Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 35
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These stages are generally more complex than theircounterparts in mitosis. The stagesFigure 3.6The life cycle of corn, Zea mays. As is typical in higher plants, the diploid spore-producing (sporophyte)generation is conspicuous, whereas the gamete-producing (gametophyte) generation is microscopic.
Theegg-producing spore is the megaspore, and the sperm-producing spore is the microspore. Nuclei participatingin meiosis and fertilization are shown in yellow and green.Page 90Page 91Figure 3.7Diagram illustrating the major features of meiosis in an organism with two pairs of homologous chromosomes. At each stage, thesmall diagram represents the entire cell and the larger diagram is an expandedview of the chromosomes at that stage.Page 92Figure 3.8Substages of prophase of the first meiotic division in microsporocytes of a lily (Lilium longiflorum): (A) leptotene,in which condensation of the chromosomes is initiated and bead-like chromosomes are visible along thelength of the chromosomes; (B) zygotene, in which pairing (synopsis) of homologous chromosomesoccurs (paired and unpaired regions can be seen particularly at the lower left in this photograph);(C) pachytene, in which crossing-over between homologous chromosomes occurs; (D) diplotene,characterized by mutual repulsion of the paired homologous chromosomes, which remain held togetherat one or more cross points (chiasmata) along their length; (E) diakinesis, in which the chromosomesreach their maximum contraction; (F) zygotene (at higher magnification in another cell) showing pairedhomologs and matching of chromomeres during synapsis.[Courtesy of Marta Walters (parts A, B, C, E, and F) and Herbert Stern (part D).]and substages can be visualized with reference to Figures 3.7 and 3.8.1.
Prophase I This long stage lasts several days in most higher organisms and is commonly divided into fivesubstages: leptotene, zygotene, pachytene, diplotene, and diakinesis. These terms describe the appearance of thechromosomes at each substage.In leptotene, which literally means ''thin thread," the chromosomes first become visible as long, thread-likestructures. The pairs of sister chromatids can be distinguished by electron microscopy. In this initial phase ofcondensation of the chromosomes, numerous dense granules appear at irregular intervals along their length.
Theselocalized contractions, called chromomeres, have a characteristic number, size, and position in a givenchromosome (Figure 3.8A).The zygotene period is marked by the lateral pairing, or synapsis, of homologous chromosomes, beginning at thechromosome tips. (The term zygotene means "paired threads.") As the pairing process proceeds along the length ofthe chromosomes, it results in a precise chromomere-by-chromomere association (Figure 3.8B and F). Each pair ofsynapsed homologous chromosomes is referred to as a bivalent.During pachytene (Figure 3.8C), condensation of the chromosomes continues.Page 93Pachytene literally means "thick thread" and, throughout this period, the chromosomes continue to shorten andthicken (Figure 3.7). By late pachytene, it can sometimes be seen that each bivalent (that is, each set of pairedchromosomes) actually consists of a tetrad of four chromatids, but the two sister chromatids of each chromosomeare usually juxtaposed very tightly.
The important event of genetic exchange, which is called crossing-over, takesplace during pachytene, but crossing-over does not become apparent until the transition to diplotene. In Figure 3.7,the sites of exchange are indicated by the points where chromatids of different colors cross over each other.At the onset of diplotene, the synapsed chromosomes begin to separate. Diplotene means "double thread," and thediplotene chromosomes are clearly double (Figure 3.8D and F). However, the homologous chromosomes remainheld together at intervals along their length by cross-connections resulting from crossing-over. Each crossconnection, called a chiasma (plural, chiasmata), is formed by a breakage and rejoining between nonsisterchromatids. As shown in the chromosome and diagram in Figure 3.9, a chiasma results from physical exchangebetween chromatids of homologous chromosomes.
In normal meiosis, each bivalent usually has at least onechiasma, and bivalents of long chromosomes often have three or more.The final period of prophase I is diakinesis, in which the homologous chromosomes seem to repel each other andthe segments not connected by chiasmata move apart. Diakinesis means "moving apart." It is at this substage thatthe chromosomes attain their maximum condensation (Figure 3.8E). The homologous chromosomes in a bivalentremain connected by at least one chiasma, which persists until the first meiotic anaphase. Near the end ofdiakinesis, the formation of a spindle is initiated, and the nuclear envelope breaks down.2.
Metaphase I The bivalents become positioned with the centromeres of the two homologous chromosomes onopposite sides of the metaphase plate (Figure 3.10A). As each bivalent moves onto the metaphase plate, itscentromeres are oriented at random with respect to the poles of the spindle. As shown in Figure 3.11, the bivalentsformed from nonhomologous pairs of chromosomes can be oriented on the metaphase plate in either of two ways.The orientation of the centromeres determines which member of each bivalent will subsequently move to eachpole. If each of the nonhomologous chromosomes is heterozygous for a pair of alleles, then one type of alignmentresults in AB and ab gametes and the other type results in Ab and aB gametes (Figure 3.11). Because the metaphasealignment takes place at random, the two types of alignment—andFigure 3.9Light micrograph (A) and interpretative drawing (B) of a bivalent consisting of a pair of homologous chromosomes.This bivalent was photographed at late diplotene in a spermatocyte of the salamander Oedipina poelzi.It shows two chiasmata where the chromatids of the homologous chromosomes appear to exchangepairing partners.[From F.
W. Stahl. 1964. The Mechanics of Inheritance. Prentice-Hall, Inc.; courtesy of James Kezer.]Page 94Figure 3.10Later meiotic stages in microsporocytes of the lily Lilium longiflorum: (A) metaphase I; (B) anaphase I; (C)metaphase II; (D) anaphase II; (E) telophase II. Cell walls have begun to form in telophase, which will lead tothe formation of four pollen grains.[Courtesy of Herbert Stern.]therefore the four types of gametes—are equally frequent. The ratio of the four types of gametes is 1:1:1:1, whichmeans that the A, a and B, b pairs of alleles undergo independent assortment. In other words,Genes on different chromosomes undergo independent assortment because nonhomologous chromosomesalign at random on the metaphase plate in meiosis I.3.
Anaphase I In this stage, homologous chromosomes, each composed of two chromatids joined at an undividedcentromere, separate from one another and move to opposite poles of the spindle (Figure 3. 10B). Chromosomeseparation at anaphase is the cellular basis of the segregation of alleles:The physical separation of homologous chromosomes in anaphase is the physical basis of Mendel'sprinciple of segregation.4.
Telophase I At the completion of anaphase I, a haploid set of chromosomes consisting of one homolog fromeach bivalent is located near each pole of the spindle (Figure 3.6). In telophase, the spindle breaks down and,depending on the species, either a nuclear envelope briefly forms around each group of chromosomes or thechromosomes enter the second meiotic division after only a limited uncoiling.The Second Meiotic Division: EquationThe second meiotic division (meiosis II) is sometimes called the equational division because the chromosomenumber remains the same in each cell before and after the second division.
In some species, the chromosomes passdirectly from telophase I to prophase II without loss of condensation; in others, there is a brief pause between thetwo meiotic divisions and the chromo-Page 95Figure 3.11Random alignment of nonhomologous chromosomes at metaphase I results in theindependent assortment of genes on nonhomologous chromosomes.somes may "decondense" (uncoil) somewhat. Chromosome replication never takes place between the twodivisions; the chromosomes present at the beginning of the second division are identical to those present at the endof the first division.After a short prophase (prophase II) and the formation of second-division spindles, the centromeres of thechromosomes in each nucleus become aligned on the central plane of the spindle at metaphase II (Figure 3.10C).In anaphase II, the centromeres divide longitudinally and the chromatids of each chromosome move to oppositepoles of the spindle (Figure 3.10D).
Once the centromere has split at anaphase II, each chromatid is considered tobe a separate chromosome.Telophase II (Figure 3.10E) is marked by a transition to the interphase condition of the chromosomes in the fourhaploid nuclei, accompanied by division of the cytoplasm.
Thus the second meiotic division superficiallyresembles a mitotic division. However, there is an important difference: The chromatids of a chromosome areusually not genetically identical sisters along their entire length because of crossing-over associated with theformation of chiasmata during prophase of the first division.Page 96Connection Grasshopper, GrasshopperE.
Eleanor Carothers 1913University of Kansas,Lawrence, KansasThe Mendelian Ratio in Relation to Certain Orthopteran ChromosomesAs an undergraduate researcher, Carothers showed that nonhomologous chromosomes undergoindependent assortment in meiosis. For this purpose she studied a grasshopper in which one pair ofhomologous chromosomes had members of unequal length. At the first anaphase of meiosis in males, shecould determine by observation whether the longer or the shorter chromosome went in the same directionas the X chromosome. As detailed in this paper, she found 154 of the former and 146 of the latter, a resultin very close agreement with the 1:1 ratio expected from independent assortment.
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