4 From RNA to protein: translation

The base sequence of an RNA molecule, which is copied from the DNA, is converted into the amino acid sequence of a protein chain. In doing so, the cell must translate the four-character language of RNA (the four bases, A, C, G and U) into the corresponding 20-character language of a protein (the 20 naturally occurring amino acids). The code in mRNA consists of consecutive three-base sequences, or triplets. Each triplet is called a codon. Each codon codes for a particular amino acid.

Let’s explore this with a particular example. Suppose that a sequence of bases in a tiny section of an mRNA molecule is

…AUGCCUGCUGUUGGAAAG…

This message forms a series of three-letter ‘words’, or codons. In this example they would be

–AUG–CCU–GCU–GUU–GGA–AAG–

Each of these six codons codes for a different amino acid. For example, the RNA codon AUG codes for the amino acid methionine (abbreviated to Met) and CCU codes for the amino acid proline (Pro). The sequence of codons within a molecule of RNA enables the sequence of amino acids in the protein to be determined, as shown in Figure 3.

Figure 3 mRNA to protein codon usage © The Open University

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The diagram shows translation from the four-character language of RNA to the twenty-character language of a protein. mRNA code is separated into six groups of three. The groups are labelled as Codon one to six, and each codon code corresponds with a particular amino acid in the protein. The acids and their corresponding bases are "Met" (AUG), "Gly" (GGG), "Leu" (CUA), "Glu" (GAA), "Ala" (GCA) and "Leu" (CUG).

Figure 3 mRNA to protein codon usage © The Open University