Meiosis and Sexual Reproduction

Sexual reproduction is a form of reproduction that involves fusion of gametes, producing offspring that differ genetically from either parent. This fusion is called fertilization, which is the union of sperm and egg. Fertilization takes place when a haploid male gamete (sperm) unites with a haploid female gamete (ovum) to produce a single fertilized cell, or diploid zygote. This cell then undergoes many rounds of mitosis as it develops into an organism.

The joining of male and female gametes produces diploid zygotes; this is the process through which most plants, animals, and all humans develop. Since the zygote is formed from the fusion of two gametes, these specialized cells must be haploid; if they were diploid like the other cells of the organism, the resulting zygote would have double the number of chromosomes it needed.

Mitosis is the usual process of cell division in animals and produces cells identical to the parent cell. A separate specialized process is required for the formation of gametes. In meiosis there are two cell divisions. The first division is preceded by chromosome duplication, just as happens in mitosis. At the end of the first division, the two resulting cells are haploid but contain duplicated chromosomes composed of two chromatids. Meiosis I involves the separation of homologous chromosome pairs; in meiosis II the separated cells are sister chromatids. The second division takes place without further duplication of chromosomes. This results in each cell having half the number of chromosomes of the original cell. Because both sperm and egg cells are haploid cells, when the sperm and egg join during fertilization, the correct diploid number (2n) is reached.