Cracking the code

All proteins are made up of combinations of 20 different amino acids. The RNA code (like DNA) is written in just four different chemical 'letters' - bases. How can a code with four letters identify 20 different amino acids? By reading it three letters at a time - CAU, CAG, GGA, etc. This system has 64 different combinations, or codons. Most amino acids are represented by several alternative codons. Some codons are not instructions for amino acids at all, but signal the end of the gene ('stop' codons).