The RecQ DNA helicases： Jacks-of-all-trades or master-tradesmen？

Homologous recombination occurs when a damaged chromosome uses an intact homologous chromosome as a template for its repair. The main steps of recombination are most readily illustrated for the repair of a DNA doublestrand-break （DSB）. First, DSB-ends are processed to form single-stranded tails, which assemble into nucleoprotein complexes comprising an oligomeric filament of a RecA-family protein （Rad51 in eukaryotes） and associated factors. Rad51 filaments catalyze homologous pairing and strand-exchange between a DSB-end and a double-stranded template to form a joint molecule intermediate. This structure allows de novo priming of DNA synthesis to restore sequences that were lost or damaged at the site of the original lesion. Recombination also underpins chromosome replication by facilitating the repair of broken replication forks. In this case, joint molecule formation allows rep- lication to reinitiate. At the final step of recombination, strand-exchange ＂Holliday＂ junctions that connect the involved chromosomes are resolved so that segregation can ensue. Joint molecule resolution can occur with one of two outcomes： a crossover, in which chromosome arms are exchanged; or a noncrossover without exchange.