Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 34
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The beginning of prophase is marked by the condensation ofchromosomes to form visibly distinct, thin threads within the nucleus. Each chromosome is already longitudinallydouble, consisting of two closely associated subunits called chromatids. The longitudinally bipartite nature of eachchromosome is readily seen later in prophase. Each pair of chromatids is the product of the duplication of onechromosome in the S period of interphase. The chromatids in a pair are held together at a specific region of thechromosome called the centromere.
As prophase progresses, the chromosomes become shorter and thicker as aresult of intricate coiling. At the end of prophase, the nucleoli disappear and the nuclear envelope, a membranesurrounding the nucleus, abruptly disintegrates.2. Metaphase At the beginning of metaphase, the mitotic spindle forms. The spindle is a bipolar structureconsisting of fiber-like bundles of microtubules that extend through the cell between the poles of the spindle. Eachchromosome becomes attached to several spindle fibers in the region of the centromere. The structure associatedwith the centromere to which the spindle fibers attach is technically known as the kinetochore. After thechromosomes are attached to spindle fibers, they move toward the center of the cell until all the kinetochores lie onan imaginary plane equidistant from the spindle poles. This imaginary plane is called the metaphase plate.Aligned on the metaphase plate, the chromosomes reach their maximum contraction and are easiest to count andexamine for differences in morphology.Proper chromosome alignment is an important cell cycle control checkpoint at metaphase in both mitosis andmeiosis.
In a cell in which a chromosome is attached to only one pole of the spindle, the completion of metaphaseis delayed. By grasping such a chromosome with a micromanipulation needle and pulling, one can mimic thetension that the chromosome would experience were it attached on both sides; the mechanical tension allows themetaphase checkpoint to be passed, and the cell enters the next stage of division.
The signal for chromosomealignment comes from the kinetochore, and the chemical nature of the signal seems to be the dephosphorylation ofcertain kinetochore-associated proteins. The role of the kinetochore is demonstrated by the finding that metaphaseis not delayed by an unattached chromosome whose kinetochore has been destroyed by a focused laser beam. Therole of dephosphorylation is demonstrated through the use of an antibody that reacts specifically with somekinetochore proteins only when they are phosphorylated.
Unattached kinetochores combine strongly with theantibody, but attachment to the spindle weakens the reaction. In chromosomes that have been surgically detachedfrom the spindle, the antibody reaction with the kinetochore reappears. Through the signaling mechanism, when allof the kinetochores are under tension and aligned on the metaphase plate, the metaphase checkpoint is passed andthe cell continues the process of division.3. Anaphase In anaphase, the centromeres divide longitudinally, and the two sister chromatids of eachchromosome move toward opposite poles of the spindle. Once the centromeres divide, each sister chromatid isregarded as a separate chromosome in its own right. Chromosome movement results in part from progressiveshortening of the spindle fibers attached to the centromeres, which pulls the chromosomes in opposite directionstoward the poles.
At the completion of anaphase, the chromosomes lie in two groups near opposite poles of thespindle. Each group contains the same number of chromosomes that was present in the original interphase nucleus.Page 874. Telophase In telophase, a nuclear envelope forms around each compact group of chromosomes, nucleoli areformed, and the spindle disappears. The chromosomes undergo a reversal of condensation until they are no longervisible as discrete entities.
The two daughter nuclei slowly assume a typical interphase appearance as the cytoplasmof the cell divides into two by means of a gradually deepening furrow around the periphery. (In plants, a new cellwall is synthesized between the daughter cells and separates them.)3.3—MeiosisMeiosis is a mode of cell division in which cells are created that contain only one member of each pair ofchromosomes present in the premeiotic cell. When a diploid cell with two sets of chromosomes undergoes meiosis,the result is four daughter cells, each genetically different and each containing one haploid set of chromosomes.Meiosis consists of two successive nuclear divisions.
The essentials of chromosome behavior during meiosis areoutlined in Figure 3.4. This outline affords an overview of meiosis as well as an introduction toFigure 3.4Overview of the behavior of a single pair of homologous chromosomes inmeiosis. (A) The homologous chromosomes form a pair by coming together;each chromosome consists of two chromatids joined at a single centromere.(B) The members of each homologous pair separate. (C) At the end ofthe first meiotic division, each daughter nucleus carries one or the other ofthe homologous chromosomes.
(D) In the second meiotic division, in each of thedaughter nuclei formed in meiosis I, the sister chromatids separate. (E) Theend result is four products of meiosis, each containing one of each pair ofhomologous chromosomes. For clarity, this diagram does not incorporatecrossing-over, an interchange of chromosome segments that takes place atthe stage depicted in part A. If crossing-over were included, each chromatidwould consist of one or more segments of red and one or more segments ofblue. (Crossing-over is depicted in Figure 3.7.)Page 88the process as it takes place in a cellular context.1.
Prior to the first nuclear division, the members of each pair of chromosomes become closely associated alongtheir length (Figure 3.4). The chromosomes that pair with each other are said to be homologous chromosomes.Because each member of a pair of homologous chromosomes is already replicated, each member consists of twosister chromatids joined at the centromere.
The pairing of the homologous chromosomes therefore produces a fourstranded structure.2. In the first nuclear division, the homologous chromosomes are separated from each another, one member of eachpair going to opposite poles of the spindle (Figure 3.4B). Two nuclei are formed, each containing a haploid set ofduplex chromosomes (Figure 3.4C) with two chromatids.3.
The second nuclear division loosely resembles a mitotic division, but there is no chromosome replication. Atmetaphase, the chromosomes align on the metaphase plate; and at anaphase, the chromatids of each chromosomeare separated into opposite daughter nuclei (Figure 3.4D).
The net effect of the two divisions in meiosis is thecreation of four haploid daughter nuclei, each containing the equivalent of a single sister chromatid from each pairof homologous chromosomes (Figure 3.4E).Figure 3.4 does not show that at the time of chromosome pairing, the homologous chromosomes can exchangegenes. The exchanges result in the formation of chromosomes that consist of segments from one homologouschromosome intermixed with segments from the other. In Figure 3.4, the exchanged chromosomes would bedepicted as segments of alternating color. The exchange process is one of the critical features of meiosis, and it willbe examined in the next section.In animals, meiosis takes place in specific cells called meiocytes, a general term for the primary oocytes andspermatocytes in the gamete-forming tissues (Figure 3.5).
The oocytes form egg cells, and the spermatocytes formsperm cells. Although the process of meiosis is similar in all sexually reproducing organisms, in the female of bothanimals and plants, only one of the four products develops into a functional cell (the other three disintegrate). Inanimals, the products of meiosis form gametes (sperm or eggs).Figure 3.5The life cycle of a typical animal. The number n is the number of chromosomes in the haploidchromosome complement. In males, the four products of meiosis develop into functional sperm;in females, only one of the four products develops into an egg.Page 89In plants, the situation is slightly more complicated:1. The products of meiosis typically form spores, which undergo one or more mitotic divisions to produce ahaploid gametophyte organism.
The gametophyte produces gametes by mitotic division of a haploid nucleus(Figure 3.6).2. Fusion of haploid gametes creates a diploid zygote that develops into the sporophyte plant, which undergoesmeiosis to produce spores and so restarts the cycle.Meiosis is a more complex and considerably longer process than mitosis and usually requires days or even weeks.The entire process of meiosis is illustrated in its cellular context in Figure 3.7. The essence is that meiosis consistsof two divisions of the nucleus but only one duplication of the chromosomes.
The nuclear divisions—called thefirst meiotic division and the second meiotic division—can be separated into a sequence of stages similar to thoseused to describe mitosis. The distinctive events of this important process occur during the first division of thenucleus; these events are described in the following section.The First Meiotic Division: ReductionThe first meiotic division (meiosis I) is sometimes called the reductional division because it divides thechromosome number in half. By analogy with mitosis, the first meiotic division can be split into the four stages ofprophase I, metaphase I, anaphase I, and telophase I.
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