遺伝子変換

Gene conversion repairs double-strand breaks created by Spo11 enzyme during meiosis. The damaged acceptor DNA strand invades a homologous donor DNA duplex, forming a displacement loop. DNA polymerase extends the invading strand, creating heteroduplex DNA regions where donor and acceptor strands pair. This process ultimately resolves into either crossover or non-crossover products through Holliday junction cleavage.

Holliday junctions are four-strand DNA structures formed during gene conversion when the extended D-loop pairs with the free 3' tail. Two Holliday junctions create an intermediate structure that resolvase enzymes cleave in two possible orientations. Horizontal cleavage produces non-crossover products where parental strands remain intact, while vertical cleavage generates crossover products with recombined donor and acceptor strands.

Non-crossover products result from horizontal Holliday junction cleavage, keeping parental strands with their original partners without major strand exchange. Crossover products arise from vertical cleavage, causing regions flanking the damage to switch, so donor strands recombine with acceptor strands. Both outcomes contribute to genomic diversity but through different recombination patterns.

Gene conversion increases genomic diversity by recombining parental DNA sets during meiosis. Offspring inherit one gene set from each parent, and sister chromatids undergo gene conversion, creating novel chromosomes different from either parent. This recombination of sequences forms the molecular basis of genomic evolution, generating new amalgamated sequences that determine cell fitness and survival.

Spo11 enzyme deliberately creates double-strand breaks during meiosis by cleaving the phosphodiester backbone, unlike accidental breaks in mitosis. The MRX protein complex then trims the broken helical ends, preparing them for repair. Together, these proteins initiate the gene conversion process that repairs damage while promoting genetic recombination and diversity.

Heteroduplex DNA forms when a strand from donor DNA pairs with a complementary strand from acceptor DNA during gene conversion. This occurs after the acceptor strand invades the donor duplex and creates a displacement loop. The heteroduplex regions represent mismatched base pairing between the two DNA sources, which DNA polymerase extends to complete the repair process.

Gene conversion alters the distribution of alleles between homologous chromosomes, changing a gene's form or manifestation in offspring. Minor sequence differences between homologous chromosomes don't affect gene function but determine which allele is expressed. For example, a hair color gene's allele determines whether hair is black, blonde, or red, and gene conversion redistributes these allelic variants among offspring.