Two types of nuclear division, mitosis and meiosis, occur in cell biology. Most human cells (called diploid cells) are formed through mitosis and contain forty-six chromosomes in twenty-three matched pairs. By contrast, meiosis produces haploid cells, each containing a single set of twenty-three unpaired chromosomes. Sex cells (sperm and ovum) are haploid.

Prior to meiosis, DNA is replicated within a diploid cell, resulting in four copies of each chromosome (now numbering ninety-two). Two successive divisions of the nuclear material occur during meiosis. As part of the process, homologous chromosomes—paired maternal and paternal chromosomes—exchange segments, thus recombining their genes. Four daughter haploid cells are formed, each with one-quarter of the genetic material (twenty-three chromosomes) of the original diploid cell. When haploid cells unite in sexual reproduction, each contributes half of the genetic material that creates the offspring. Meiosis, therefore, contributes to the genetic diversity within species.

See also: MITOSIS

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