Meiosis

For thoughtful child sponsors

The cellular process at the basis of sexual reproduction is called meiosis. Meiosis is a process of cell division in eukaryotes by which a diploid parent cell produces four haploid daughter cells. It consists of two cycles (meiosis I and meiosis II) of nuclear division (also called chromosome segregation), usually accompanied by cell division (especially in multicellular forms, where it is generally used to produce gametes (gametogenesis), preceded by DNA replication. In most eukayrotic species, the proliferation of organisms occurs through sexual reproduction. Meiosis enables the genomes of two parents to be combined in such a way to generate offspring whose genetic makeup is related both to that of their parents and the other siblings, while remaining unique.

The mechanistic differences between mitosis, which produces somatic cells, and meiosis, is best understood by considering mitosis first. (All jargon used in this article is defined in the article on mitosis.) During a mitotic division, chromosomes are duplicated but remain closely aligned, and these twin copies are called sister chromatids. Note that diploid cells have two sets of homologous chromosomes. DNA replication generates sister chromatids from each chromosome, and in the interval between DNA replication and cell division, the cell bears four copies of each chromosome. At metaphase, sister chromatids align on the mitotic spindle. At anaphase, these sister chromatids separate, each migrating toward an opposite pole of the spindle. The two new cells that result bear one copy of each homologous chromosome.

As described earlier, meiotic nuclear division consists of two stages, called meiosis I and meiosis II. It starts with a cell in the same state as does a mitotic division. However, the alignment of chromosomes for prophase is different. Homologous chromosomes join into tetrads (so called because each tetrad contains four chromatids), and the tetrads line up on the metaphase plane.

During prophase I (the prophase of meiosis I), various phenomena unique to meiosis may occur, such as recombination or crossing over. These mechanisms also contribute to the genetic variation in the gametes:
During metaphase I, the tetrads line up on the metaphase plate or equatorial plate.
During anaphase I, they are pulled apart into their constituent homologous chromosomes.

Meiosis II is identical to mitosis.

Meiosis is a figure of speech. See meiosis (figure of speech)