Helicase

A helicase is a motor protein that travels along DNA or RNA and pulls apart the two strands of a double helix by disrupting the hydrogen bonds that hold paired bases together, using energy from the breakdown of ATP. The best-known helicases act at the start of DNA replication. Proteins first mark an origin of replication, a helicase is loaded there, and as it unwinds the DNA it opens Y-shaped replication forks that move outward in both directions.

At each fork the helicase works with a team of other proteins. Single-strand binding proteins cling to the exposed strands so they do not pair up again, primase lays down short RNA primers, and DNA polymerase builds the new strands. Unwinding overtwists the DNA ahead of the fork, a strain that topoisomerases relieve by briefly cutting and rejoining it. Replicative helicases are usually rings of six subunits that encircle one strand, such as DnaB in Escherichia coli and the MCM complex in eukaryotes.

Other helicases unwind DNA for repair, recombination and transcription, and RNA helicases reshape RNA molecules. Inherited faults in some helicases cause disease: faulty copies of WRN cause Werner syndrome, with early signs of aging, and faulty copies of BLM cause Bloom syndrome; both conditions raise cancer risk.