Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 33
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The importance of the nucleus in inheritance wasreinforced by the nearly simultaneous discovery that the nuclei of two gametes fuse in the process of fertilization.The next major advance came a decade later with the discovery of chromosomes, which had been made visible bylight microscopy when stained with basic dyes. A few years later, chromosomes were found to segregate by anorderly process into the daughter cells formed by cell division as well as into the gametes formed by the division ofreproductive cells.
Finally, three important regularities were observed about the chromosome complement (thecomplete set of chromosomes) of plants and animals.1. The nucleus of each somatic cell (a cell of the body, in contrast with a germ cell, or gamete) contains a fixednumber of chromosomes typical of the particular species. However, the numbers vary tremendously among speciesand bear little relation to the complexity of the organism (Table 3.1).2. The chromosomes in the nuclei of somatic cells are usually present in pairs.
For example, the 46 chromosomesof human beings consist of 23 pairs (Figure 3.1). Similarly, the 14 chromosomes of peas consist of 7 pairs. Cellswith nuclei of this sort, containing two similar sets of chromosomes, are called diploid. The chromosomes arepresent in pairs because one chromosome of each pair derives from the maternal parent and the other from thepaternal parent of the organism.Table 3.1 Somatic chromosome numbers of some plant and animal speciesOrganismChromosomenumberOrganismChromosomenumberField hosetail216Yeast (Saccharomyces cerevisiae)32Bracken fern116Fruit fly (Drosophilia melanogaster)Giant sequoia22Nematode (Caenorhabditis elegans)Macaroni wheat28House fly12Bread wheat42Scorpion4Fava bean12Geometrid mothGarden pea14Common toad22Wall cress (Arabidopsis thaliana)10Chicken78Corn (Zea mays)20Mouse40Lily12Gibbon44Snapdragon16Human being468224Page 833. The germ cells, or gametes, that unite in fertilization to produce the diploid state of somatic cells have nuclei thatcontain only one set of chromosomes, consisting of one member of each of the pairs.
The gamete nuclei arehaploid.In multicellular organisms that develop from single cells, the presence of the diploid chromosome number insomatic cells and the haploid chromosome number in germ cells indicates that there are two different processes ofnuclear division. One of these, mitosis, maintains the chromosome number; the other, meiosis, halves the number.These two processes are examined in the following sections.3.2—MitosisMitosis is a precise process of nuclear division that ensures that each of two daughter cells receives a diploidcomplement of chromosomes identical with the diploid complement of the parent cell.
Mitosis is usuallyaccompanied by cytokinesis, the process in which the cell itself divides to yield two daughter cells. The essentialdetails of mitosis are the same in all organisms, and the basic process is remarkably uniform:1. Each chromosome is already present as a duplicated structure at the beginning of nuclear division. (Theduplication of each chromosome coincides with the replication of the DNA molecule contained within it.)2.
Each chromosome divides longitudinally into identical halves that become separated from each other.3. The separated chromosome halves move in opposite directions, and each becomes included in one of the twodaughter nuclei that are formed.In a cell not undergoing mitosis, the chromosomes are not visible with a light microscope.
This stage of the cellcycle is called interphase. In preparation for mitosis, the genetic material (DNA) in the chromosomes is replicatedduring a period of interphase called S (Figure 3.2). (The S stands for synthesis of DNA.) DNA replication isaccompanied by chromosome dupli-Figure 3.1Chromosome complement of a human male. There are 46 chromosomes,present in 23 pairs. At the stage of the division cycle in which thesechromosomeswere observed, each chromosome consists of twoidentical halves lying side by sidelongitudinally. Except for themembers of one chromosome pair (the pair thatdetermines sex), themembers of each of the other chromosome pairs are thesame color because they contain DNA molecules that werelabeled with the same mixture of fluorescent dyes. The colorsdiffer from one pair to the next becausethe dye mixtures for each chromosome differ in color.
Insome cases, the long and the short arms have been labeledwith different colors.[Courtesy of David C. Ward and Michael R. Speicher.]cation. Before and after S, there are periods, called G1 and G2, respectively, in which DNA replication does nottake place. The cell cycle, or the life cycle of a cell, is commonly described in terms of these three interphaseperiods followed by mitosis, M. The order of events is therefore G1 SG2M, as shown in Figure 3.2. In thisrepresentation, cytokinesis, the division of the cytoplasm into two approximately equal parts containing thedaughter nuclei, is included in the M period. The length of time required for a complete life cycle varies with celltype.
In higher eukaryotes, the majority of cells require from 18 to 24 hours. The relative duration of the differentperiods in the cycle also varies considerably with cell type. Mitosis, requiring from 1/2 hour to 2 hours, is usuallythe shortest period.Page 84Figure 3.2The cell cycle of a typical mammalian cell growing in tissue culture with a generation time of 24 hours.The critical control points for the G1S and G2M transitions are governed by a p34 kinase that is activatedby stage-specific cyclins and that regulates the activity of its target proteins through phosphorylation.The cell cycle itself is under genetic control. The mechanisms of control appear to be essentially identical in alleukaryotes.
There are two critical transitions—from G1 into S and from G2 into M (Figure 3.2). The G1/S and G2/Mtransitions are called "checkpoints" because the transitions are delayed unless key processes have been completed.For example, at the G1/S checkpoint, either sufficient time must have elapsed since the preceding mitosis (in somecell types) or the cell must have attained sufficient size (in other cell types) for DNA replication to be initiated.Similarly, the G2/M checkpoint requires that DNA replication and repair of any DNA damage be completed for theM phase to commence. Both major control points are regulated in a similar manner and make use of a specializedprotein kinase (called the p34 kinase subunit in Figure 3.2) that regulates the activity of target proteins byphosphorylation (transfer of phosphate groups).
The p34 kinase is one of numerous types of protein kinases that areused to regulate cellular processes. To become activated, the p34 polypeptide subunit must combine with severalother polypeptide chains that are known as cyclins because their abundance cycles in phase with the cell cycle. Atthe G1/S control point, one set of cyclins combines with the p34 subunit to yield the active kinase that triggersDNA replication and other events of the S period.
Similarly, at the G2/M control point, a second set of cyclinscombines with the p34 subunit to yield the active kinase that initiates condensation of the chromosomes,breakdown of the nuclear envelope, and reorganization of the cytoskeleton in preparation for cytokinesis.Illustrated in Figure 3.3 are the essential features of chromosome behavior inPage 85Figure 3.3Diagram of mitosis in an organism with two pairs of chromosomes (red/rose versus green/blue). At each stage, the smaller innerdiagram represents the entire cell, and the larger diagram is an exploded view showing the chromosomes at that stage.Interphase is usually not considered part of mitosis proper; it is typically much longer than the rest of the cell cycle,and the chromosomes are not yet visible. In early prophase, the chromosomes first become visible as fine strands,and the nuclear envelope and one or more nucleoli are intact.
As prophase progresses, the chromosomescondense and each can be seen to consist of two sister chromatids; the nuclear envelope and nucleoli disappear.In metaphase, the chromosomes are highly condensed and aligned on the central plane of the spindle, which forms atthe end of prophase. In anaphase, the centromeres split longitudinally, and the sister chromatids of each chromosomemove to opposite poles of the spindle. In telophase, the separation of sister chromatids is complete, the spindle breaks down,new nuclear envelopes are formed around each group of chromosomes, the condensationprocess of prophase is reversed, and the cell cycles back into interphase.Page 86mitosis.
Mitosis is conventionally divided into four stages: prophase, metaphase, anaphase, and telophase. (Ifyou have trouble remembering the order, you can jog your memory with peas make awful tarts.) The stages havethe following characteristics:1. Prophase In interphase, the chromosomes have the form of extended filaments and cannot be seen with a lightmicroscope as discrete bodies. Except for the presence of one or more conspicuous dark bodies (nucleoli), thenucleus has a diffuse, granular appearance.
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