The four nucleotides of messenger RNA are arranged in 64 three-letter words called codons. The genetic code is the language consisting of these 64 codons. 61 of the 64 codons code for a specific amino acid, while the remaining three instruct the ribosome to stop the translation process. Let's see how this table is used to determine the amino acid of a given codon, CAG. First, on the left side of the table, find the first letter of the codon - C. Then, on the top of the table, find the second letter of the codon - A. Finally, locate the codon that has G as its last letter and you will see that the amino acid coded for by CAG is Glutamine. Can you determine which amino acid is coded for by UCA? Serine is correct! Now can you determine the effect of UAA? The codon UAA signals termination of the elongation of the polypeptide. It is a stop codon.