In molecular genetics, which enzyme complex unwinds short stretches of the DNA double helix at the replication fork so that polymerases can copy the strands?

Introduction / Context:
DNA replication requires coordinated enzymes that open, stabilize, and copy parental strands. A common point of confusion is distinguishing the enzyme that directly unwinds base pairs at the fork from enzymes that relax supercoils or stabilize single strands. This question checks precise roles at the replication fork.

Given Data / Assumptions:

• The site of interest is the replication fork.
• We are focusing on the enzyme that separates the two DNA strands (breaks base pairing by ATP-driven translocation).
• Other fork-associated proteins (topoisomerase, SSB, primase) may be present but have different tasks.

Concept / Approach:
Helicases are motor proteins that hydrolyze ATP to move along nucleic acid and disrupt hydrogen bonds between complementary bases, creating single-stranded templates. Topoisomerases (including gyrase) relieve torsional strain ahead of the fork by transiently breaking and rejoining DNA but do not separate base pairs at the fork. SSB proteins bind exposed single strands to prevent re-annealing. Primase synthesizes short RNA primers for initiation but does not unwind DNA.

Step-by-Step Solution:
Identify the action: strand separation at the fork requires ATP-driven motor activity.Match function to enzyme: helicase performs strand separation.Differentiate from topoisomerase: relieves supercoils but does not unzip base pairs at the fork.Differentiate from SSB: stabilizes single strands after helicase action.Differentiate from primase: lays RNA primers on the exposed template.

Verification / Alternative check:
Biochemistry texts describe DnaB (prokaryotic replicative helicase) encircling one strand and advancing 5′→3′ (or 3′→5′ depending on system) to open the fork while topoisomerase acts further upstream to prevent overwinding.

Why Other Options Are Wrong:

• DNA polymerases: synthesize DNA; they require an already unwound template.
• SSB proteins: bind single strands; they do not unwind duplex DNA.
• Topoisomerases: relax supercoils; they are not the primary strand-separating motor at the fork.
• Primase: makes RNA primers; no unwinding activity.

Common Pitfalls:
Equating “unwinding ahead of the fork” with topoisomerase activity. Topoisomerase prevents torsional buildup; helicase performs the base-pair separation at the fork itself.

Final Answer:
Helicases