Which base pairing forms two hydrogen bonds?

• A. A-T/U
• B. C-G
• C. A-C
• D. G-U

Answer: (A)

Two hydrogen bonds form between adenine and thymine (in DNA) or adenine and uracil (in RNA). This pairing works because adenine has a hydrogen-bond donor on its amino group and an acceptor at a ring nitrogen, while thymine (or uracil) presents a complementary acceptor and donor in the right positions. The result is two favorable hydrogen-bond interactions that stitch the two strands together in the standard Watson–Crick geometry. By contrast, cytosine–guanine pairing involves three hydrogen bonds, making it more stable, and guanine–uracil can form a two-hydrogen-bond wobble pair but is noncanonical and context-dependent. So the canonical two-H-bond pairing is adenine with thymine (or uracil).