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The Genetic Code 
To summarize what we know to this point, the cellular process of transcription produces messenger 
RNA (mRNA), which is a mobile molecular copy of one or more genes with the letters A, C, G, and U. (U). 
The mRNA template is translated into a protein product from nucleotide-based genetic information. Protein 
sequences are made up of 20 commonly occurring amino acids, so the protein alphabet is made up of 20 letters. 
The triplet codon is a three-nucleotide sequence that defines each amino acid. The relationship between a 
nucleotide codon and its corresponding amino acid is called the genetic code. Given the different numbers of 
“letters” in the mRNA and protein “alphabets,” combinations of nucleotides corresponded to single amino 
acids. Using a three-nucleotide code means that there is a total of 64 (4 × 4 × 4) possible combinations; 
therefore, a given amino acid is encoded by more than one nucleotide triplet (Figure 48).
Figure 48. 
The genetic code for translating each nucleotide triplet, or codon, in mRNA into an amino 
acid or a termination signal in a nascent protein. (credit: modification of work by NIH).
 
Three of the 64 codons stop protein synthesis and allow the polypeptide to be released from the 
translation machinery. These triplets are referred to as stop codons. AUG is another codon with a unique 
function. It not only specifies the amino acid methionine, but it also serves as the start codon for translation to 
begin. The AUG start codon near the 5' end of the mRNA determines the Reading Resource frame for 

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translation. The genetic code is global. Almost all species, with a few exceptions, use the same genetic code 
for protein synthesis, which is powerful evidence that all life on Earth shares a common origin
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