Conversion génique

Unlike mitosis where double-strand breaks are accidental, in meiosis, they are created by an enzyme called Spo11, which cleaves the phosphodiester backbone. The broken helical ends are trimmed by a protein complex called MRX and the damage is repaired by a process called gene conversion.  Here the damaged “acceptor” DNA strand invades a homologous “donor” DNA duplex to form a displacement loop. This creates regions of heteroduplex DNA where a strand from the donor DNA pairs with a complementary strand from the acceptor DNA.  DNA polymerase extends the invading strand, and the extended D-loop then pairs with the free 3′ tail. DNA synthesis at the newly captured strand, results in the formation of an intermediate with two four-strand structures called Holliday junctions. This double Holliday junction intermediate is resolved by DNA repair enzymes called resolvases and there are two orientations in which the junctions can be cleaved. In the first, resolvase nicks each junction horizontally so that the parental strands are still intact.  This results in a non-crossover product, named because the strands after the break remain with their original partner, and there is no major crossing over of the donor and acceptor strands. Alternatively, if cleavage occurs vertically the regions flanking the damage are switched leading to a crossover product, where the donor strand following the break recombines with the acceptor strand. Gene conversion has a significant impact on genomic diversity. In sexually reproducing organisms the offspring inherits one set of genes from the father and one set from the mother.  The parental DNA sets recombine and the sister chromatids undergo gene conversion. This results in the offspring having novel chromosomes compared to those of the parents.