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. In meiosis, absence of a chiasma generally results in improper chromosomal segregation and aneuploidy.
When each tetrad, which is composed of two pairs of sister chromatids, begins to split, 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.
In humans, there seems to be one chiasma per chromosome arm, and in mammals, the number of chromosome arms is a good predictor of the number of crossovers. 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.
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.
- Andersen SL, Sekelsky J (2010). "Meiotic versus mitotic recombination: two different routes for double-strand break repair: the different functions of meiotic versus mitotic DSB repair are reflected in different pathway usage and different outcomes". BioEssays. 32 (12): 1058–66. doi:10.1002/bies.201000087. PMC 3090628. PMID 20967781.
- Fledel-Alon A, Wilson DJ, Broman K, Wen X, Ober C, Coop G, Przeworski M (2009). "Broad-scale recombination patterns underlying proper disjunction in humans". PLoS Genetics. 5 (9): e1000658. doi:10.1371/journal.pgen.1000658. PMC 2734982. PMID 19763175.
- Elof Axel Carlson, Mendel's Legacy: The Origin of Classical Genetics, CSHL Press, 2004, ISBN 0-87969-675-3, p.xvii
- In pursuit of the gene: from Darwin to DNA By James Schwartz Harvard University Press (2008), p. 182 ISBN 0-674-02670-5 Retrieved 19 March 2010.
- Hassold T, Judis L, Chan ER, Schwartz S, Seftel A, Lynn A (2004). "Cytological studies of meiotic recombination in human males". Cytogenetic and Genome Research. 107 (3–4): 249–55. doi:10.1159/000080602. PMID 15467369.
- Pardo-Manuel de Villena F, Sapienza C (2001). "Recombination is proportional to the number of chromosome arms in mammals". Mammalian Genome. 12 (4): 318–22. doi:10.1007/s003350020005. PMID 11309665.
- Church, Kathleen; Wimber, Donald E. (March 1969). "Meiosis in the Grasshopper: Chiasmata Frequency After Elevated Temperature and X-Rays". Canadian Journal of Genetics and Cytology. 11 (1): 209–216. doi:10.1139/g69-025. PMID 5797806. Retrieved 7 April 2016.
- Westerman M (1971). "The effect of x-irradiation on chiasma frequency in Chorthippus brunneus". Heredity (Edinb). 27 (1): 83–91. doi:10.1038/hdy.1971.73. PMID 5289295.
- Recombination and the formation of chiasmata in meiosis
- Whitby, M. (2009), "Recombination and the formation of chiasmata in meiosis", in Millar, J. (ed.), The Cell Division Cycle: Controlling when and where cells divide and differentiate, The Biomedical & Life Sciences Collection, Henry Stewart Talks Ltd, London (online at http://hstalks.com/?t=BL0422198)