Most higher plants and animals exist in a diploid state in which their somatic cells have two of each type of chromosome. Because mitosis produces new cells with exactly the same chromosomal content as the parent cell, a union of gametes produced by mitosis would double the normal number of chromosomes in each cell. Yet, the number of chromosomes remains constant within a species from generation to generation. Meiosis is the process of cell division by which gametes with half the number of chromosomes are created so when sperm and egg unite in sexual reproduction the diploid number of chromosomes is reconstituted. Meiosis does not randomly separate chromosomes but insures that each homologous chromosome pair in a diploid cell is divided so that each gamete contains one of each type of chromosome. Thus, gametes, said to be haploid, are cells with only one of each type of chromosome. Gametes join to produce a diploid zygote which receives half of its genetic material from the male parent and half from the female parent.
In the process of meiotic cell division, there are two division sequences, each with the same four phases as mitosis including prophase, metaphase, anaphase, and telophase. The first division sequence separates the pairs of homologous chromosomes so that each of the two daughter cells receives one of each type of chromosome in the form of chromatid pairs; the second division sequence separates the pairs of chromatids so that each of the four resulting gametic cells receives one of each type of
chromatid (a single stranded chromosome). When two gametic cells join together
