How many hydrogen bonds stabilize A-T and G-C base pairs?

• A. A-T pairs form one hydrogen bond; G-C pairs form two.
• B. A-T pairs form three hydrogen bonds; G-C pairs form two.
• C. A-T pairs form two hydrogen bonds; G-C pairs form three.
• D. A-T pairs form four hydrogen bonds; G-C pairs form five.

Answer: (C)

The stability of a DNA double helix comes from hydrogen bonding between complementary bases. Adenine pairs with thymine using two hydrogen bonds, while guanine pairs with cytosine using three hydrogen bonds. The extra bond in G-C makes those pairs more stable, contributing to higher melting temperatures in GC-rich regions. That’s why A-T is stabilised by two hydrogen bonds and G-C by three.