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8.2.2.1. Meiosis I

First meiotic division: reductional division

The first meiotic division (or meiosis I) is a reductional division, since the number of chromosomes is reduced by half.

The main events of meiosis I are:


Prophase I

The prophase I is the longest stage of meiosis and it occurs on mating of homologous chromosomes, forming the tetrad, being formed by the two chromatids of each chromosome. Then there is a crossover between homologous chromosomes, which consists of an exchange of chromosomal fragments (DNA) between chromatids of homologous (non-sister) chromosomes.

The karyotheca disappears at the end of prophase I, as does the nucleolus. Instead, the spindle microtubules are formed.

Prophase I is divided into five phases:

  • Leptotene. The chromatin condenses into chromosomes with two chromatids. Homologous chromosomes approximate.
  • ZygoteneSynapse occurs, where homologous chromosomes pair up to form a bivalent (because they are two homologous chromosomes joined) or tetrad (because they have four chromatids). Chromosomes unite along their entire length, except for the X and Y chromosomes, which do so only in the homologous part.
  • Paquiteno. Chromosomes finish pairing and contract, becoming shorter and thicker. There is crossover (or crossing-over) between non-sister chromatids, that is, the exchange of DNA segments (genes) between homologous chromosomes. This results in the genetic recombination of the hereditary material, and the chromosomes will no longer be paternal or maternal, as one of their chromatids will have pieces of paternal and maternal DNA. There may be, as in humans, between 2 and 3 crossovers for each tetrad.

In prophase I of meiosis there is an exchange of chromosomal segments between homologous chromosomes of paternal and maternal origin that do not necessarily have the same information, so the chromatids will be different .

  • Diplotene. The homologous chromosomes begin to separate (desnapsis), although they remain united by points called chiasmas, where the places where the crossover took place are reflected. That is why it is said that the chiasm is the cytological evidence, observable under the microscope, of crossing over, and its genetic consequence is gene recombination.

This stage is the longest of meiosis, as it can last for years, as in the case of human oocytes.

  • Diakinesis. The bivalents, united by the chiasmas, present their highest degree of condensation. Now is when the two chromatids of each homologuecan be distinguished, forming tetrads in which the sister chromatids are joined by the centromeres and the non-sisters by the chiasmas.  

The nuclear membrane and nucleolus disappear. The centrioles begin to separate, forming the spindle between the diplosomes.

Metaphase I

The bivalents (pairs of homologous chromosomes) are placed in the center of the spindle together. The chiasms of each tetrad are arranged in the equatorial plane (not the centromeres). The arrangement in the equatorial plane of the bivalents is also called the parent star.

The main difference from the metaphase of mitosis is that the recombined homologous chromosomes are paired on the equatorial plate.

Anaphase I

In anaphase I the kinetochore detach because the kinetochoric microtubules are shortened. This is why the chiasmata rupture and each homologous chromosome migrates to the opposite pole of the cell.

The n chromosomes migrate to each pole, reducing the number of chromosomes by half.

Chromatids are not separated, as in anaphase of mitosis, but complete chromosomes made up of two chromatids in which there has been genetic recombination.


Telophase I

Each of the homologous chromosomes has reached the poles, the nuclear envelope is regenerated, the nucleolus reappears, and the spindle fibers disappear.  

The chromosomes undergo a slight decondensation and cytokinesis usually occurs.

After telophase I, the chromosomes become slightly decondensed and enter a brief interphase, in which no DNA synthesis takes place. This interphase may not even exist. Soon after, each nucleus prepares to continue with the second division.

As a result of this first meiotic division, each daughter cell is haploid, with a set of chromosomes, although these are made up of two chromatids. Chromosomes contain information from two non-sister chromatids and have been randomly distributed in gametes.