Meiosis

Meiosis is the process of dividing the nuclei of germ cells when they are converted into gametes. Meiosis includes two cell divisions, which are called respectively meiosis I and meiosis II.Each of these divisions formally consists of the same stages as mitosis: prophase, metaphase, anaphase and telophase.

Meiosis I is also called reduction division, because as a result of this division the number of chromosomes in newly formed cells decreases by a factor of 2.In the prophase I, the chromosomes already enter divided into chromatids, which are connected in a centromere. It is at this stage of meiosis that an extremely important event takes place from the point of view of creating a genetic diversity - the exchange of homologous regions of nestritic chromatids, i.e., chromatids belonging to different pairs of homologous chromosomes. This exchange is called crossing-over, or recombination. Prophase I lasts long enough, it is accepted to divide into 5 stages: leptotene, zygote, pachytene, diplotene and diakinesis. In the leptotene stage, the chromosomes begin to condense and become visible. In the zygotene, pairs of homologous chromosomes are conjugated( paired) and form a characteristic double structure, which is called synopenemal complex. Two conjugated homologous chromosomes are called bivalent. In pachytene, the chromosomes become shorter due to greater spiralization.

In each chromosome, a longitudinal slit is now visible - the chromosome is divided into chromatids. The bivalent is represented by 4 chromatids located side by side to each other: the nestritic bivalent chromatids are joined together at some points, forming the so-called chiasms. Chiasmas are a manifestation of the exchange of genetic material between chromosomes. These exchanges in formal genetics are called crossover. Each chiasma corresponds to one crossing-over event. Crossing-over is carried out with very high accuracy, therefore none of the chromatids lose or acquire genes. In diplotene, homologous chromosomes begin to diverge, retaining only at those points where chiasma is observed. Chiasm is formed more in large chromosomes, in all, there are about 40 crossovers per gamete. In diakinesis, the chromosomes condense as they diverges, the homologous chromosomes continue to be retained by chiasmas.

In metaphase I, the nuclear envelope disappears, and the chromosomes are distributed in the equatorial plane of the cell. The spindle threads are fastened to centromeres, as in mitosis, and they begin to pull them to the poles of the cell. In anaphase I, the chiases are terminated, ie, they move to the ends of the chromosomes and disappear. Homologous chromosomes move to opposite poles due to reduction of spindle threads. As a result, haploid sets of chromosomes are assembled in telophase I at the cell poles, and the embryonic cell, completing the division, gives rise to two new daughter cells, which in spermatogenesis are called secondary spermatocytes, and in oogenesis - by oocytes.

Meiosis II is similar in mechanism to the usual mitosis, but the doubled haploid set of chromosomes is mitotically divided. As a result of the second meiotic division, two spermatids are formed in male gametogenesis, and in female gametogenesis an egg is formed, since the so-called directing body is formed from the second daughter cell.

Meiosis explains many genetic phenomena, including Mendel's inheritance rules. First, as a result of meiosis, sex cells containing a haploid set of chromosomes are formed, so the child receives from each of the parents half of his chromosome set, and the contribution of each parent to the child's genotype is the same. Secondly, in meiosis 1, bivalents diverge independently from each other to different poles of the cell, which explains the independent inheritance of characters if their genes are in different chromosomes. The probability that two gametes of an individual will contain a set of identical chromosomes is very low. Third, as a result of crossing-over, each chromatid contains DNA, which is derived from the chromosomes of both parents. As a result, the probability of genetic identity of gametes is practically reduced to zero, and this is the basis of the genetic individuality of a person at the chromosome level.