The four nitrogen bases found in DNA are adenine, cytosine, guanine, and
thymine. Each of these bases are often abbreviated a single letter: A (adenine),
C (cytosine), G (guanine), T (thymine). The bases come in two categories:
thymine and cytosine are pyrimidines, while adenine and guanine are
purines ().
Figure 2.1: DNA Bases
The pyrimidine structure is produced by a six-membered, two-nitrogen molecule;
purine refers to a nine-membered, four-nitrogen molecule. As you can see, each
constituent of the ring making up the base is numbered to help with specificity
of identification.