Ingenetics, achiasma (pl.:chiasmata) is the point of contact, the physical link, between two (non-sister)chromatids belonging tohomologous chromosomes. At a given chiasma, an exchange of genetic material can occur between both chromatids, what is called achromosomal crossover, but this is much more frequent duringmeiosis thanmitosis.[1] Inmeiosis, absence of a chiasma generally results in improper chromosomal segregation andaneuploidy.[2]
Points of crossing over become visible as chiasma after the synaptonemal complex dissembles and the homologouschromosomes slightly apart from each other.
The phenomenon of genetic chiasmata (chiasmatypie) was discovered and described in 1909 byFrans Alfons Janssens, a Professor at theUniversity of Leuven inBelgium.[3][4]
When eachtetrad, which is composed of two pairs of sisterchromatids, begins to split, the only points of contact are at the chiasmata. The chiasmata become visible during the diplotene stage of prophase I ofmeiosis, but the actual "crossing-overs" of genetic material are thought to occur during the previouspachytene stage. 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.
In humans, there seems to be one chiasma per chromosome arm,[5] and in mammals, the number of chromosome arms is a good predictor of the number of crossovers.[6] Yet, in humans and possibly other species, evidence shows that the number of crossovers is regulated at the level of an entire chromosome and not an arm.[2]
The grasshopperMelanoplus femurrubrum was exposed to an acute dose of X-rays during each individual stage of meiosis, and chiasma frequency was measured.[7] 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 grasshopperChorthippus brunneus, exposure to X-irradiation during the zygotene-early pachytene stages caused a significant increase in mean cell chiasma frequency.[8] Chiasma frequency was scored at the later diplotene-diakinesis stages of meiosis. These results suggest that X-rays induceDNA damages, likely including double-strand breaks, and these damages are repaired by acrossover pathway leading to chiasma formation.