Meiosis is the process by which the nucleus divides in all sexually reproducing organisms during the production of spores or gametes. These cells have a single set of chromosomes and are called haploid, as opposed to diploid cells with two sets. In humans, for example, gametes have one set of 23 chromosomes and are formed through meiosis from special diploid cells found in the testes and ovaries. When meiosis begins, each of the 46 chromosomes in these cells consists of two identical chromatids, just as in body cells about to divide by mitosis. However, in meiosis, there are two cell divisions instead of one, so that four daughter cells are produced, instead of two. At the start of the first meiotic division, homologous chromosomes (which have genes for the same traits in the same position) form pairs and exchange genetic material in the process known as crossing over. This process does not occur in mitosis. Then during the first meiotic division, one member of each pair of homologous chromosomes moves to each end of the cell, and the cell itself divides. Each of the two cells produced by the first division has just one set of 23 chromosomes. However, every chromosome still consists of two chromatids at this stage. The two daughter cells then undergo the second meiotic division, which is similar to mitosis. One chromatid from each of the 23 chromosomes moves to each of end of the cell, and the daughter cell itself divides. The chromatids form the chromosomes of the new cells produced by the second meiotic division, and each cell has a single set of 23 chromosomes, normally with slight genetic variation from the original parent cell. In the human female, just one of the four daughter cells will become a functional gamete (the ovum), but in the human male, all four cells develop into gametes (sperm). At fertilization, the union of the male and female gametes restores the two full sets of chromosomes in the human zygote.

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