How Translation Works (IMAGE) European Molecular Biology Laboratory Caption The genetic code is made up of three-letter sequences called codons. Each one codes for an amino acid, except for three 'stop' codons, which signal that an amino acid chain is complete. The Lemke group were able to develop a cell organelle that uses a reprogrammed stop codon, so that it codes for a new amino acid - not one of the 20 that occur naturally in living organisms. Credit IMAGE: Aleks Krolik/EMBL Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.