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8.2.2. Meiosis

Meiosis

Most multicellular organisms reproduce sexually, through fertilization, consisting of the union of two gametes, one from the father and the other from the mother. Therefore, to keep the number of chromosomes constant, these gametes have to have half the chromosomes so that, when they join, they give rise to an egg or zygote cell with the correct number of chromosomes. This process of chromosome reduction is called meiosis.

The meiosis (from the Greek μείωσις meiosis "decline") is the type of cell reproduction, which is done in the sex glands to produce gametes. Meiosis is a cell division process in which a diploid (2n) cell or gametogonia undergoes two successive divisions, to produce four haploid (n) cells or gametes. This is how eggs and sperm are formed, which when unite, will give rise to a diploid individual.

The main characteristics of meiosis are:

  • It occurs in all biological cycles that have sexual reproduction.
  • Genetic material is duplicated once, but then two successive divisions occur.
  • Four haploid daughter cells (n) are obtained with half as many chromosomes as the progenitor cell (2n).

As in mitosis, during the interphace the DNA is duplicated, and each chromatid originates a homologous chromatid (twin), remaining united by the centromere.

Then there are two nuclear and cytoplasmic divisions:

Fundamental ideas about meiosis

Meiosis

  • It is carried out in sex cells (germ or gametes).
  • Interphace.
    • Genetic material is duplicated.
    • The cell goes from being diploid (2n) to being 4n. Each chromatid causes a chromatid homologous (same, if no error), being joined by centromere.  
    • Then, after two divisions, 4 n cells will be obtained.
  • Meiosis I (reductional) .
    • The homologous chromosomes are paired, one from the father and the other from the mother, and the number of chromosomes (n chromosomes) is reduced by half, forming tetrads or bivalent, although each chromosome would have 4 chromatids, 2 from the paternal homologous chromosome and another 2 of the maternal homolog.
    • Then the chromosomes separate, and n chromosomes migrate to each pole. N or chromatids are separated, as in the  anaphase  of mitosis, but chromosomes complete composed of two chromatids in which have been genetic recombination.
    • After telophase I, the 2n cell, which after interphase had duplicated its 4n genetic material, will have divided into two daughter cells with n chromosomes but two chromatids, with homologous paternal and maternal chromosome segments by genetic recombination.
  • Meiosis II (equational):
    • It is similar to a normal mitosis 
    • The sister chromatids of each chromosome are divided between the two daughter cells.
    • At the beginning of meiosis II, each cell had n chromosomes with two chromatids, and at the end, it will have n chromosomes with 1 chromatid.