Nucleotide

In the double helix of DNA, guanine nucleotides are base-paired opposite cytosine nucleotides. Adenine nucleotides are base-paired opposite thymine nucleotides. This pairing is due to the complementary natures of the structures involved. Note that G is a two-ringed purine, while its partner C is a The molecular structures of the five nitrogenous bases. one-ringed pyrimidine. Similarly, A is a purine and T is a pyrimidine. These pairings give the interior of the helix a fixed diameter, without bulges or gaps. Just as importantly, the arrangement of atoms in the rings allows the partners to form sets of weak attractions, called hydrogen bonds, across the interior of the helix. The hydrogen bonds contribute greatly to the stability of the double helix, and the specificity of the G-C, A-T pairing is the structural basis of faithful replication of DNA.