The Genetic Code: Codons

The genetic code is a “dictionary” that identifies the correspondence between a sequence of nucleotide bases and a sequence of amino acids. Each individual “word” in the code is composed of three nucleotide bases. These genetic words are called codons.
Codons
Codons are presented in the mRNA language of adenine (A), guanine (G), cytosine (C), and uracil (U). Their nucleotide sequences are always written from the 5′-end to the 3′-end. The four nucleotide bases are used to produce the three-base codons. Therefore, 64 different combinations of bases exist, taken three at a time (a triplet code), as shown in the table in Figure 1.

Figure 1:  Use of the genetic code table to translate the codon AUG. A = adenine; G = guanine; C = cytosine; U = uracil. The three-letter abbreviations for many common amino acids are shown as examples.
1. How to translate a codon: This table can be used to translate any codon and, thus, to determine which amino acids are coded for by an mRNA sequence. For example, the codon AUG codes for methionine ([Met] see Fig. 1). [Note: AUG is the initiation (start) codon for translation.] Sixty-one of the 64 codons code for the 20 standard amino acids .
2. Termination codons: Three of the codons, UAA, UAG, and UGA, do not code for amino acids but, rather, are termination (also called stop, or nonsense) codons. When one of these codons appears in an mRNA sequence, synthesis of the polypeptide coded for by that mRNA stops.