meiosis defined in 1951 yearmeiosis - meiosis (reduction division);
meiosis - Two successive cell divisions of special kind, starting in a diploid cell. Both divisions resemble mitoses, but the chromosomes are duplicated only once, before the first division, so that the number of chromosomes present in each of the four daughter-cells is half that of diploid cell (i.e. the daughter-cells are haploid). Meiosis occurs at some time in the life cycle of all sexually reproducing organisms, because gametes must be haploid to compensate for the chromosome doubling produced by fertilization. In animals, and some algae, it occurs during formation of gametes. In many fungi and green algae and in Sporo-zoa, it occurs immediately after fertilization or on germination of zygote. In most other plants it occurs some time after fertilization but some time before gamete formation, during formation of spores (See also: Alternation of generations).
The course of meiosis is extremely similar wherever it occurs. Chief differences from two successive mitoses are as follows (Compare with: Mitosis). When chromosomes first appear (in prophase) they are separate fine threads; this is the leptotene stage. The two homologous members of each pair of chromosomes then associate closely side by side, corresponding loci adhering together, a process called pairing or synopsis; each associated pair is a bivalent; Fig. 6B (ab); this is the zygotene stage. The bivalents shorten and thicken. Each individual chromosome is now manifestly double (the actual doubling occurred in the previous inter-phase); hence each bivalent consists of four chromatids (ac) (packy-tene stage). The two chromatids derived from one chromosome remain paired but they separate from the other two chromatids derived from the homologous chromosome (diplotene stage, followed by diakinesis). At certain places, however, they are held together by interchanges (chiasmata) occurring between chromatids derived from homologous chromosomes. Chiasmata are the visible expression of crossing-over of genes. There are usually one or more chiasmata per chromosome pair per meiosis. In a chiasma two chromatids, one from each of the original chromosomes, have apparently broken at corresponding places; and the broken ends of one chromatid have fused with the broken ends of the other. Where there is more than one chiasma in a single bivalent they may involve different pairs of chromatids, but always one from each of the original chromosomes. At anaphase two of the four chromatids from each bivalent go to one pole of the spindle, and the other two to the other pole. The chromatids go in pairs because the spindle attachment of each original chromosome has not yet duplicated, so that two chromatids are united at that point. Owing to the effects of chiasmata, however, the united chromatids are not usually derived throughout their length from the same chromosome (though they always are in the immediate neighbourhood of the spindle attachment) but are mixtures of one or more pieces from each of the original chromosomes. It is a matter of chance, and quite uninfluenced by behaviour of other bivalents, which spindle attachment with its Ghromatids, goes to which pole of the spindle. After anaphase there may be a short telophase (5) and resting stage, or the second meiotic division may follow immediately, usually in both the daughter-cells formed by the first division. It is like a mitosis, except that it starts prophase with only half the normal number of chromosomes, each already divided into two chromatids before the previous prophase. These two chromatids separate at anaphase (4'), one going to each daughter-cell. Again, the distribution of the chromatids is a matter of chance, except that the two from each chromosome must go to opposite poles. On this chance distribution of the chromatids at both anaphases depends the law of Independent Assortment. Each daughter-cell therefore eventually receives only the haploid number of chromosomes (5'). The reduction from the diploid number of the original cell to the haploid number of the four products is one of the bases of gene tic segregation.
near meiosis in Knolik
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