Proteins are organic compounds formed from chains of amino acids. How many amino acids are there?

• A. 20
• B. 15
• C. 25
• D. 12

Answer: (A)

Proteins are built from amino acids, and the standard set used to assemble most proteins consists of twenty different amino acids. The variety of side chains among these twenty provides the chemical diversity that drives protein folding and function. The genetic code specifies codons that encode these twenty amino acids during translation, so in the typical context the count is twenty. There are rare exceptions where organisms incorporate unusual amino acids like selenocysteine or pyrrolysine, but these are not part of the usual 20-character repertoire. Twenty is therefore the best answer.