In which step is DNA ligase used during recombinant-DNA construction?

Correct answer: Joining the cut vector and insert

In recombinant-DNA construction, restriction enzymes first cut the vector and the desired DNA insert at selected sites. The resulting ends are brought together, and DNA ligase then seals the sugar-phosphate backbones by forming phosphodiester bonds. This produces a covalently closed recombinant plasmid or another recombinant DNA molecule. Thus option A is correct because it identifies the joining or ligation step. Option B describes the earlier cutting step performed by a restriction enzyme, not by ligase. Gel electrophoresis separates DNA fragments for analysis, and boiling a protein has no role in ligating DNA fragments. The sequence of cutting followed by ligation is central to cloning.