12 Sexual reproduction and meiosis Concept outline 12.1 Meiosis produces haploid cells from diploid cells. Discovery of Reduction Division. Sexual reproduction does not increase chromosome number because gamete production by meiosis involves a decrease in chromosome number. Individuals produced from sexual reproduction inherit chromosomes from two parents 12.2 Meiosis has three unique features Unique Features of Meiosis. Three unique features of meiosis are synapsis, homologous recombination, and reduction division 12.3 The sequence of events during meiosis involves two nuclear divisions Prophase I. Homologous chromosomes pair intimately, and undergo crossing over that locks them together Metaphase I. Spindle microtubules align the chromosomes in the central plane of the cell. Completing Meiosis. The second meiotic division is like FIGURE 12.1 a mitotic division, but has a very different outcome. Plant cells undergoing meiosis(600x). This preparation of Why Sex? Sex may have evolved as a mechanism to repair clots and then fracturing them. It shows several stagesor go ollen cells of a spiderwort, Tradescantia, was made by freezin 12.4 The evolutionary origin of sex is a puzzle. the cells DNA, or perhaps as a means for contagious elements to spread. Sexual reproduction increases genetic variability by huffing combinations of mor ost animals and plants reproduce sexually. Gametes of opposite sex unite to form a cell that, dividing re- peatedly by mitosis, eventually gives rise to an adult body with some 100 trillion cells. The gametes that give rise to the initial cell are the products of a special form of cell divi sion called meiosis(figure 12. 1), the subject of this chapter Far more intricate than mitosis. the details of meiosis are not as well understood. The basic process, however, is clear. Also clear are the profound consequences of sexual reproduction: it plays a key role in generating the tremen dous genetic diversity that is the raw material of evolution 225
225 12 Sexual Reproduction and Meiosis Concept Outline 12.1 Meiosis produces haploid cells from diploid cells. Discovery of Reduction Division. Sexual reproduction does not increase chromosome number because gamete production by meiosis involves a decrease in chromosome number. Individuals produced from sexual reproduction inherit chromosomes from two parents. 12.2 Meiosis has three unique features. Unique Features of Meiosis. Three unique features of meiosis are synapsis, homologous recombination, and reduction division. 12.3 The sequence of events during meiosis involves two nuclear divisions. Prophase I. Homologous chromosomes pair intimately, and undergo crossing over that locks them together. Metaphase I. Spindle microtubules align the chromosomes in the central plane of the cell. Completing Meiosis. The second meiotic division is like a mitotic division, but has a very different outcome. 12.4 The evolutionary origin of sex is a puzzle. Why Sex? Sex may have evolved as a mechanism to repair DNA, or perhaps as a means for contagious elements to spread. Sexual reproduction increases genetic variability by shuffling combinations of genes. Most animals and plants reproduce sexually. Gametes of opposite sex unite to form a cell that, dividing repeatedly by mitosis, eventually gives rise to an adult body with some 100 trillion cells. The gametes that give rise to the initial cell are the products of a special form of cell division called meiosis (figure 12.1), the subject of this chapter. Far more intricate than mitosis, the details of meiosis are not as well understood. The basic process, however, is clear. Also clear are the profound consequences of sexual reproduction: it plays a key role in generating the tremendous genetic diversity that is the raw material of evolution. FIGURE 12.1 Plant cells undergoing meiosis (600×). This preparation of pollen cells of a spiderwort, Tradescantia, was made by freezing the cells and then fracturing them. It shows several stages of meiosis
12.1 Meiosis produces haploid cells from diploid cells Discovery of Reduction Division number of chromosomes in each cell would become impos- sibly large. For example, in just 10 generations, the 46 Only a few tpa 1882, Beldgtad ditologist Pien discovery of chromosomes present in human cells would increase to Irs after Walther Fler Beneden was surprised to find different numbers of chro The number of chromosomes does not explode in this mosomes in different types of cells in the roundworm As way because of a special reduction division that occurs caris. Specifically, he observed that the gametes(eggs and durin sperm)each contained two chromosomes, while the so- norm gamete formation, producing cells with half the I number of chromosomes. The subsequent fusion matic(nonreproductive)cells of embryos and mature indi- of two of these cells ensures a consistent chromosome viduals each contained fou number from one generation to the next. This reduction division process, known as meiosis, is the subject of this Fertilization From his observations, van Beneden proposed in 1887 that The Sexual Life Cycle an egg and a sperm, each containing half the complet of chromosomes found in other cells, fuse to produce a sin- Meiosis and fertilization together constitute a cycle of re- gle cell called a zygote. The zygote, like all of the somatic production. Two sets of chromosomes are present in the cells ultimately derived from it, contains two copies of each somatic cells of adult individuals, making them diploid chromosome. The fusion of gametes to form a new cell is cells( Greek diploos, "double"+ eidos, "form"), but only one called fertilization, or syngamy set is present in the gametes, which are thus haploid ( Greek baploo,“ single”+ photon,“ vessel”). Reproduction Reduction Division that involves this alternation of meiosis and fertilization is called sexual reproduction. Its outstanding characteristic It was clear even to early investigators that gamete forma that offspring inherit chromosomes from two parents tion must involve some mechanism that reduces the num -(figure 12. 2). You, for example, inherited 23 chromosomes ber of chromosomes to half the number found in other from your mother, contributed by the egg fertilized at your ells, If it did not the chromosome number would double conception, and 23 from your father, contributed by the with each fertilization, and after only a few generations, the sperm that fertilized that egg. FIGURE 12.2 Diploid cells carry chromosomes from two Diploid zygote parents. a diploid cell contains two versions o each chromosome, one contributed by the haploid egg of the mother, the other by the haploid sperm 226 Part IV Reproduction and Heredity
number of chromosomes in each cell would become impossibly large. For example, in just 10 generations, the 46 chromosomes present in human cells would increase to over 47,000 (46 × 210). The number of chromosomes does not explode in this way because of a special reduction division that occurs during gamete formation, producing cells with half the normal number of chromosomes. The subsequent fusion of two of these cells ensures a consistent chromosome number from one generation to the next. This reduction division process, known as meiosis, is the subject of this chapter. The Sexual Life Cycle Meiosis and fertilization together constitute a cycle of reproduction. Two sets of chromosomes are present in the somatic cells of adult individuals, making them diploid cells (Greek diploos, “double” + eidos, “form”), but only one set is present in the gametes, which are thus haploid (Greek haploos, “single” + ploion, “vessel”). Reproduction that involves this alternation of meiosis and fertilization is called sexual reproduction. Its outstanding characteristic is that offspring inherit chromosomes from two parents (figure 12.2). You, for example, inherited 23 chromosomes from your mother, contributed by the egg fertilized at your conception, and 23 from your father, contributed by the sperm that fertilized that egg. 226 Part IV Reproduction and Heredity Discovery of Reduction Division Only a few years after Walther Fleming’s discovery of chromosomes in 1882, Belgian cytologist Pierre-Joseph van Beneden was surprised to find different numbers of chromosomes in different types of cells in the roundworm Ascaris. Specifically, he observed that the gametes (eggs and sperm) each contained two chromosomes, while the somatic (nonreproductive) cells of embryos and mature individuals each contained four. Fertilization From his observations, van Beneden proposed in 1887 that an egg and a sperm, each containing half the complement of chromosomes found in other cells, fuse to produce a single cell called a zygote. The zygote, like all of the somatic cells ultimately derived from it, contains two copies of each chromosome. The fusion of gametes to form a new cell is called fertilization, or syngamy. Reduction Division It was clear even to early investigators that gamete formation must involve some mechanism that reduces the number of chromosomes to half the number found in other cells. If it did not, the chromosome number would double with each fertilization, and after only a few generations, the 12.1 Meiosis produces haploid cells from diploid cells. Haploid egg Diploid zygote Haploid sperm FIGURE 12.2 Diploid cells carry chromosomes from two parents. A diploid cell contains two versions of each chromosome, one contributed by the haploid egg of the mother, the other by the haploid sperm of the father
Haploid(n) multicellular organism Mitosis Sperm(n)Egg(n) (n)cells Meiosis Fertilization Diploid (2n) ploid (2n) zygote FIGURE 12.3 Germ cell Alternation of generations. In sexual reproduction multicellular organism haploid cells or organisms alternate with diploid cells or organisms. Somatic Tissues. The life cycles of all sexually reproduc O Diploid(2n) ing organisms follow the same basic pattern of alternation O Haploid(n) between the diploid and haploid chromosome numbers (figures 12.3 and 12.4). After fertilization, the resulting zy- gote begins to divide by mitosis. This single diploid cell Grows into eventually gives rise to all of the cells in the adult. These adult male or adult female cells are called somatic cells from the Latin word for body. Except when rare accidents occur, or in specia variation-creating situations such as occur in the immune system, every one of the adults somatic cells is genetically identical to the zygote In unicellular eukaryotic organisms, including most pro- tists, individual cells function as gametes, fusing with other Meiosis gamete cells. The zygote may undergo mitosis, or it may divide immediately by meiosis to give rise to haploid indi- viduals. In plants, the haploid cells that meiosis produces divide by mitosis, forming a multicellular haploid phase. Certain cells of this haploid phase eventually differentiate Into eggs or sperm Fertilization Germ-Line Tissues. In animals. the cells that will eventu-(haploid)n ally undergo meiosis to produce gametes are set aside from somatic cells early in the course of development. These cells are often referred to as germ-line cells. Both the somatic cells and the gamete-producing germ-line cells are diploid, but while somatic cells undergo mitosis to form genetically (diploid)2n identical, diploid daughter cells, gamete-producing germ FIGURE 12. 4 line cells undergo meiosis, producing haploid gametes The sexual life cycle. In animals, the completion of meiosis is followed soon by fertilization. Thus, the vast majority of the life Meiosis is a process of cell division in which the number cycle is spent in the diploid stage of chromosomes in certain cells is halved during gamete formation In the sexual life cycle, there is an alternation of diploid and haploid generations. Chapter 12 Sexual Reproduction and Meiosis 22
Somatic Tissues. The life cycles of all sexually reproducing organisms follow the same basic pattern of alternation between the diploid and haploid chromosome numbers (figures 12.3 and 12.4). After fertilization, the resulting zygote begins to divide by mitosis. This single diploid cell eventually gives rise to all of the cells in the adult. These cells are called somatic cells, from the Latin word for “body.” Except when rare accidents occur, or in special variation-creating situations such as occur in the immune system, every one of the adult’s somatic cells is genetically identical to the zygote. In unicellular eukaryotic organisms, including most protists, individual cells function as gametes, fusing with other gamete cells. The zygote may undergo mitosis, or it may divide immediately by meiosis to give rise to haploid individuals. In plants, the haploid cells that meiosis produces divide by mitosis, forming a multicellular haploid phase. Certain cells of this haploid phase eventually differentiate into eggs or sperm. Germ-Line Tissues. In animals, the cells that will eventually undergo meiosis to produce gametes are set aside from somatic cells early in the course of development. These cells are often referred to as germ-line cells. Both the somatic cells and the gamete-producing germ-line cells are diploid, but while somatic cells undergo mitosis to form genetically identical, diploid daughter cells, gamete-producing germline cells undergo meiosis, producing haploid gametes. Meiosis is a process of cell division in which the number of chromosomes in certain cells is halved during gamete formation. In the sexual life cycle, there is an alternation of diploid and haploid generations. Chapter 12 Sexual Reproduction and Meiosis 227 Haploid (n) Gametes Sperm (n) Egg (n) Diploid (2n) Diploid (2n) multicellular organism Diploid (2n) zygote Diploid (2n) germ-line cells Meiosis Mitosis Gamete formation Germ cell formation Mitosis Haploid (n) cells Haploid (n) multicellular organism Fertilization FIGURE 12.3 Alternation of generations. In sexual reproduction, haploid cells or organisms alternate with diploid cells or organisms. Male (diploid) 2n Meiosis Grows into adult male or adult female Sperm (haploid) n Diploid (2n) Zygote (diploid) 2n Fertilization Female (diploid) 2n Meiosis Haploid (n) Egg (haploid) n FIGURE 12.4 The sexual life cycle. In animals, the completion of meiosis is followed soon by fertilization. Thus, the vast majority of the life cycle is spent in the diploid stage
2.2 Meiosis has three unique features Unique Features of meiosis SYNAPSIS The mechanism of cell division varies in important details different organisms. This is particularly true of chromo- somal separation mechanisms, which differ substantially in and fungi from the process in plants and animals will describe here. Meiosis in a diploid organism of two rounds of division, mitosis of one. Although meiosis and mitosis have much in common meiosis has ue features: synapsis, homologous recombina Region of close ssociation where ynapsIs occurs The first unique feature of meiosis happens early durin the first nuclear division. Following chromosome replica tion OIS ChRono mes, or homologues(see chapter 11), pairall along their length. The process of forming these complexes of homologous chromosomes is called synapsis Homologous Recombination The second unique feature of meiosis is that genetic ex occurs between the homologous chromosomes while they(a) Homologue Homologue lus physically joined(figure 12. 5a). The exchange that occurs between paired chromosomes is called rossing over. Chromosomes are then drawn together REDUCTION along the equatorial plane of the dividing cell; subse DIVISION quently, homologues are pulled by microtubules toward opposite poles of the cell. When this process is complete, the cluster of chromosomes at each pole contains one of Chromosome the two homologues of each chromosome. Each pole is duplication from each other in the first nuclear division. so each homologue is still composed of two chromatids Reduction division Meiosis I The third unique feature of meiosis is that the chromosomes do not replicate between the two muclear divisions, so that at the end of meiosis, each cell contains only half the original complement of chromosomes(figure 12.56). In most re- spects, the second meiotic division is identical to a normal Haploid gametes Meiosis il mitotic division. However, because of the crossing over that occurred during the first division the sister chromatids in meiosis ii are not identical to each other Meiosis is a continuous process, but it is most easily stud- ied when we divide it into arbitrary stages. The stages of meiosis are traditionally called meiosis I and meiosis Il. Like FIGURE 12.5 mitosis, each stage is subdivided further into prophase, Unique features of meiosis. (a) Synapsis draws homologous metaphase, anaphase, and telophase(figure 12.6). In meio- sis, however, prophase I is more complex than in mitosis chromosomes can physically exchange parts, a process called crossing over.(b)Reduction division, by omitting a chromosome In meiosis, homologous chromosomes become duplication before meiosis Il, produces haploid gametes, thus intimately associated and do not replicate between the ensuring that chromosome number remains stable during the two nuclear divisions 228 Part IV Reproduction and Heredity
Unique Features of Meiosis The mechanism of cell division varies in important details in different organisms. This is particularly true of chromosomal separation mechanisms, which differ substantially in protists and fungi from the process in plants and animals that we will describe here. Meiosis in a diploid organism consists of two rounds of division, mitosis of one. Although meiosis and mitosis have much in common, meiosis has three unique features: synapsis, homologous recombination, and reduction division. Synapsis The first unique feature of meiosis happens early during the first nuclear division. Following chromosome replication, homologous chromosomes, or homologues (see chapter 11), pair all along their length. The process of forming these complexes of homologous chromosomes is called synapsis Homologous Recombination The second unique feature of meiosis is that genetic exchange occurs between the homologous chromosomes while they are thus physically joined (figure 12.5a). The exchange process that occurs between paired chromosomes is called crossing over. Chromosomes are then drawn together along the equatorial plane of the dividing cell; subsequently, homologues are pulled by microtubules toward opposite poles of the cell. When this process is complete, the cluster of chromosomes at each pole contains one of the two homologues of each chromosome. Each pole is haploid, containing half the number of chromosomes present in the original diploid cell. Sister chromatids do not separate from each other in the first nuclear division, so each homologue is still composed of two chromatids. Reduction Division The third unique feature of meiosis is that the chromosomes do not replicate between the two nuclear divisions, so that at the end of meiosis, each cell contains only half the original complement of chromosomes (figure 12.5b). In most respects, the second meiotic division is identical to a normal mitotic division. However, because of the crossing over that occurred during the first division, the sister chromatids in meiosis II are not identical to each other. Meiosis is a continuous process, but it is most easily studied when we divide it into arbitrary stages. The stages of meiosis are traditionally called meiosis I and meiosis II. Like mitosis, each stage is subdivided further into prophase, metaphase, anaphase, and telophase (figure 12.6). In meiosis, however, prophase I is more complex than in mitosis. In meiosis, homologous chromosomes become intimately associated and do not replicate between the two nuclear divisions. 228 Part IV Reproduction and Heredity 12.2 Meiosis has three unique features. SYNAPSIS Homologue Homologue Region of close association, where crossing over occurs (a) Centromere Sister chromatids REDUCTION DIVISION Diploid germ-line cell Haploid gametes Chromosome duplication Meiosis I Meiosis II (b) FIGURE 12.5 Unique features of meiosis. (a) Synapsis draws homologous chromosomes together, creating a situation where the two chromosomes can physically exchange parts, a process called crossing over. (b) Reduction division, by omitting a chromosome duplication before meiosis II, produces haploid gametes, thus ensuring that chromosome number remains stable during the reproduction cycle
MEIOSIS MITOSIS Paternal homologue Chromosome Pairing of Synapsis and Cell division FIGURE 12.6 A comparison of meiosis and mitosis. Meiosis involves two nuclear divisions with no DNA replication betwee four daughter cells, each with half the original number of chromosomes. Crossing over occurs in prophase I of i Involves a single nuclear division after DNA replication. It thus produces two daughter cells, each containing the original Chapter 12 Sexual Reproduction and Meiosis 229
Chapter 12 Sexual Reproduction and Meiosis 229 Cell division Cell division Cell division Synapsis and crossing over Pairing of homologous chromosomes Chromosome replication Chromosome replication Paternal homologue Maternal homologue MEIOSIS MITOSIS MEIOSIS I MEIOSIS II FIGURE 12.6 A comparison of meiosis and mitosis. Meiosis involves two nuclear divisions with no DNA replication between them. It thus produces four daughter cells, each with half the original number of chromosomes. Crossing over occurs in prophase I of meiosis. Mitosis involves a single nuclear division after DNA replication. It thus produces two daughter cells, each containing the original number of chromosomes