nitrogenous base

If DNA is a message written in a four-letter alphabet, the nitrogenous bases are the letters themselves. They are the part of each nucleotide that actually differs from one position to the next, and so they are what carries the genetic information. Everything else in DNA is essentially the paper the letters are printed on.

Chemically, a nitrogenous base is a ring-shaped molecule rich in nitrogen atoms. DNA uses four of them: adenine, guanine, cytosine, and thymine (RNA swaps thymine for uracil). They fall into two families by ring structure — the larger, double-ringed purines (adenine and guanine) and the smaller, single-ringed pyrimidines (cytosine and thymine).

Each base attaches to the sugar of its nucleotide and points inward toward the center of the double helix, where it can form hydrogen bonds with a base on the opposite strand. The pairing is selective: a purine always pairs with a pyrimidine, and specifically A with T and G with C, which keeps the helix a uniform width.

Because only the base varies along a strand, reading the order of bases is the same as reading the genetic information. That sequence is what a cell interprets to make proteins, and what we sequence when we read a genome.