homologous recombination

Homologous recombination is the cell's accurate way of repairing a double-strand break — and, in a different setting, the engine that shuffles genes between chromosomes. Its single great idea is to use a matching, intact copy of the DNA as a template, so that broken information can be rebuilt perfectly rather than guessed at. 'Homologous' simply means 'matching in sequence'.

When a break occurs, the cell first chews back the broken ends to expose single strands. A key protein coats one of these single strands and uses it to search the genome for a region with the same sequence — typically the identical sister chromatid made during replication, or the matching homologous chromosome. Once found, the searching strand invades that intact duplex, pairing with its complement and forming a crossover-shaped structure (the Holliday junction). A polymerase then copies the missing sequence off the intact template, the structure is resolved, and the break is healed with no loss of information.

This same template-copying machinery does double duty. During meiosis — the cell divisions that make eggs and sperm — homologous recombination is triggered deliberately to swap segments between the maternal and paternal copies of each chromosome, the event called crossing over. That reshuffling is why every child is a genetic mosaic of its grandparents rather than a clone of one parent, making homologous recombination both a guardian of genome integrity and a primary source of the genetic variety on which evolution feeds. Its importance to health is underscored by the BRCA1 and BRCA2 genes, whose protein products are essential for this repair: inheriting a broken copy sharply raises the risk of breast and ovarian cancer.