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:
- Pairing of homologous chromosomes.
- Genetic recombination.
- Homologous chromosome segregation. Chromatids are not distributed, but whole chromosomes.
- Phases of meiosis 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.
Prophase I is divided into five phases:
- Leptotene. The chromatin condenses into chromosomes with two chromatids. Homologous chromosomes approximate.
- Zygotene. Synapse 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.