What is the function of DNA ligase in gene cloning?

• A. Cuts DNA at specific sites.
• B. Unwinds the DNA.
• C. Synthesizes RNA.
• D. Seals the sugar-phosphate backbone to join DNA fragments.

Answer: (D)

DNA ligase’s job is to seal the sugar-phosphate backbone to join DNA fragments into a continuous strand. In gene cloning, after fragments are cut and prepared, ligase form phosphodiester bonds between the ends, using energy from ATP (or NAD+ in some systems) to drive the reaction. This creates a stable recombinant plasmid that can be replicated in a host cell. Without this sealing step, the fragments would remain separate and couldn’t be propagated. The other activities belong to different enzymes: cutting DNA at specific sites is done by restriction enzymes, unwinding DNA is done by helicase, and synthesizing RNA is the job of RNA polymerase.