Which enzyme seals nicks between Okazaki fragments on the lagging strand?

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

Answer: (A)

Sealing nicks between Okazaki fragments on the lagging strand is done by DNA ligase. On the lagging strand, DNA is synthesized in short fragments called Okazaki fragments, each starting with an RNA primer. DNA polymerase III extends each fragment, and DNA polymerase I removes the RNA primers and fills in the gaps with DNA. This leaves nicked sugar-phosphate backbones at the junctions, which must be joined to create a continuous strand. DNA ligase is the enzyme that forms the final phosphodiester bond, effectively “gluing” the fragments together. In bacteria, ligase uses NAD+ as a cofactor, while in eukaryotes it uses ATP. Primase makes the primers, and polymerase III builds the DNA; polymerase I fills gaps but does not seal the nicks.