DNA Replication—Opening the Replication Bubble Which protein class is primarily responsible for opening a replication bubble by separating parental DNA strands?

Introduction / Context:
DNA replication begins at origins where a replication bubble forms as parental strands are separated. Understanding which enzymes initiate strand separation versus those that stabilize or process DNA is fundamental to replication biology.

Given Data / Assumptions:

• Replication requires helicase, SSB, topoisomerase, primase, polymerase, clamp, and ligase.
• Question asks for the protein class that actively opens (unwinds) the duplex.

Concept / Approach:
Helicases use NTP hydrolysis to translocate along DNA and separate complementary strands, generating single-stranded templates. SSB binds and stabilizes exposed single strands; topoisomerase relieves torsional stress; ligase seals nicks after synthesis; sliding clamp enhances polymerase processivity. Only helicase directly unwinds to create the bubble.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical assays show helicase-dependent DNA unwinding; inhibition of helicase blocks bubble formation despite presence of SSB or topoisomerase.

Why Other Options Are Wrong:

• SSB stabilizes but does not unwind.
• Ligase seals nicks; no unwinding function.
• Topoisomerase resolves supercoils but does not separate base pairs.
• Sliding clamp increases polymerase processivity; not an unwinding factor.

Common Pitfalls:
Attributing the opening to SSB because it binds single-stranded DNA; overlooking that SSB acts after helicase action.

Final Answer:
DNA helicases