Practice question

RecA family recombinases exhibit strong preference for single-stranded DNA as assembly platform, even though they can weakly associate with duplex. ATP-bound RecA binds cooperatively to ssDNA generated by RecBCD or MRX resection, forming extended nucleoprotein filament where DNA is stretched 1.5-fold and organized in triplets for homology sampling. This filament discriminates against dsDNA because secondary structure melting and exposure of bases is required. In vivo, mediator proteins assist replacement of SSB or RPA on ssDNA to facilitate filament nucleation and subsequent homology search and strand invasion functions.