DNA uses the four-letter alphabet A, C, G and T to encode genetic information. These letters represent the bases adenine, cytosine, guanine, and thymine.
Each DNA nucleotide contains one of these bases, a deoxyribose sugar, and a phosphate group. In double-stranded DNA, adenine pairs with thymine, while cytosine pairs with guanine. The order of bases along a strand carries biological instructions.
The letter U belongs to RNA rather than DNA: RNA normally uses uracil in place of thymine. This difference is useful when identifying whether a sequence is described as DNA or RNA, although some specialized biological systems have unusual variations.
The alphabet is small, but sequences can be extraordinarily informative. A human chromosome contains millions or hundreds of millions of base pairs, and even a single-base change can sometimes affect a trait or cause disease.