Genetic code: The instructions in a gene that tell the cell how to make a specific protein. A, T, G, and C are the 'letters' of the DNA code and represent the chemicals adenine, thymine, guanine, and cytosine, respectively. These make up the nucleotide bases of DNA. Each gene's code combines these four chemicals in various ways to spell out three-letter 'words' that specify which amino acid is needed at every step in making a protein. The discovery of the genetic code ranks as one of the premiere events of biology and medicine.