Which enzyme performs the joining step in making a recombinant plasmid?

Correct answer: DNA ligase

The governing principle of recombinant-DNA construction is that a vector and an insert must be joined into one continuous DNA molecule. After restriction enzymes create compatible ends, DNA ligase brings the ends together and catalyses phosphodiester-bond formation in the sugar-phosphate backbones. Thus option A, DNA ligase, performs the joining step and helps produce the recombinant plasmid. DNA helicase separates strands rather than joining them. RNA polymerase synthesises RNA from a DNA template, and ribonuclease degrades RNA. Those enzymes may have important cellular functions, but none carries out covalent joining of the plasmid vector and inserted DNA in this procedure.