The sexual reproduction occurs when two sex cells or gametes from two different individuals come together to form a new cell, the egg or zygote. The zygote has the same number of chromosomes as the parent organisms, since the gametes that originated it have half as many chromosomes as the rest of the cells of the parent organisms.
We have seen that by mitosis cells with the same number of chromosomes as the progenitor cells originated, so there must be another mechanism by which cells with half the chromosomes are generated so that, when fertilization occurs, they give rise to a zygote with the same chromosomal endowment as that of the parents. Logically, if the same number of chromosomes were not maintained, each generation would double the number of chromosomes of the species.
The meiosis is the cell division mechanism consisting of two consecutive cell divisions called first and second meiotic division, respectively. They are obtained four haploid daughter cells (n), genetically distinct, and half of chromosomes as the stem cell diploid (2n).
As in mitosis, before meiosis begins, at the interphase, the DNA in the nucleus of the dividing cell is duplicated. At this time, before the first meiotic division, each chromosome is made up of two identical chromatids.
Then the two divisions occur: