(in nucleotide) Nitrogen-containing, ring-shaped molecules in DNA and RNA. There are four different bases in a nucleic acid: two with a double-ring structure, called adenine (A) and guanine (G); and three with a single-ring structure, cytosine (C) and either thymine (T), in DNA, or uracil (U), in RNA. It is the sequence of bases which determines the information content of the molecule.