Homologous recombination is a biological process in which genetic information is exchanged or copied between DNA molecules containing identical or closely similar sequences. It is a major mechanism of DNA repair, particularly for double-strand breaks, and underlies much of the genetic exchange occurring during meiosis. Rather than simply joining broken ends, it uses a matching DNA sequence to guide repair or rearrangement. Its products may involve reciprocal exchange of chromosome segments, localized copying of information, or restoration of damaged DNA without a crossover. (ncbi.nlm.nih.gov)
Biological functions and templates
Homologous recombination helps preserve the integrity of the genome when both strands of a DNA molecule are broken or when problems arise during DNA replication. A matching, undamaged sequence supplies information that may be unavailable at the damaged site. Recombination also participates in repairing single-stranded gaps and in processing some lesions that obstruct replication. These functions connect chromosome maintenance with the accurate transmission of genetic information. (pmc.ncbi.nlm.nih.gov)
In dividing eukaryotic cells, the preferred repair template is usually a sister chromatid, the replicated copy of a chromosome. Consequently, repair by homologous recombination is closely associated with the post-replication stages of the cell cycle. During meiosis, however, recombination is directed toward a homologous chromosome, allowing exchange between the maternally and paternally inherited copies. The template therefore need not be completely identical to the damaged molecule. (pmc.ncbi.nlm.nih.gov)
Molecular mechanism
A common starting point is a DNA double-strand break. Processing of the broken ends, termed end resection, removes DNA from the 5′-ending strands and exposes single-stranded tails with 3′ ends. Recombination proteins assemble on these tails and promote the search for a matching sequence in an intact DNA duplex. The sequence comparison depends on complementary base pairing rather than recognition of an entire chromosome as a unit. (ncbi.nlm.nih.gov)
In bacteria, the principal strand-exchange protein is RecA. In eukaryotes, the related RAD51 protein performs the central strand-exchange function. These proteins form nucleoprotein filaments on single-stranded DNA. In humans, BRCA2 helps regulate RAD51 activity and its recruitment to damaged DNA. In Escherichia coli, the RecBCD complex processes double-strand breaks before RecA-mediated pairing. (medlineplus.gov)
During strand invasion, a single-stranded tail pairs with the complementary strand of the template duplex, displacing the other strand and forming a displacement loop, or D-loop. The invading 3′ end can then be extended by DNA polymerase, copying the intact template. Subsequent steps recover the second broken end and restore continuous DNA molecules, although the exact intermediates depend on the pathway followed. (ncbi.nlm.nih.gov)
Pathways and genetic outcomes
Homologous recombination is a family of related pathways, not a single obligatory sequence of reactions. In the double-strand-break repair model, engagement of both broken ends can produce two Holliday junctions: four-way DNA structures connecting the participating molecules. Processing these junctions separates the molecules. Enzymatic cleavage can yield crossover or noncrossover products, whereas dissolution of a double junction yields noncrossover products. (pubmed.ncbi.nlm.nih.gov)
In synthesis-dependent strand annealing, the newly extended invading strand leaves its template and anneals with the other broken end. This route generally produces noncrossover repair without forming a stable double Holliday junction. Thus, successful homologous repair does not necessarily exchange the chromosome regions flanking the repaired site. (pubmed.ncbi.nlm.nih.gov)
A distinct outcome is gene conversion, the nonreciprocal transfer of sequence information. It can occur when repair synthesis copies a template carrying a different allele, or when DNA mismatch repair resolves differences within heteroduplex DNA formed during recombination. Gene conversion may accompany either crossover or noncrossover events; it is not synonymous with reciprocal exchange. (ncbi.nlm.nih.gov)
Meiosis and bacterial genetic exchange
During meiosis, recombination is commonly initiated by programmed breaks made by the Spo11 enzyme. Repair using the homologous chromosome produces some events that become chromosomal crossovers. Together with sister-chromatid cohesion, these exchanges help maintain connections between homologs until their separation at the first meiotic division. They also create new combinations of inherited alleles, contributing to genetic diversity. Breaks and crossovers are regulated and are not distributed uniformly along chromosomes. (ncbi.nlm.nih.gov)
Bacterial recombination serves both chromosome repair and genetic exchange. DNA acquired through horizontal gene transfer can become integrated into a recipient chromosome when it contains sufficiently similar sequences. The requirement for sequence similarity distinguishes this mechanism from processes that insert DNA primarily through recognition of short, specific sites. (ncbi.nlm.nih.gov)
Accuracy and alternative repair
Homologous recombination can restore missing information accurately when the correct template is used, but it is not intrinsically error-free. Recombination between similar sequences at inappropriate positions can generate deletions, duplications, or other rearrangements. Regulation of template choice and intermediate processing is therefore important for genome stability. (pmc.ncbi.nlm.nih.gov)
The pathway differs from non-homologous end joining, which reconnects DNA ends without requiring an extensive homologous template. The relative use of these pathways depends on the organism, cell type, cell-cycle stage, and nature of the damage. Neither pathway can be characterized adequately as universally accurate or universally mutagenic. (pmc.ncbi.nlm.nih.gov)
Research applications
Homologous recombination enables targeted replacement or modification of a gene using introduced DNA with matching flanking sequences. Its application in mouse embryonic stem cells established a foundation for gene-targeted and knockout mice. These developments were recognized by the 2007 Nobel Prize in Physiology or Medicine. (nobelprize.org)
In genome editing, targeted nucleases such as Cas9 can stimulate template-directed modification by creating a break near the intended change. An introduced donor sequence can supply the replacement information. The resulting edit depends on cellular repair, however: nuclease cleavage alone does not ensure homologous recombination or precise incorporation of the donor. (pmc.ncbi.nlm.nih.gov)