centromeres toward the poles, drag￾ging the chromosomes along with them. Because the mi￾crotubules are attached to kinetochores on only one side of each centromere, the individual centromeres are not pulled apart to form two daughter centromeres, as they are in mitosis. Instead, the entire centromere moves to one pole, taking both sister chromatids with it. When the spindle fibers have fully contracted, each pole has a com￾plete haploid set of chromosomes consisting of one mem￾ber of each homologous pair. Because of the random ori￾entation of homologous chromosomes on the metaphase plate, a pole may receive either the maternal or the pater￾nal homologue from each chromosome pair. As a result, the genes on different chromosomes assort indepen￾dently; that is, meiosis I results in the independent as￾sortment of maternal and paternal chromosomes into the gametes. Telophase I By the beginning of telophase I, the chromosomes have segregated into two clusters, one at each pole of the cell. Now the nuclear membrane re-forms around each daugh￾ter nucleus. Because each chromosome within a daughter nucleus replicated before meiosis I began, each now con￾tains two sister chromatids attached by a common cen￾tromere. Importantly, the sister chromatids are no longer iden￾tical, because of the crossing over that occurred in prophase I (figure 12.14). Cytokinesis may or may not occur after telophase I. The second meiotic division, meiosis II, occurs after an interval of variable length. The Second Meiotic Division After a typically brief interphase, in which no DNA synthe￾sis occurs, the second meiotic division begins. Meiosis II resembles a normal mitotic division. Prophase II, metaphase II, anaphase II, and telophase II follow in quick succession. Prophase II. At the two poles of the cell the clusters of chromosomes enter a brief prophase II, each nuclear envelope breaking down as a new spindle forms. Metaphase II. In metaphase II, spindle fibers bind to both sides of the centromeres. Anaphase II. The spindle fibers contract, splitting the centromeres and moving the sister chromatids to oppo￾site poles. Telophase II. Finally, the nuclear envelope re-forms around the four sets of daughter chromosomes. The final result of this division is four cells containing haploid sets of chromosomes (figure 12.15). No two are alike, because of the crossing over in prophase I. Nuclear envelopes then form around each haploid set of chromo￾somes. The cells that contain these haploid nuclei may de￾velop directly into gametes, as they do in animals. Alterna￾tively, they may themselves divide mitotically, as they do in plants, fungi, and many protists, eventually producing greater numbers of gametes or, as in the case of some plants and insects, adult individuals of varying ploidy. During meiosis I, homologous chromosomes move toward opposite poles in anaphase I, and individual chromosomes cluster at the two poles in telophase I. At the end of meiosis II, each of the four haploid cells contains one copy of every chromosome in the set, rather than two. Because of crossing over, no two cells are the same. These haploid cells may develop directly into gametes, as in animals, or they may divide by mitosis, as in plants, fungi, and many protists. 234 Part IV Reproduction and Heredity FIGURE 12.14 After meiosis I, sister chromatids are not identical. So-called “harlequin” chromosomes, each containing one fluorescent DNA strand, illustrate the reciprocal exchange of genetic material during meiosis I between sister chromatids