Meiosis

Importance of Meiosis

In the context of a multicellular organism it is necessary that all cells (in order not to recognize each other as foreign) have the same hereditary patrimony. This is achieved by mitosis, dividing chromosomes among the daughter cells, in which the "equality of genetic information is ensured by the DNA reduplication mechanism, in a cell continuity that goes from the zygote to the last cells of the organism, in what is called the somatic line of cell generations.
However, if the same mechanism were adopted in the generation of descendants, the entire species would tend to be composed of genetically equal individuals. Such a lack of genetic variability could easily compromise the survival of the species as environmental conditions change. Therefore it is necessary that the species , in the context of the variability of the genetic material that it admits, may give rise to a reassortment, a mixing, not in the context of the single organism, but in the passage from one generation to another. This is done by the phenomena of sexuality and the particular cell division mechanism called meiosis.

What is meiosis

Meiosis occurs only in germline cells. When a long series of mitotic divisions has sufficiently multiplied the number of germ cells available, the latter enter meiosis, thus preparing the gametes. Gametes, merging into fertilization, pool their chromosomal material. If the gametes were diploid, like the other cells of the organism, their fusion in the zygote would give children with 4n heritage; these would give 8n children and so on.
To keep the number of chromosomes of the species constant, the gametes must be haploid, that is, with number n instead of 2n of chromosomes. This is achieved with meiosis.
Meiosis can be understood as the succession of two mitotic divisions without interleaving a reduplication.
In each of the two successive divisions, which originate four haploid cells from a diploid germ cell, there is a succession of prophase, metaphase, anaphase, telophase and cytodieresis.
However, the prophase of the first meiotic division is particularly complicated, giving rise to a succession of moments that take the respective names of leptotene, zygotene, pachytene, diplotene and diakinesis.
We consider these moments one by one, following the behavior of a single chromosome pair.
Leptotene. It is the beginning of meiosis. The chromosomes begin to see each other, still not very spiralized.
Zygotene. Chromosomes are more clearly identified, and homologous chromosomes are seen to come closer. (Recall that the filaments that tend to approach, parallel to each other, are 4: two chromatids for each of the two homologous chromosomes).
Pachytene. The four chromatidic filaments adhere along the entire length, exchanging strokes, by breaking and welding.
Diplotene. As the spiraling and therefore thickening increases, the chromosomes tend to assume their separate individuality: with each centromere joining a double strand.
The points where the exchange by breaking and welding took place (chiasma) still hold the filaments (chromonemes) together in different sections. The four chromonemes, joined in pairs by the centromeres and variously adherent in the chiasms, form the tetrodes.
Diacinesis. The tetrads tend to arrange themselves at the equator of the spindle; the nuclear membrane has disappeared; the separation of the centromeres begins. As this happens, the chromosomes, already united in the chiasms, separate.
After the next metaphase the two centromeres (not yet doubled) migrate towards the opposite poles of the spindle.
This is followed in rapid succession by the anaphase, telophase and cytodieresis of the first division, and immediately afterwards the second division.
While after the metaphase of the first division the centromeres migrated to the poles of the spindle dragging two filaments, in the second metaphase each centromere doubles. The two cells resulting from the first division received n centromeres with 2n filaments, but their subsequent division results in 4 cells, each with n filaments (ie, at this point, n chromosomes).
This general scheme explains three different and parallel phenomena:

1. the reduction of the chromosomal kit from the diploid (2n) of the "organism to the haploid (n) of the gamete".
   
2. The random attribution to the gamete of one or the other chromosome, of maternal or paternal origin.
   
3. The exchange of genetic material between homologous chromosomes of paternal and maternal origin (with mixing of the genetic material, not only at the level of entire chromosomes, but also within the chromosomes themselves).