In an organism with some number of chromosomes, the process of meiosis, with its two cell divisions, already provides for a large variability among organisms. Humans have 23 pairs of chromosomes, so each individual could produce 223 different gametes just through different assortments of chromosomes. However, the actual variability in gametes is much larger than this because of chromosomal crossing-over. This process exchanges DNA between homologous chromosomes during meiosis.
During interphase prior to meiosis, each homologous chromosome is duplicated. During prophase I of meiosis, these duplicated homologous chromosomes are paired, forming a bivalent, a structure consisting of two sets of sister chromatids (from replicated homologous chromosomes) connected by the synaptonemal, ladderlike complex prior to the first meiotic division. The two pairs of sister chromatids are in close proximity during prophase I, allowing for a chiasma, or point of attachment between two (nonsister) chromatids, to form. A chiasma (plural, chiasmata) is the location on homologous chromosomes in a bivalent where crossing-over occurs. At the chiasmata, the double helices of the DNA strand of the individual chromatids are broken and then re-formed. The re-formation of the helices results in two new permanent connections between the attached chromatids. The chromatids still contain the same genes, in the same sequences as before, but they have exchanged alleles. An allele is a version of a gene. The process of swapping alleles between homologous chromosomes is called genetic recombination.Each bivalent has at least one chiasma and may have multiple chiasmata. In human chromosomes there are an average of four chiasmata per tetrad. Furthermore, each pair of sister chromatids may make different chiasmata with either of the other sister chromatids. Each individual chromatid can thus generate a unique genetic sequence through genetic recombination. At the end of meiosis, each now-unique sister chromatid becomes a chromosome in one of the four gametes produced. Since this crossing-over occurs individually in the bivalent formed from each pair of homologous chromosomes, the genetic variability it provides is extremely high. It is also a random occurrence and so contributes to the genetic variation among individuals of a species, which allows for evolution through natural selection.