Rad51

Rad51 is the eukaryotic homolog of bacterial RecA, a recombinase protein that catalyzes the central reaction of homologous recombination: the search for homology and the exchange of DNA strands between a damaged duplex and an intact repair template. It is essential for the repair of DNA double-strand breaks in mitosis and for crossing over during meiosis, where it works alongside its meiosis-specific paralog Dmc1.

The protein forms a right-handed helical nucleoprotein filament on single-stranded DNA, coating approximately six nucleotides per monomer. This filament is the active search structure: it probes double-stranded DNA for sequence homology through a combination of three-dimensional diffusion and one-dimensional scanning, then catalyzes strand invasion to form a displacement loop (D-loop). The process is regulated by dozens of mediator proteins — including BRCA2, RAD52, and the Rad51 paralogs (RAD51B, RAD51C, RAD51D, XRCC2, XRCC3) — that control filament assembly, disassembly, and the choice of homologous template.

Rad51 is not merely a repair enzyme. It is a system coordinator whose activity is integrated with cell cycle checkpoints, replication fork stability, and telomere maintenance. In mitosis, it ensures genomic fidelity; in meiosis, it generates the diversity that drives evolution. The same protein, repurposed by regulatory context, performs opposite functions: conservation and innovation. This is the molecular demonstration that the genome's stability machinery is also its change machinery — a duality that confounds simple distinctions between repair and recombination.