1.2 Each amino acid is given a three-letter abbreviation and a one-letter symbol. They often use the first three letter and the first letter. When there is confusion an alternative is used. These must be remembered 1.3 All proteins in all species(from bacteria to human) are constructed from the same set of 20 amino acids All proteins, no matter how different they are in structure and function are made from the 20 standard amino acids This fundamental alphabet of the protein language is at least two billion years old.1.2 Each amino acid is given a three-letter abbreviation and a one-letter symbol. They often use the first three letter and the first letter. When there is confusion, an alternative is used. These must be remembered. 1.3 All proteins in all species (from bacteria to human) are constructed from the same set of 20 amino acids. •All proteins, no matter how different they are in structure and function, are made from the 20 standard amino acids. •This fundamental alphabet of the protein language is at least two billion years old