During DNA replication, what is the primary role of DNA ligase?

Introduction / Context:
DNA replication on the lagging strand proceeds discontinuously as short Okazaki fragments. Converting these fragments into a continuous strand requires a sealing step that completes the sugar-phosphate backbone.

Given Data / Assumptions:

• Okazaki fragments are produced after priming and polymerization.
• RNA primers are removed and replaced with DNA.
• A final enzymatic step must close remaining nicks.

Concept / Approach:
DNA ligase catalyzes phosphodiester bond formation between a 3'-OH and a 5'-phosphate at single-strand breaks (nicks), using an energy cofactor (ATP in eukaryotes, NAD+ in many bacteria). This reaction yields a continuous DNA strand.

Step-by-Step Solution:

Verification / Alternative check:
In vitro replication assays show loss of ligase yields discrete fragments; adding ligase restores high-molecular-weight DNA, confirming its function.

Why Other Options Are Wrong:

• Synthesizes lagging strand from scratch: DNA polymerases synthesize; ligase does not polymerize de novo.
• Copies mRNA to DNA: that is reverse transcriptase, not ligase.
• Degrades mRNA: RNases perform this function.
• Unwinds helix: helicase performs unwinding.

Common Pitfalls:
Confusing sealing of nicks (ligase) with primer removal (RNase H/Pol I flap removal) or with helicase action.

Final Answer:
Seals nicks by joining adjacent DNA fragments