A codon is a trinucleotide sequence of DNA or RNA that corresponds to a specific amino acid in a protein

. Codons are made up of any triplet combination of the four nitrogenous bases adenine (A), guanine (G), cytosine (C), or uracil (U)

. There are 64 different codons: 61 specify amino acids and 3 are used as stop signals

. In the context of messenger RNA (mRNA), codons are read in groups of three, and each group of three bases forms a codon

. The mRNA contains many such codons, with a series of three bases followed by three more bases, and then three more bases, and so on

. Each codon instructs the cell to start the creation of a protein chain, to add a specific amino acid to the growing protein chain, or to stop the creation of the protein chain

. For example, a messenger RNA codon, GCA, signals the addition of the amino acid alanine to the protein chain, while the messenger RNA stop codon, UAG, signals the end of that protein's production