RecA-mediated DNA strand exchange. (a) The DNA substrates, intermediates, and products of the DNA strand exchange reaction. The circular DNAs, derived from bacteriophages, have a total length of 5,000 to 8,000 nucleotides or base pairs. With DNAs of this size, the reaction time is measured in tens of minutes at 37°C. (b) Intermediates (as diagrammed to the right of the agarose gel) generated by the initial strand invasion. Complete exchange of strands requires ATP hydrolysis. No branch migration occurs when ATPγS, a nonhydrolyzable ATP analog, replaces ATP (lanes 1 to 5).