What enzyme seals nicks in the sugar-phosphate backbone after replication?

• A. DNA ligase
• B. DNA polymerase
• C. Helicase
• D. Primase

Answer: (A)

DNA ligase is the enzyme responsible for sealing nicks in the sugar-phosphate backbone after replication. On the lagging strand, DNA is built in short fragments called Okazaki fragments. After the RNA primers are removed and replaced with DNA, gaps remain between these fragments. DNA ligase catalyzes the formation of the missing phosphodiester bonds, linking the fragments into a continuous strand. It uses energy from ATP in eukaryotes (or NAD+ in prokaryotes) to drive the reaction. Other enzymes have different roles: DNA polymerase builds new DNA, helicase unwinds the double helix, and primase lays down the RNA primers. But only DNA ligase completes the backbone by sealing the nicks.