Appendix 1: Codon Table

Each three-letter sequence of mRNA nucleotides corresponds to a specific amino acid, or to a stop codon.

• AUG is the codon for methionine, and is also the start codon.
• UGA, UAA, and UAG are stop codons.

To see how the codon table works, let’s walk through an example. Suppose that we are interested in the codon CAG and want to know which amino acid it specifies.

1. First, we look at the left side of the table. The axis on the left side refers to the first letter of the codon, so we find C along the left axis. This tells us the (broad) row of the table in which our codon will be found.
2. Next, we look at the top of the table. The upper axis refers to the second letter of the codon, so we find A along the upper axis. This tells us the column of the table in which our codon will be found.

The row and column from steps 1 and 2 intersect in a set of boxes in the codon table, one half containing four codons and the other half containing the mapped amino acid(s). It’s often easiest to simply look at these four codons and see which one is the one you’re looking for.

If you want to use the structure of the table to the maximum, however, you can use the third axis (on the right side of the table) corresponding to the intersect box. By finding the third nucleotide of the codon on this axis, you can identify the exact row within the box where your codon is found. For instance, if we look for G on this axis in our example above, we find that CAG encodes the amino acid glutamine (gln).^[1]