 During DNA replication , double @-@ strand breaks can sometimes be encountered at replication forks as DNA helicase unzips the template strand . These defects are repaired in the break @-@ induced replication ( BIR ) pathway of homologous recombination . The precise molecular mechanisms of the BIR pathway remain unclear . Three proposed mechanisms have strand invasion as an initial step , but they differ in how they model the migration of the D @-@ loop and later phases of recombination . 
