How many different amino acids exist?

• A. 18
• B. 22
• C. 25
• D. 20

Answer: (D)

Proteins are built from twenty standard amino acids. These are the amino acids encoded by the genetic code and used during translation to assemble polypeptide chains in nearly all organisms. There are two additional amino acids, selenocysteine and pyrrolysine, that can be incorporated in specific contexts, but they are not part of the standard set used to synthesize most proteins. Other amino acids arise only through post-translational modifications or in special organisms, but the canonical count for amino acids in protein synthesis is twenty.