DNA is a chain of nucleotide units; each unit consists of a backbone made of a sugar and a phosphate group, with a nitrogenous base attached. The base in a unit is one of

• adenine (A),

• guanine (G),

• cytosine (C), or

• thymine (T).

In RNA, uracil (U) is used instead of thymine. A and G belong to the chemical class called purines; C, T, and U. are pyrimidines.