homologous recombination

Homologous recombination is the cell's way of exchanging or repairing DNA by lining up two stretches of sequence that closely match, then swapping or copying between them. The shared similarity acts like matching edges of a torn page, letting the pieces be rejoined in exactly the right place.

In meiosis, this is the mechanism behind crossing over: matching regions of homologous chromosomes pair up and trade segments, generating recombinant chromosomes. The same machinery is also a high-fidelity repair pathway: when both strands of DNA are broken, the cell uses an undamaged matching copy, such as a sister chromatid, as a template to rebuild the missing sequence accurately.

Because it relies on an intact matching sequence as a guide, homologous recombination is usually error-free, unlike repair pathways that simply rejoin broken ends. This same template-copying principle is harnessed in the lab: genome-editing tools introduce changes by supplying a designed DNA template that the cell copies during homology-directed repair.