the search process induces stretching of the dna duplex, which enhances sequence complementarity recognition (a mechanism termed conformational proofreading). the reaction initiates the exchange of strands between two recombining dna double helices. after the synapsis event, in the heteroduplex region a process called "branch migration" begins. in branch migration an unpaired region of one of the single strands displaces a paired region of the other single strand, moving the branch point without changing the total number of base pairs. spontaneous branch migration can occur, however as it generally proceeds equally in both directions it is unlikely to complete recombination efficiently. the reca protein catalyzes unidirectional branch migration and by doing so makes it possible to complete recombination, producing a region of heteroduplex dna that is thousands of base pairs long.