Helicase

The helicase is an enzyme that promotes the separation of nucleic acid strands breaking the hydrogen bonds between base pairs so that replication may occur. This enzyme moves along the DNA double helix using energy from ATP hydrolysis. Helicases are also utilized to separate a self-annealed RNA molecule.
During this process helicases are helped by topoisomerases, enzymes that regulate the overwinding or underwinding of DNA. DNA becomes overwound ahead of a replication fork and if topoisomerase don’t act the replication process could get to a halt due to the tension.

Structure of E. coli's helicase RuvA

http://www.dnatube.com/video/56/Structure-of-DNA-helicase