What nucleotide pairs with guanine?

The story behind the answer

Cytosine pairs with guanine in DNA because their shapes and chemical groups are complementary, allowing them to form three hydrogen bonds. This G–C pairing (a purine with a pyrimidine) keeps the DNA double helix uniform in width and contributes to its stability. In contrast, adenine pairs with thymine (A–T) in DNA and with uracil (A–U) in RNA, each with two hydrogen bonds.

Why it matters: Regions rich in G–C pairs have higher melting temperatures because three bonds hold them together more tightly than A–T pairs. A handy memory aid: “G–C = Greater Count” of bonds (3), while A–T has 2. This pattern was central to Chargaff’s rules, which helped Watson and Crick deduce DNA’s structure.