Which enzyme repairs breaks in the DNA backbone?

• A. DNA Polymerase
• B. Helicase
• C. DNA Ligase
• D. RNA Ligase

Answer: (C)

Sealing breaks in the DNA backbone is accomplished by DNA ligase, which forms the necessary phosphodiester bonds to join broken DNA ends and restore a continuous sugar-phosphate backbone. It does this by activating the ends with energy from ATP (in eukaryotes) or NAD+ (in bacteria) and then catalyzing the bond formation. DNA polymerase synthesizes new DNA and can fill in small gaps, but it does not seal the backbone. Helicase unwinds the DNA double helix rather than repairing backbone breaks. RNA ligase joins RNA strands, not DNA. So the enzyme that repairs breaks in the DNA backbone is DNA ligase.