In recombinant plasmid construction, which enzyme joins the wanted gene to the plasmid?

• A. Reverse transcriptase
• B. DNA ligase
• C. DNA polymerase
• D. RNA polymerase

Answer: (B)

The key idea is that joining separate DNA pieces to form a single, continuous molecule is done by DNA ligase. After the gene fragment and the plasmid are prepared with compatible ends, DNA ligase catalyzes the formation of phosphodiester bonds between the sugar-phosphate backbones, sealing the nicks and creating a stable recombinant plasmid. This sealing step is what makes the inserted gene a permanent part of the plasmid so it can be replicated in the host. Other enzymes listed have different roles: reverse transcriptase makes DNA from an RNA template, DNA polymerase copies or extends DNA strands but does not fuse separate DNA pieces, and RNA polymerase transcribes RNA from a DNA template.