Which enzyme seals Okazaki fragments on the lagging strand?

• A. Helicase
• B. DNA polymerase
• C. Primase
• D. Ligase

Answer: (D)

During DNA replication, the lagging strand is made in short segments called Okazaki fragments because DNA polymerase can only synthesize DNA in one direction (5' to 3'). After the RNA primers are removed and the gaps filled, these fragments must be joined to form a continuous strand. DNA ligase performs this final sealing step by forming phosphodiester bonds between the fragments, effectively covalently linking them and closing the backbone nick. Helicase unwinds the DNA, primase lays down the RNA primers, and DNA polymerase synthesizes and fills in the DNA, but the action that actually seals the fragments into one continuous strand is ligase.