Chromosomal crossover 64045 219999112 2008-06-17T20:36:09Z DOI bot 6652755 Citation maintenance. Initiated by [[User:Fconaway|Fconaway]]. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. {{redirect|Crossing over|other uses|Crossing Over}} [[Image:Morgan crossover 1.jpg|thumb|Thomas Hunt Morgan's illustration of crossing over (1916)]] '''Chromosomal crossover''' (or '''crossing over''') is the process by which two [[chromosomes]] pair up and exchange sections of their [[DNA]]. This often occurs during [[prophase]] 1 of [[meiosis]] in a process called [[synapsis]]. Synapsis begins before the [[synaptonemal complex]] develops, and is not completed until near the end of prophase 1. Crossover usually occurs when matching regions on matching [[chromosome]]s break and then reconnect to the other chromosome. The result of this process is an exchange of [[gene]]s, called [[genetic recombination]]. Chromosomal crossovers also occur in asexual organisms and in [[somatic cell]]s, since they are important in some forms of [[DNA repair]].<ref>{{cite journal |author=Li X, Heyer WD |title=Homologous recombination in DNA repair and DNA damage tolerance |journal=Cell Res. |volume=18 |issue=1 |pages=99–113 |year=2008 |pmid=18166982 | doi = 10.1038/cr.2008.1}}</ref> [[Image:Morgan crossover 2.jpg|thumb|A double crossing over]] [[Image:Chromosomal Recombination.svg|thumb|Recombination involves the breakage and rejoining of parental chromosomes]] Crossing over was first described, in theory, by [[Thomas Hunt Morgan]]. The physical basis of crossing over was first demonstrated by [[Harriet Creighton]] and [[Barbara McClintock]] in [[1931]].<ref>{{cite journal |author=Creighton H, McClintock B |title=A Correlation of Cytological and Genetical Crossing-Over in Zea Mays |journal=Proc Natl Acad Sci U S A |volume=17 |issue=8 |pages=492–7 |year=1931 |pmid=16587654 |doi=10.1073/pnas.17.8.492}} (Original paper)</ref> ==Chemistry== [[Image:Holliday Junction.png|thumb|200px|right|Holliday Junction]] [[Image:Holliday junction.jpg|thumb|200px|right|Molecular structure of a Holliday junction.]] Meiotic recombination initiates with double-stranded breaks that are introduced into the DNA by the Spo11 protein.<ref>Keeney S, Giroux CN, and Kleckner N (1997). "Meiosis-specific DNA double-stranded breaks are catalyzed by Spo11, a member of a widely conserved protein family." ''Cell'' '''88'''(3):375-384. PMID 9039264 {{doi|10.1016/S0092-8674(00)81876-0}}</ref> One or more exonucleases then digest the 5’ ends generated by the double-stranded breaks to produce 3’ single-stranded DNA tails. The meiosis-specific recombinase [[Dmc1]] and the general recombinase [[Rad51]] coat the single-stranded DNA to form nucleoprotein filaments.<ref>Sauvageau S, Stasiak AZ, Banville I, Ploquin M, Stasiak A, and Masson JY (2005). "[http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1140613 Fission yeast rad51 and dmc1, two efficient DNA recombinases forming helical nucleoprotein filaments]." ''Mol Cell Biol'' '''25'''(11):4377-4387. PMID 15899844 {{doi|10.1128/MCB.25.11.4377-4387.2005}}</ref> The recombinases catalyze invasion of the opposite [[chromatid]] by the single-stranded DNA from one end of the break. Next, the 3’ end of the invading DNA primes DNA synthesis, causing displacement of the complementary strand, which subsequently anneals to the single-stranded DNA generated from the other end of the initial double-stranded break. The structure that results is a ''cross-strand exchange'', also known as a [[Holliday junction]]. The contact between two chromatids that will soon undergo crossing-over is known as a ''[[chiasma (genetics)|chiasma]]''. The Holliday junction is a [[Tetrahedron|tetrahedral]] structure which can be 'pulled' by other recombinases, moving it along the four-stranded structure. ==Consequences== In most [[eukaryote]]s, a [[cell (biology)|cell]] carries two copies of each [[gene]], each referred to as an [[allele]]. Each parent passes on one allele to each offspring. An individual [[gamete]] inherits a complete haploid complement of alleles on chromosomes that are independently selected from each pair of [[Chromatid|chromatids]] lined up on the metaphase plate. Without recombination, all alleles for those genes linked together on the same chromosome would be inherited together. Meiotic recombination allows a more independent selection between the two alleles that occupy the positions of single genes, as recombination shuffles the allele content between sister [[Chromatid|chromatids]]. Recombination does not have any influence on the statistical probability that another offspring will have the same combination. This theory of "[[Mendelian inheritance|independent assortment]]" of alleles is fundamental to genetic inheritance. However, there is an exception that requires further discussion. [[Image:Conversion and crossover.jpg|thumb|right|300px|The difference between [[gene conversion]] and '''chromosomal crossover'''. Blue is the two [[chromatid]]s of one chromosome and red is the two chromatids of another one.]] The frequency of recombination is actually not the same for all gene combinations. This leads to the notion of "[[centiMorgan|genetic distance]]", which is a measure of recombination frequency averaged over a (suitably large) sample of pedigrees. Loosely speaking, one may say that this is because recombination is greatly influenced by the proximity of one gene to another. If two genes are located close together on a chromosome, the likelihood that a recombination event will separate these two genes is less than if they were farther apart. [[Genetic linkage]] describes the tendency of genes to be inherited together as a result of their location on the same chromosome. [[Linkage disequilibrium]] describes a situation in which some combinations of genes or genetic markers occur more or less frequently in a population than would be expected from their distances apart. This concept is applied when searching for a gene that may cause a particular [[disease]]. This is done by comparing the occurrence of a specific [[DNA sequence]] with the appearance of a disease. When a high correlation between the two is found, it is likely that the appropriate gene sequence is really closer. ==Problems== Although crossovers typically occur between homologous regions of matching chromosomes, similarities in sequence can result in mismatched alignments. These processes are called unbalanced recombination. Unbalanced recombination is fairly rare compared to normal recombination, but severe problems can arise if a gamete containing unbalanced recombinants becomes part of a [[zygote]]. The result can be a local [[Gene_duplication|duplication]] of genes on one chromosome and a [[Gene_deletion|deletion]] of these on the other, a [[Chromosomal_translocation|translocation]] of part of one chromosome onto a different one, or an [[Chromosomal_inversion|inversion]]. ==References== <references /> ==See also== *[[Mitotic crossover]] *[[Recombinant frequency]] *[[Independent assortment]] *[[Genetic distance]] {{Genetic recombination}} [[Category:Cellular processes]] [[Category:Molecular genetics]] [[cs:Crossing-over]] [[de:Crossing over]] [[es:Recombinación genética]] [[fr:Enjambement (génétique)]] [[id:Pindah silang]] [[it:Crossing-over]] [[lt:Persikryžiavimas]] [[hu:Crossing over]] [[nl:Crossing-over]] [[ja:乗換え (生物学)]] [[pl:Crossing-over]] [[pt:Recombinação]] [[ru:Кроссинговер]] [[sr:Кросинг-овер]] [[fi:Tekijäinvaihdunta]] [[sv:Överkorsning]] [[tr:Sarılma (genetik)]] [[uk:Кросинговер]] [[zh:染色體互換]]