Nitrogenous bases can be either purines ot pyrimidines;
each atom in the ring is assigned a number.
The double-ring purines are adenine (A) and guanine (G).
The single-ring pyrimidines are cytosine (C), thymine (T), and uracil (U).
Both DNA and RNA contain A, C, and G; only DNA contains T, while only RNA contains U.