Various other proteins play roles in other replication tasks; some of these are shown in Focus: Key Figure 13.13. The first event at the origin of replication is the localized unwinding and separation (denaturation) of the DNA strands. As we discussed in Key Concept 13.2, the two strands are held together by hydrogen bonds and van der Waals forces. An enzyme called DNA helicase uses energy from ATP hydrolysis to unwind and separate the strands, and single-strand binding proteins bind to the unwound strands to keep them from reassociating into a double helix. This process makes each of the two template strands available for complementary base pairing.