what is the base pairing rule for dna

The base pairing rule for DNA states that adenine (A), a purine, always pairs with thymine (T), a pyrimidine, and guanine (G), a purine, always pairs with cytosine (C), a pyrimidine. These pairs are held together by hydrogen bonds: A and T form two hydrogen bonds, while G and C form three hydrogen bonds. This complementary pairing is known as Watson-Crick base pairing and ensures the DNA double helix maintains a consistent width and stable structure. The rule also explains why the amount of adenine equals thymine and the amount of guanine equals cytosine in DNA, a relationship known as Chargaff's rule