Base pairing in RNA. Nucleobases in blue. Nucleobases (or nucleotide bases) are the parts of DNA and RNA that may be involved in pairing (see also base pairs). The main ones are cytosine, guanine, adenine (DNA and RNA), thymine (DNA) and uracil (RNA), abbreviated as C, G, A, T, and U, respectively. They're usually simply called bases in genetics. Because A, G, C, and T appear in the DNA, these molecules are called DNA-bases; A, G, C, and U are called RNA-bases. Uracil replaces thymine in RNA. T… (More on Nucleobase)