Meiosis is a kind of cell division that occurs only in sexually-reproducing organisms. It is the process by which they produce gametes. Meiosis is that division in which the number of chromosomes in the resulting cells are reduced by half. Due to this reason, meiosis occurs in a two step division process viz: Meiosis I and II.
Meiosis produces genetically different reproductive cells. This is as a result of a phenomenon unique to only meiosis called CROSSING-OVER. Crossing-over is a process whereby non-sister chromatids of homologous chromosomes exchange a segment of their genetic material. It occurs only in Prophase 1 of meiosis where the TETRAD structure is formed. This exchange of genetic material causes a recombination of DNA in the resulting cell (gamete).
Crossing-over only occurs between non-sister chromatids of homologous chromosomes, hence, it can only occur in Prophase 1. Prophase II involves sister chromatids.
