codon

n.
A sequence of three adjacent nucleotides constituting the genetic code that determines the insertion of a specific amino acid in a polypeptide chain during protein synthesis or the signal to stop protein synthesis.

[COD(E) + -ON¹.]

What is a codon?

A codon is the three-unit sequence (AUG, AGC, etc.) of mRNA nucleotides that codes for a specific amino acid. Since there are only twenty commonly used amino acids and sixty-four (4x4x4) possible codon sequences, the genetic code is described as both degenerate and unambiguous. Each codon codes for only one amino acid, but each amino acid may have more than one matching codon.

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A triplet in a chain of nucleic acids in mRNA that specifies the order in which amino acids are added. The codon triplet pairs with a sequence of three complementary nucleotides, called the anticodon, present in the anticodon arm of tRNA. Called also triplet. See also deoxyribonucleic acid.

• start c. — see initiation codon.
• stop c's — three codons, UAG, UAA and UGA, also referred to as amber, ochre and opal codons, in mRNA which terminate translation.
• termination c. — see stop codons (above).

(koh-don)

A group of three bases on the DNA molecule. Each codon determines the identity of one amino acid in proteins made by the cell.