A
chiasma (plural:
chiasmata), in
genetics, is thought to be the point where two
homologous non-sister
chromatids exchange genetic material during
chromosomal crossover during
meiosis (sister chromatids also form chiasmata between each other (also known as a chi structure), but because their genetic material is identical, it does not cause any change in the resulting daughter cells). The chiasmata become visible during the diplotene stage of prophase I of
meiosis, but the actual "crossing-over" of genetic material is thought to occur during the previous
pachytene stage. When each tetrad, which is composed of two pairs of sister
chromatids, begins to split, the only points of contact are at the chiasmata.