page_title: Gene conversion
section_title: Mechanism
hierarchical_section_title: Gene conversion / Mechanism
caption_attribution_description: English: A current model of meiotic recombination, initiated by a double-strand break or gap, followed by pairing with an homologous chromosome and strand invasion to initiate the recombinational repair process. Repair of the gap can lead to crossover (CO) or non-crossover (NCO) of the flanking regions. CO recombination is thought to occur by the Double Holliday Junction (DHJ) model, illustrated on the right, above. NCO recombinants are thought to occur primarily by the Synthesis Dependent Strand Annealing (SDSA) model, illustrated on the left, above. Most recombination events appear to be the SDSA type.
context_page_description: Gene conversion is the process by which one DNA sequence replaces a homologous sequence such that the sequences become identical after the conversion event. Gene conversion can be either allelic, meaning that one allele of the same gene replaces another allele, or ectopic, meaning that one paralogous DNA sequence converts another.
context_section_description: Conversion of one allele to the other is often due to base mismatch repair during homologous recombination: if one of the four chromatids during meiosis pairs up with another chromatid, as can occur because of sequence homology, DNA strand transfer can occur followed by mismatch repair.  This can alter the sequence of one of the chromosomes, so that it is identical to the other.
  Meiotic recombination is initiated through formation of a double-strand break (DSB). The 5’ ends of the break are then degraded, leaving long 3’ overhangs of several hundred nucleotides. One of these 3’ single stranded DNA segments then invades a homologous sequence on the homologous chromosome, forming an intermediate which can be repaired through different pathways resulting either in crossovers (CO) or noncrossovers (NCO). At various steps of the recombination process, heteroduplex DNA (double-stranded DNA consisting of single strands from each of the two homologous chromosomes which may or may not be perfectly complementary) is formed. When mismatches occur in heteroduplex DNA, the sequence of one strand will be repaired to bind the other strand with perfect complementarity, leading to the conversion of one sequence to another. This repair process can follow either of two alternative pathways as illustrated in the Figure.  By one pathway, a structure called a double Holliday junction (DHJ) is formed, leading to the exchange of DNA strands. By the other pathway, referred to as Synthesis Dependent Strand Annealing (SDSA), there is information exchange but not physical exchange. Gene conversion will occur during SDSA if the two DNA molecules are heterozygous at the site of the recombinational repair.  Gene conversion may also occur during recombinational repair involving a DHJ, and this gene conversion may be associated with physical recombination of the DNA duplexes on the two sides of the DHJ.
caption: A current model of meiotic recombination, initiated by a double-strand break or gap, followed by pairing with a homologous chromosome and strand invasion to initiate the recombinational repair process. Repair of the gap can lead to crossover (CO) or non-crossover (NCO) of the flanking regions. CO recombination is thought to occur by the Double Holliday Junction (DHJ) model, illustrated on the right, above. NCO recombinants are thought to occur primarily by the Synthesis Dependent Strand Annealing (SDSA) model, illustrated on the left, above. Most recombination events appear to be the SDSA type.
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