Which enzyme catalyzes the joining of Okazaki fragments on the lagging strand?

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

Answer: (D)

Okazaki fragments on the lagging strand are connected by DNA ligase, which seals the gaps between fragments to form a continuous strand. After primers are laid down and DNA is synthesized in short fragments, the RNA primers are removed and replaced with DNA, leaving nicks at the junctions. DNA ligase catalyzes the formation of phosphodiester bonds to join the ends, completing the sugar-phosphate backbone. The other enzymes have different jobs: helicase unwinds the DNA, primase makes the RNA primers, and DNA polymerase I fills in gaps after primer removal. But the step that literally links the fragments together is performed by DNA ligase.