Meiosis is the process by which diploid cells divide to produce haploid daughter cells. In humans, each diploid cell contains 46 chromosomes, half from the mother and half from the father. Following meiosis, the resulting haploid eggs or sperm only contain 23 chromosomes; however, each of these chromosomes contains a unique combination of parental information that results from the meiotic process of crossing over.
Although meiosis shares similarities with mitosis—both rely on microtubules to partition chromosomes to opposite sides of a cell, which then divides to form a daughter cell pair—meiosis is only observed in the sex organs, while mitosis occurs in other tissue types of the body. In addition, the cells resulting from mitosis are genetically indistinguishable (save for random mutations) from their predecessor: crossing over does not occur, and all the daughter cells are diploid. In contrast, meiosis produces four cells that not only have half the number of chromosomes from their predecessor, but they also contain unique combinations of genetic material. No two meiotic products are identical, which helps account for the appearance and personality differences often seen between siblings in the same family.
Diploid cells within either the human testes or ovaries produce haploid sperm or eggs by undergoing two divisions during the process of meiosis.
The first division during meiosis is meiosis I, which begins with diploid cells in which chromosomes have replicated, appearing x like in shape. Critical here is the existence of such paternally and maternally inherited structures, which exchange information between them during a process called crossing over, creating genetic diversity.
Other changes during this period result in a microtubule-based framework that separates the chromosomes so that they are randomly but evenly partitioned between opposite ends of the cell, which then splits. The end result is a novel pair of haploid cells. Importantly, although every x in these newly formed structures is individually termed a chromosome, each is actually composed of two copies of the same chromosome, termed sister chromatids.
During the second division, meiosis II, a similar arrangement of microtubules breaks apart sister chromatids. This again yields haploid cells, with the key difference being that the chromosomes in them are no longer replicated and appears as I's when condensed.
Depending on whether meiosis occurs in the testes or ovaries, either four sperm or one mature egg and smaller cells that dissolve result. When an egg is fertilized this produces a unique diploid individual, genetically distinct from its parents due to meiotic events.
