RecA protein filaments. RecA and other recombinases in this class function as filaments of nucleoprotein. (a) Segment of a RecA filament with four helical turns (24 RecA subunits). Notice the bound double-stranded DNA in the center. The core domain of RecA is structurally related to the domains in helicases. (b) Electron micrograph of a RecA filament bound to DNA. (c) Filament formation proceeds in discrete nucleation and extension steps. Extension occurs by adding RecA subunits so that the filament grows in the 5′→3′ direction. When disassembly occurs, subunits are subtracted from the trailing end.