Chiasma (genetics)

In genetics, a chiasma (pl. chiasmata) is the point where two homologous non-sister chromatids exchange genetic material during chromosomal crossover in 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 noticeable 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.

where recombination frequency is:

The phenomenon of genetic chiasmata (chiasmatypie) was discovered and described in 1909 by Frans Alfons Janssens, a Professor at the University of Leuven in Belgium. "Bivalent" refers to the two homologous chromosomes (4 chromatids); "chiasma" refers to the actual break of the phosphodiester bond during crossing over. The larger the number of map units between the genes, the more crossing over occurs.

The grasshopper Melanoplus femurrubrum was exposed to an acute dose of X-rays during each individual stage of meiosis, and chiasma frequency was measured. Irradiation during the leptotene-zygotene stages of meiosis, that is, prior to the pachytene period in which crossover recombination occurs, was found to increase subsequent chiasma frequency. Similarly, in the grasshopper Chorthippus brunneus, exposure to X-irradiation during the zygotene-early pachytene stages caused a significant increase in mean cell chiasma frequency. Chiasma frequency was scored at the later diplotene-diakinesis stages of meiosis. These results suggest that X-rays induce DNA damages, likely including double-strand breaks, and these damages are repaired by a crossover pathway leading to chiasma formation (see Chromosomal crossover).

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