What is the role of DNA ligase in replication?

• A. Joins the sugar-phosphate backbones of Okazaki fragments
• B. Unwinds the DNA double helix
• C. Synthesizes RNA primers
• D. Proofreads nucleotides

Answer: (A)

DNA ligase’s job is to seal the backbone of DNA by forming phosphodiester bonds between adjacent nucleotides. On the lagging strand, DNA is synthesized as short fragments called Okazaki fragments. After these fragments are completed, RNA primers are removed and the gaps filled in, and ligase knits the fragments together to make a continuous strand. This sealing step is what converts a series of fragments into a complete, intact DNA molecule. Other enzymes handle different tasks: helicase unwinds the double helix, primase lays down RNA primers to start synthesis, and DNA polymerase proofreads nucleotides during replication. Ligase uses energy (ATP in bacteria, NAD+ in eukaryotes) to drive the bond-formation process that joins the fragments.