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).
Then there are two nuclear and cytoplasmic divisions: