Two classes of nucleobases
Double ring vs. single ring
The nucleobases of DNA and RNA fall into two classes. Purines, adenine (A) and guanine (G), consist of two fused rings that contain nitrogen. Pyrimidines, cytosine (C), thymine (T), and uracil (U), have a single ring with two nitrogen atoms. Thymine occurs in DNA and uracil in RNA; thymine is also called 5-methyluracil. In the double helix, a purine always pairs with a pyrimidine, A with T or U and G with C, so every base pair has the same width and the helix keeps a constant diameter.