During initiation of DNA replication, which protein actively unwinds the parental double helix ahead of the replication fork?

Introduction / Context:
DNA replication requires coordinated action of multiple enzymes and accessory proteins. A critical early step is separation of the two parental strands to allow templated synthesis of new DNA.

Given Data / Assumptions:

• We are focusing on the enzyme that actively breaks hydrogen bonds between bases.
• The replication fork advances directionally along duplex DNA.

Concept / Approach:
Helicases are ATP-dependent motors that translocate along DNA and unwind the duplex by disrupting base pairing. Other proteins assist: SSB stabilizes exposed single strands, topoisomerase relieves torsional stress, ligase seals nicks, and primase lays down RNA primers—but only helicase performs the active unwinding.

Step-by-Step Solution:

Verification / Alternative check:
In vitro reconstitution of replication forks shows that without helicase, fork progression stalls even if polymerases, primase, and ligase are present. Addition of helicase restores unwinding and synthesis.

Why Other Options Are Wrong:

• SSB: Binds single-stranded DNA to prevent reannealing but does not separate strands.
• DNA ligase: Joins nicks after synthesis; no unwinding activity.
• Topoisomerase: Relieves supercoils by transient breaks; does not unwind base pairs directly.
• Primase: Synthesizes RNA primers; no helicase function.

Common Pitfalls:
Equating stabilization of single strands with active unwinding, or attributing helicase-like roles to topoisomerases.

Final Answer:
DNA helicase.