Stages of Meiosis

Meiosis is the division of diploid cells, as a result of which haploid cells are formed. This means that of each pair of homologous chromosomes of the mother cell, only one chromosome is included in the daughter cells. Meiosis is the basis of the formation of sexual cells - gametes. As a result of the fusion of male and female gametes, the diploid set is restored. Thus, one of the important meanings of meiosis is to ensure the constancy of the number of chromosomes of the species during sexual reproduction.

In the cell that proceeds to meiotic division, duplication (replication) of chromosomes has already occurred, as well as in the interphase of mitosis. So each chromosome consists of two chromatids, and the number of chromosomes is diploid. Thus, according to the amount of genetic information, the cells entering into mitosis and meiosis are the same.

Unlike mitosis, meiosis includes two divisions. As a result of the first division, the homologous chromosomes of each pair diverge into different daughter cells, and two cells with a haploid number of chromosomes are formed, but each chromosome consists of two chromatids. The second division also proceeds as a mitotic, since the chromatids of each chromosome are separated, and one chromatid of each chromosome departs into the daughter cells.

Thus, as a result of meiosis, four cells are formed with a haploid set of chromosomes. In males, all four become spermatozoa. But in females only one becomes an ovum, others die. This is due to the fact that only one cell concentrates the stock of nutrients.

Stages, or phases, of the first meiotic division:

1. Prophase I. Spiralization of chromosomes. Homologous chromosomes are located parallel to each other and exchange some homologous sites (conjugation of chromosomes and crossing-over, which results in recombination of genes). The nuclear envelope is destroyed, the spindle of division begins to form.
2. Metaphase I. The pairs of homologous chromosomes are located in the equatorial plane of the cell. The microtubule of the spindle attaches to the centromere of each chromosome. One homologous chromosome is connected to the filament from one pole of the cell, and the other from the other.
3. Anaphase I. Each chromosome from a pair of homologous go to its pole of the cell. In this case, each chromosome continues to consist of two chromatids.
4. Telophase I. Two cells are formed, they contain a haploid set of doubled chromosomes.

Stages, or phases, of the second meiotic division:

1. Prophase II. Destruction of nuclear shells, spindle formation.
2. Metaphase II. Chromosomes are locating in the equatorial plane, they are joined by filaments of the spindle of division. To each centromere are attached two strands - one from one pole, the other from the other.
3. Anaphase II. Chromatids of each chromosome are separated in the centromere region, and each of the pair of sister chromatids goes to its pole.
4. Telophase II. Formation of nuclei, untwisting of chromosomes, division of cytoplasm.

The diagram shows the behavior of only one pair of homologous chromosomes. There are more of them in real cells. So in human cells there are 23 pairs. The diagram shows that the daughter cells are genetically different from each other. This is an important difference between meiosis and mitosis.

It should be noted another important meaning of meiosis (the first, as already indicated, is the provision of a mechanism for sexual reproduction). As a result of crossing-over, new combinations of genes are created. They are also created as a result of an independent discrepancy between chromosomes during meiosis. Therefore, meiosis underlies the combinative variability of organisms, which in turn is one of the sources of natural selection, i.e., evolution.