Genetic Code

The genetic code is the set of 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. They stand for the chemicals adenine, thymine, guanine, and cytosine, respectively, that make up the nucleotide bases of DNA.

The DNA code for each gene combines the four chemicals in various ways to spell out 3-letter "words" (codons) that specify which amino acid is needed at every step in making a protein.

Sources:  definition from the National Human Genome Research Institute (NHGRI) Glossary of Genetic Terms

More:  ~20 genetic codes in GenBank

The translation table above does not apply uniformly to all organisms.

GenBank/EMBL/DDBJ, which contains nucleotide sequences from more than 125,000 organisms, uses ~20 genetic codes to ensure correct DNA --> protein translation.

The NCBI Taxonomy Browser shows which genetic code(s) is used for each organism. For example, the taxonomy entry for yeast shows that the standard genetic code is used to translate genes present on the chromosomes, and the yeast mitochondrial genetic code is used to translate genes found in the mitochondrion.