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

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

Answer: (D)

During replication on the lagging strand, DNA is made in short segments called Okazaki fragments. After each fragment is extended, the RNA primer is removed and replaced with DNA, leaving a nick between fragments. DNA ligase is the enzyme that seals this nick by forming a phosphodiester bond to join the adjacent fragments, making the strand continuous. The other enzymes have different roles: helicase unwinds the double helix, RNA polymerase (primase) lays down the RNA primers, and DNA polymerase I fills in the DNA where primers were removed but does not seal the backbone. So, the sealing of the gaps is done by DNA ligase.