Which enzyme seals nicks in the sugar-phosphate backbone during replication?

• A. DNA polymerase I
• B. DNA polymerase III
• C. Helicase
• D. DNA ligase

Answer: (D)

Sealing nicks in the sugar-phosphate backbone during replication is done by DNA ligase. After the lagging strand is synthesized in short Okazaki fragments, the RNA primers are removed and gaps are filled with DNA, leaving nicks between fragments. DNA ligase catalyzes the formation of a new phosphodiester bond to join the ends and create a continuous strand. This step uses energy from a cofactor (NAD+ in bacteria or ATP in eukaryotes). Other enzymes have different roles: helicase unwinds the DNA, and polymerases synthesize new DNA (polymerase III mainly for synthesis; polymerase I can replace primers and fill gaps but does not seal the backbone). The ligase is the one responsible for the final stitching that completes the backbone.