Cells are produced by either mitosis or meiosis. Human somatic (body) cells are diploid cells; that is, they have 46 chromosomes that are arranged in 23 pairs. One pair carries the sex chromosomes, and is either XX or XY. Diploid somatic cells have 46 chromosomes in 23 pairs.

Mitosis is the process by which one diploid somatic cell divides to produce two diploid somatic cells identical to the original. The DNA in the chromosomes of the original cell replicates itself. After division, each new cell has 46 chromosomes.

Meiosis is the process of cell division by which one diploid somatic cell produces four haploid gamete cells (Figure 12-1). Haploid cells have half the number of chromosomes of the original diploid somatic cell—that is, 23 single chromosomes instead of 46.

Two successive cell divisions occur in meiosis. First, the 46 chromosomes in the diploid somatic cell replicate. Then, rather than separate into two identical cells as in mitosis, the chromosomes intertwine and exchange genetic material. This exchange is responsible for genetic variability, such as eye color or height. When the cell divides, it forms two cells, each containing 23 chromosomes that are doubled in structure, but in combinations that are different from the original cell.