Meiosis

Meiosis is a special type of cell division. Unlike mitosis, the way normal body cells divide, meiosis results in cells that only have half the usual number of chromosomes, one from each pair. For that reason, meiosis is often called reduction division. In the long run, meiosis increases genetic variation, in a way which will be explained later.^[1]

Sexual reproduction takes place when a sperm fertilizes an egg. The eggs and sperm are special cells called gametes, or sex cells. Gametes are haploid; they have only half the number of chromosomes as a normal body cell (called a somatic cell). Fertilization restores the chromosomes in body cells to the diploid number.

The basic number of chromosomes in the body cells of a species is called the somatic number and is labelled 2n. In humans 2n = 46: we have 46 chromosomes. In the sex cells the chromosome number is n (humans: n = 23).^[2] So, in normal diploid organisms, chromosomes are present in two copies, one from each parent (23x2=46). The only exception are the sex chromosomes. In mammals, the female has two X chromosomes, and the male one X and one Y chromosome.

All eukaryotes that reproduce sexually use meiosis. This also includes many single-celled organisms. Meiosis does not occur in archaea or bacteria, which reproduce by simple cell division.

1. ↑ Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. 2002. "Meiosis". Molecular Biology of the Cell (4th ed). Garland Science.
2. ↑ White M.J.D. 1973. The chromosomes. 6th ed, Chapman & Hall, London. p28