Guanine forms how many hydrogen bonds with cytosine?

Guanine forms how many hydrogen bonds with cytosine?

• A. Two
• B. Three
• C. Four
• D. One

Answer: (B)

Guanine and cytosine form three hydrogen bonds in DNA base pairing. This happens because the two bases have complementary donors and acceptors that line up to create three distinct hydrogen-bond interactions, making GC pairs relatively strong. The result is that GC pairs are more stable than AT pairs, which share only two hydrogen bonds. The other counts don’t match the actual pairing: two would correspond to A–T, four would indicate an interaction that doesn’t occur in standard Watson–Crick pairing, and one would be far too weak to stabilize the double helix.

Guanine and cytosine form three hydrogen bonds in DNA base pairing. This happens because the two bases have complementary donors and acceptors that line up to create three distinct hydrogen-bond interactions, making GC pairs relatively strong. The result is that GC pairs are more stable than AT pairs, which share only two hydrogen bonds. The other counts don’t match the actual pairing: two would correspond to A–T, four would indicate an interaction that doesn’t occur in standard Watson–Crick pairing, and one would be far too weak to stabilize the double helix.