Meiotic Roles of Crossing-over and of Gene Conversion

Recombination is a process that is, with few exceptions, initiated with deliberate aforethought only once during the life cycle of eukaryotes: during prophase I of meiosis. (Exceptions include genomic rearrangements for immunoglobulin synthesis in mammals and mating-type switching in yeast; ordinary somatic recombination is not an exception since it appears to be a by-product of repair of spontaneous or induced chromosome damage.) Homologous meiotic recombination events perform a crucial function when they involve crossing-over; they are essential for the formation of chiasmata, which are in turn essential for reductional segregation of homologs at the first meiotic division in most forms. If joined by a chiasma, the two homologs remain associated as a bivalent, orient to opposite poles at metaphase I, and segregate to opposite poles at anaphase I, producing euploid gametes. In the absence of chiasmata, the two homologs fail to remain associated, orient at random at metaphase I, and...