Major components of DNA. They are made up of nitrogen and carbon atoms in a ring structure. There are two types of bases: purines (adenine and guanine, known as A and G) and pyrimidines (cytosine and thymine, known as C and T). The bases stick to each other in pairs in the DNA double helix. A binds to T, and C binds to G. The order of bases in the DNA molecule determines the order of amino acids in the proteins it encodes.