Everyone learns that the nitrogen bases in DNA are 'pyrimidines' and 'purines'. Cytosine and thymine are pyrimidines. Adenine and guanine are purines. Adenine base pairs with thymine. Guanine base pairs with cytosine. What is a pyrimidine? What is a purine?

One very important point to remember is that pyrimidines and purines are aromatic. In the DNA double helix, the two polynucleotide chains coil around a common axis with the planes of the bases on the inside of the helix perpendicular to the helix axis. Because they are aromatic, the rings of purines and pyrimidines are planar. This planarity is essential to base pairing.

For the record, a pyrimidine is a six membered aromatic ring with nitrogen at positions 1 and 3. A purine is what you get if you fuse a pyrimidine ring with an imidizole ring (an imidizole is a five membered aromatic ring with two nitrogens).