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ru>Cullenvens: Modified "ionising-radiation induced" to "ionizing radiation-induced" to match the spelling of the title/heading of the linked article on "Ionizing Radiation" and to accurately indicate that the "DSBs" are induced by ionizing radiation; thus, they are "radiation-induced" changes.

2025-10-08T12:34:10Z

Modified "ionising-radiation induced" to "ionizing radiation-induced" to match the spelling of the title/heading of the linked article on "Ionizing Radiation" and to accurately indicate that the "DSBs" are induced by ionizing radiation; thus, they are "radiation-induced" changes.

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{{Short description|Point of contact among homologous chromosomes}}
[[File:Meiosis crossover.png|thumb|Crossing over during meiosis, with chiasma shown.]]

In [[genetics]], a '''chiasma''' ({{plural form}}: '''chiasmata''') is the point of contact, the physical link, between two (non-sister) [[chromatid]]s belonging to [[homologous chromosome]]s. 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]].<ref>{{cite journal |vauthors=Andersen SL, Sekelsky J |title=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 |journal=BioEssays |volume=32 |issue=12 |pages=1058–66 |year=2010 |pmid=20967781 |pmc=3090628 |doi=10.1002/bies.201000087}}</ref> In [[meiosis]], absence of a chiasma generally results in improper chromosomal segregation and [[aneuploidy]].<ref name=Fledel-Alon2009>{{cite journal |vauthors=Fledel-Alon A, Wilson DJ, Broman K, Wen X, Ober C, Coop G, Przeworski M |title=Broad-scale recombination patterns underlying proper disjunction in humans |journal=PLOS Genetics |volume=5 |issue=9 |article-number=e1000658 |year=2009 |pmid=19763175 |pmc=2734982 |doi=10.1371/journal.pgen.1000658 |doi-access=free }}</ref>

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 [[Catholic University of Leuven (1834–1968)|University of Leuven]] in [[Belgium]].<ref>Elof Axel Carlson, ''Mendel's Legacy: The Origin of Classical Genetics'', CSHL Press, 2004, {{ISBN|0-87969-675-3}}, p.xvii</ref><ref>[https://books.google.com/books?id=5ULXJQR4aEUC&dq=Frans+Alfons+Janssens&pg=PA182 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.</ref>

Following [[synapsis]], each homologous pair of synapsed chromosomes consists of four chromatids called a [[Tetrad (chromosomal formation)|tetrad]], this is referred to interchangeably as a [[Bivalent (genetics)|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.<ref>{{Cite book |last=Pierce |first=Benjamin A. |title=Genetics: a conceptual approach |date=2020 |publisher=Macmillan International, Higher Education |isbn=978-1-319-30831-5 |edition=Seventh |location=New York, NY}}</ref>

In humans, there seems to be one chiasma per chromosome arm,<ref>{{cite journal |vauthors=Hassold T, Judis L, Chan ER, Schwartz S, Seftel A, Lynn A |title=Cytological studies of meiotic recombination in human males |journal=Cytogenetic and Genome Research |volume=107 |issue=3–4 |pages=249–55 |year=2004 |pmid=15467369 |doi=10.1159/000080602|s2cid=1306255 }}</ref> and in mammals, the number of chromosome arms is a good predictor of the number of crossovers.<ref>{{cite journal |vauthors=Pardo-Manuel de Villena F, Sapienza C |title=Recombination is proportional to the number of chromosome arms in mammals |journal=Mammalian Genome |volume=12 |issue=4 |pages=318–22 |year=2001 |pmid=11309665 |doi=10.1007/s003350020005|s2cid=38172472 }}</ref> 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.<ref name=Fledel-Alon2009 />

The grasshopper ''[[Melanoplus femurrubrum]]'' was exposed to an acute dose of X-rays during each individual stage of meiosis, and chiasma frequency was measured.<ref name="pmid5797806">{{cite journal|last1=Church|first1=Kathleen|last2=Wimber|first2=Donald E.|title=Meiosis in the Grasshopper: Chiasmata Frequency After Elevated Temperature and X-Rays|journal=Canadian Journal of Genetics and Cytology|date=March 1969|volume=11|issue=1|pages=209–216|doi=10.1139/g69-025|pmid=5797806}}</ref> 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.<ref name="pmid5289295">{{cite journal |vauthors=Westerman M |title=The effect of x-irradiation on chiasma frequency in Chorthippus brunneus |journal=Heredity (Edinb) |volume=27 |issue=1 |pages=83–91 |year=1971 |pmid=5289295 |doi= 10.1038/hdy.1971.73|doi-access=free |bibcode=1971Hered..27...83W }}</ref> Chiasma frequency was scored at the later diplotene-diakinesis stages of meiosis. These results show that [[ionizing radiation]]-induced [[DNA repair|double stranded DNA breaks]] (DSBs) were subsequently repaired by a crossover pathway leading to chiasma formation.

== See also ==
* [[Genetic recombination]]
* [[Chromosomal crossover]]
* [[Bivalent (genetics)]]
* [[Holocentric chromosome]]

== References ==
{{reflist}}

== External links ==
* [http://hstalks.com/main/view_talk.php?t=1274 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)

[[Category:Genetics]]
[[Category:Cell cycle]]

Chiasma (genetics) - История изменений

Admin: 1 версия импортирована

ru>Cullenvens: Modified "ionising-radiation induced" to "ionizing radiation-induced" to match the spelling of the title/heading of the linked article on "Ionizing Radiation" and to accurately indicate that the "DSBs" are induced by ionizing radiation; thus, they are "radiation-induced" changes.