Which of the Following Amino Acids is Not One of the 20

Question

Which of the following amino acids is not one of the 20 common amino acids found in cells?
A)leucine
B)isoleucine
C)glutamine
D)proline
E)pyrrolysine

Accepted Answer

Pyrrolysine is not one of the 20 standard amino acids encoded directly by the universal genetic code. It is considered a special case, found in some archaea and bacteria, incorporated into proteins by a unique mechanism.