Which statement about PCNA is true?

• A. PCNA serves as the sliding clamp to increase polymerase processivity.
• B. PCNA is the helicase that unwinds DNA.
• C. PCNA seals nicks in the DNA backbone.
• D. PCNA loads RNA primers.

Answer: (A)

PCNA acts as a sliding clamp that encircles DNA and keeps DNA polymerase attached to the template, dramatically increasing its processivity during DNA replication. By forming a ring around the DNA, PCNA tethers the polymerase so it can synthesize many nucleotides without frequently dissociating, which speeds up replication and helps both the leading and lagging strands. This clamping action also provides a platform to recruit and coordinate other replication and repair proteins that interact with polymerases, enhancing overall efficiency.

The other described roles belong to different proteins: unwinding DNA is the job of helicases, not PCNA; sealing nicks in the backbone is done by DNA ligase; and loading RNA primers is the activity of primase. PCNA does not perform these functions, but it can help position proteins that do during replication and repair.