How Many Different Kinds of Naturally Occurring Amino Acids Are

Question

How many different kinds of naturally occurring amino acids are specified by the genetic code?  
A) 3
B) 6
C) 12
D) 20
E) 28

Accepted Answer

The genetic code specifies the sequence of amino acids in a protein. There are 20 different naturally occurring amino acids that can be incorporated into a protein chain. Some textbooks may mention that there are 21 amino acids, but selenocysteine, the 21st amino acid, is not directly coded for by the genetic code. Instead, it is encoded by a stop codon and requires a specialized mechanism for translation. Therefore, the correct answer is D, 20.