Which enzyme seals nicks in the sugar-phosphate backbone after primer replacement?

• A. DNA polymerase I
• B. DNA helicase
• C. RNA primase
• D. DNA ligase

Answer: (D)

Sealing nicks in the sugar-phosphate backbone after primer replacement is performed by DNA ligase. During replication, especially on the lagging strand, RNA primers laid down by primase are replaced with DNA, leaving small gaps between newly synthesized fragments. DNA polymerase I fills in the gaps, but the backbone remains nicked at the junctions. DNA ligase completes the job by forming a phosphodiester bond that links adjacent DNA fragments into a continuous strand. The other enzymes are involved in unwinding the DNA (helicase), laying down primers (primase), or synthesizing DNA (polymerases) but do not seal the backbone themselves. DNA ligase is the enzyme that finalizes the connection, restoring backbone integrity.