 In this pathway , a three @-@ subunit enzyme complex called RecBCD initiates recombination by binding to a blunt or nearly blunt end of a break in double @-@ strand DNA . After RecBCD binds the DNA end , the RecB and <unk> subunits begin unzipping the DNA duplex through helicase activity . The RecB subunit also has a nuclease domain , which cuts the single strand of DNA that emerges from the unzipping process . This unzipping continues until RecBCD encounters a specific nucleotide sequence ( 5 ' <unk> @-@ 3 ' ) known as a Chi site . 
