CoachThe genetic code maps triplets of messenger bases, codons, onto amino acids. Of the 64 codons, 61 specify the twenty amino acids and three (UAA, UAG, UGA) are stop signals; AUG specifies methionine and is also the start codon, so every new polypeptide begins with methionine. The code is degenerate — most amino acids have several codons, differing mostly at the third position — non-overlapping, read without punctuation in a reading frame fixed by the start codon, and almost universal, the same in bacteria, plants and animals with minor variants in mitochondria: a human gene put into a bacterium is read correctly.
| first | second base | third | |||
| base | U | C | A | G | base |
| U | Phe Phe Leu Leu | Ser Ser Ser Ser | Tyr Tyr stop stop | Cys Cys stop Trp | U C A G |
| C | Leu Leu Leu Leu | Pro Pro Pro Pro | His His Gln Gln | Arg Arg Arg Arg | U C A G |
| A | Ile Ile Ile Met | Thr Thr Thr Thr | Asn Asn Lys Lys | Ser Ser Arg Arg | U C A G |
| G | Val Val Val Val | Ala Ala Ala Ala | Asp Asp Glu Glu | Gly Gly Gly Gly | U C A G |