Which statement correctly describes the role of complementary base pairing in joining DNA fragments with sticky ends?

• A. It allows sticky ends to anneal
• B. It prevents ligation
• C. It destabilizes DNA ends
• D. It is unrelated to ligation

Answer: (A)

Complementary base pairing provides the specificity that lets sticky ends recognize and bind to their matching overhangs. The short single-stranded overhangs produced by restriction enzymes are designed to be complementary, so they form hydrogen bonds and anneal, creating a unified, correctly oriented junction between the fragments. This annealed alignment brings the DNA ends into close proximity and the proper geometry, which is essential for DNA ligase to seal the backbone and finish joining the fragments. So the role of base pairing here is to enable the ends to pair up and anneal, setting the stage for ligation. It does not prevent ligation or destabilize the ends; it directly facilitates the process by providing the initial, specific connection that ligase then fixes with a phosphodiester bond.