What is DNA ligase?

• A. Enzyme that copies DNA.
• B. Enzyme that unwinds DNA.
• C. Enzyme that cuts DNA.
• D. Enzyme that connects the DNA piece to the plasmid.

Answer: (D)

DNA ligase is the enzyme that seals breaks in the DNA backbone, forming the phosphodiester bond between adjacent nucleotides. In plasmid cloning, after a DNA fragment and a plasmid are cut to create compatible ends, ligase connects them to produce a single, continuous DNA molecule that can be replicated in cells. It uses energy from ATP (in many organisms) or NAD+ (in others) to make this bond. This is why the statement is about connecting the DNA piece to the plasmid. Copying DNA is done by DNA polymerase, unwinding DNA is done by helicase, and cutting DNA is done by restriction enzymes or nucleases, not ligase.