adenine

Adenine is one of the four letters in the DNA alphabet, usually written simply as A. Whenever you see a genetic sequence printed as a run of letters, every A in it stands for an adenine sitting at that spot along the strand.

Structurally, adenine is a purine: a relatively large base built from two fused rings. In the double helix it pairs with thymine across two hydrogen bonds, and in RNA it pairs with uracil. This consistent A-with-T (or A-with-U) pairing is part of what lets one strand specify the sequence of the other.

Beyond storing information in DNA and RNA, adenine is a component of several of the cell's most important small molecules. It forms the core of ATP, the energy currency of the cell, and of signaling and helper molecules such as cyclic AMP, NAD, and coenzyme A.

So adenine wears two hats: as a genetic letter it is one of four interchangeable symbols, but as a chemical it is a recurring, multipurpose part throughout the cell's machinery.