1. Purines: These are double-ring structures consisting of a pyrimidine ring fused to an imidazole ring. The purines found in DNA and RNA are adenine (A) and guanine (G).

2. Pyrimidines: These are single-ring structures. The pyrimidines found in DNA and RNA are cytosine (C), thymine (T) (found only in DNA), and uracil (U) (found only in RNA).