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

• A. DNA polymerase
• B. RNase
• C. Helicase
• D. DNA ligase (correct)

Answer: (D)

Sealing the gaps between Okazaki fragments is about sticking the fragments together with a proper sugar-phosphate backbone. After the lagging strand is synthesized in short pieces, the RNA primers are removed and the gaps filled, leaving a nick between fragments. DNA ligase carries out the final joining step by forming a phosphodiester bond between the adjacent DNA ends, effectively stitching the fragments into one continuous strand. Energy is required for this bond formation (in bacteria it uses NAD+, in eukaryotes ATP).

Other enzymes have different jobs: DNA polymerase builds new DNA by adding nucleotides; RNase removes RNA primers; helicase unwinds the DNA helix. The key enzyme that actually seals the gaps is DNA ligase.