пятница, 8 октября 2010 г.

Reduction or Maturation Division (Meiosis)

Reduction or maturation division (meiosis) is a special form of cell division. In preparation for later fertilization, the male and female germ cells must halve their set of chromosomes (to form a haploid set), so that when ovum and sperm join, a normal double (diploid) set of chromosomes is formed. This process is known as meiosis, a cell division that comprises two steps: first and second maturation divisions.
Shortly before the first maturation division, the male and female sex cells duplicate their DNA as in mitosis, with each chromosome containing two identical chromatids. In meiosis, the prophase of the first maturation division lasts a good deal longer than the prophase of mitosis. As a rule it takes 24 days in male germ cells, while in the female it may at times take decades, because of an interpolated resting phase (dictyotene) (see Chapter 11: Development of the Ovum (Oogenesis) and Follicle Maturation). Prophase is divided into five phases: leptotene, zygotene, pachytene, diplotene, and diakinesis.

First maturation division. During the leptotene of the prophase, the chromosomes become visible as fine threads; in the ensuing zygotene, they arrange themselves side by side in pairs (chromosome pairing). During this process, the corresponding (homologous) paternal and maternal chromosomes are always arranged next to each other. Since each single chromosome contains two chromatids (sister chromatids), the chromosome pairs contain four chromatids, two maternal and two paternal, the so-called tetrad, which is especially conspicuous during the diplotene of prophase. At this point the homologous chromosomes begin to separate. During this process, homologous paternal and maternal chromatids lying in parallel next to each other can interchange homologous fragments by so-called crossing-over (forming a chiasma) and reattachment of the fragments.
In the metaphase which follows, the chromosomes arrange themselves in the equatorial plane, similarly to a mitosis. During anaphase, the separation of the homologous chromosomes by a spindle is completed. Telophase completes the first maturation division. The two daughter cells each now have only half the number of chromosomes of the initial cell, though each chromosome still consists of two chromatids.
Second maturation division. After a brief phase (interkinesis) during which the DNA is not duplicated, the second maturation division begins. This maturation division proceeds entirely like a normal mitosis, that is, during anaphase the two chromatids of each chromosome divide and are distributed to two daughter cells. Consequently, the haploid daughter cells that originated when the double set of chromosomes was halved during the first maturation division, once again halve their DNA content during the second maturation division.
The result of the two maturation divisions = mature sex cells. Thus four daughter cells (mature sex cells) result from the two-step division of meiosis. In each of these, the number of chromosomes and the DNA content have been reduced to half the original. In addition, the chromosomes have been restructured as a result of the crossing-over, and recombined as a result of the random distribution of the two homologous chromosomes into the daughter cells. The real biological significance of meiosis lies in this shuffling of genetic material.

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