A codon is a sequence of three nucleotides. Each codon provides the template for the production of a single amino acid or serves as a stop signal during protein synthesis.
DNA uses four nucleotides — adenine, cytosine, guanine and thymine — to code for the production of the 20 amino acids that make up all of the proteins found in living organisms. These four nucleotides are analogous to a four-letter alphabet that can be combined to form 64 different three-letter words called codons. These codons are arranged in long sequences that show the specific arrangement of amino acids that makes up a particular protein molecule.