Genes: DNA's instructions

Translating the genetic code

At the ribosomes, the mRNA is used as a template for assembling a protein molecule from its building blocks (amino acids). This process is called translation.

There are 20 different types of amino acids, for example, methionine (Met or M), leucine (Leu or L), phenylalanine (Phe or F) and proline (Pro or P). Translation at the ribosomes is very similar to translating from one language to another. In this case, the translation is from the four-letter language of DNA into the 20-letter language of proteins.

The DNA code is read three letters at a time: these DNA triplets are called codons. Since there are four different RNA letters (A, G, C and U), there are 4 x 4 x 4 = 64 different codon combinations. Most of the codons correspond to a specific amino acid. However, as there are only 20 different types of amino acid, some of the 64 codons code for the same amino acid. Three of the codons are used as 'stop' signals (STOP codon) - telling the cell to end the transcript there - and another is the 'start' signal (START codon).

An Example (with codon wheel)

An example of a DNA sequence might be


If you use our DNA decoder (codon wheel*), you can decode this triplet by triplet. Start from the inside of the wheel: find the first letter of your codon in the centre of the wheel and work outwards, through the second ring (with the next letter) and so on, to find the corresponding amino acid.

This would make the amino acid chain:

P - C - G - A - T - STOP


(*You can get a copy of a codon wheel from here. Have a look at more activities in our downloads section)

Codon Cracker

Check your answers from the codon wheel with our new codon cracker. Click on the coloured letter buttons to enter the triplet then click on translate to decode the triplet.

*Please note that the codon wheel and codon cracker use the sense DNA codons (5' to 3'). See example.