That code is written in nucleotides, which are components of the DNA and RNA molecules. Each nucleotide consists of a nitrogenous base linked with a sugar molecule and one or more bits of phosphate. If DNA resembles a spiral staircase supported by two helical strands, those nitrogenous bases are the stair steps connecting the strands. There are four kinds of base in DNA—molecular components known as adenine, cytosine, guanine, and thymine, and abbreviated as A, C, G, and T, little pieces in the great game of genetic Scrabble.
