Adenine and guanine are the most commonly found purines in nucleic acid.
Purines and pyrimidines
There are two kinds of nitrogen-containing bases – purines and pyrimidines. Purines consist of a six-membered and a five-membered nitrogen-containing ring fused together. Pyrimidines have only a six-membered nitrogen-containing ring. Thus, there are 4 purines and 4 pyrimidines that are of concern to us. The number of rings this base has determines whether the base is a purine (two rings) or pyrimidine (one ring).
- Adenine and guanine are found in both DNA and RNA.
- Cytosine is found in both DNA and RNA. Uracil is found only in RNA
Which purines pair with which pyrimidines are always constant, as is the number of hydrogen bonds between them:
- ADENINE pairs with THYMINE (A::T) with two hydrogen bonds
- GUANINE pairs with CYTOSINE (G::C) with three hydrogen bonds