Which enzyme is the bacterial helicase that unwinds DNA at the replication fork?

• A. DnaB
• B. DNA polymerase III
• C. Topoisomerase
• D. SSB

Answer: (A)

Unwinding the DNA double helix at the replication fork is the job of a helicase, which uses energy from ATP to separate the two strands so the polymerases can copy each template. In bacteria, the helicase responsible for this task is DnaB. It forms a ring-shaped motor that moves along the DNA to open the fork, typically acting on the lagging-strand template in a 5' to 3' direction. DnaB is loaded onto the DNA by DnaC and works together with primase DnaG to lay down RNA primers for synthesis. The other proteins you’ll encounter have different roles: DNA polymerase III carries out the actual DNA synthesis, topoisomerase relieves the torsional strain from unwinding, and SSB stabilizes the single-stranded DNA. Therefore, the bacterial helicase that unwinds DNA at the replication fork is DnaB.