Which description best matches the role of DNA ligase in constructing plasmids that contain donor DNA?

• A. Annealing
• B. Restriction of DNA
• C. Backbones joined via phosphodiester bonds
• D. Splicing

Answer: (C)

DNA ligase’s job is to seal breaks in the DNA backbone by forming phosphodiester bonds between adjacent nucleotides. In plasmid construction, after donor DNA and vector ends have been prepared (often created by restriction enzymes), there are nicks and exposed ends. The ligase catalyzes the formation of covalent sugar-phosphate links, effectively joining the backbones and producing a continuous circular plasmid that contains the donor insert. This specific action—creating phosphodiester bonds to connect the DNA strands—is why the description of backbones being joined via phosphodiester bonds is the best match.

Annealing refers to base-pairing between complementary sequences, not the chemical sealing of the backbone. Restriction of DNA describes cutting DNA at specific sites, not joining. Splicing can refer to joining fragments in some contexts, but the precise chemical event in plasmid construction is the formation of phosphodiester bonds that ligase catalyzes.