The base pairing rules for DNA are governed by the complementary base pairs: adenine (A) with thymine (T) in an A-T pairing and cytosine (C) with guanine (G) in a C-G pairing. Conversely, thymine only binds with adenine in a T-A pairing and guanine only binds with cytosine in a G-C pairing.
Deoxyribonucleic acid, or DNA, contains the entire set of information essential for the survival of an organism. This set of instructions are encoded in a double-helix stranded structure composed of nucleotide monomers. Each nucleotide carries a phosphate group, a five-carbon sugar called a deoxyribose and one of four nucleobases. The four nitrogen-containing bases found in DNA are A, T, C and G. A and G are classified as "purines," while C and T are considered as "pyrimidines." Purines are bigger in size compared to pyrimidines.
An important discovery regarding the structure of DNA was made by Edwin Chargaff in 1949. In one of his experiments, Chargaff illustrated that the quantity of A is equal to that of T, while the quantity of C is equal to that of G. He then concluded that the complementary base of A must be T and the complementary base of C must be G. Chargaff's findings formed the basis for the base pairing principle of DNA.