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In genetics, a chiasma (pl.: chiasmata) is the point of contact, the physical link, between two (non-sister) chromatids belonging to homologous chromosomes. At a given chiasma, an exchange of genetic material can occur between both chromatids, what is called a chromosomal crossover, but this is much more frequent during meiosis than mitosis.[1] In meiosis, absence of a chiasma generally results in improper chromosomal segregation and aneuploidy.[2]
Points of crossing over become visible as chiasma after the synaptonemal complex dissembles and the homologous chromosomes slightly apart from each other.
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.[3][4]
Following synapsis, each homologous pair of synapsed chromosomes consists of four chromatids called a tetrad, this is referred to interchangeably as a bivalent, which is one pair of chromosomes. As the tetrad begins to split in diplotene, the only points of contact are at the chiasmata. The chiasmata become visible during the diplotene stage of prophase I of meiosis, but the actual "crossing-overs" of genetic material are thought to occur during the previous pachytene 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.[5]
In humans, there seems to be one chiasma per chromosome arm,[6] and in mammals, the number of chromosome arms is a good predictor of the number of crossovers.[7] 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]
バッタMelanoplus femurrubrum は、減数分裂の各段階で急性 X 線照射を受け、キアズマ頻度が測定された。[ 8 ] 減数分裂のレプトテン期からジゴテン期、すなわちクロスオーバー組換えが起こるパキテン期の前に照射すると、その後のキアズマ頻度が増加することがわかった。同様に、バッタChorthippus brunneusでは、ジゴテン期からパキテン期初期に X 線照射を受けると、平均細胞キアズマ頻度が有意に増加した。[ 9 ] キアズマ頻度は、減数分裂の後期ジプロテン期からダイアキネシス期でスコアされた。これらの結果は、電離放射線によって誘発された二本鎖 DNA 切断(DSB) が、その後、キアズマ形成につながるクロスオーバー経路によって修復されることを示している。