How many amino acids are specified by the genetic code?

• A. 64
• B. 20
• C. 22
• D. 18

Answer: (B)

Proteins are built from twenty standard amino acids, and the genetic code is written in triplets of nucleotides called codons. There are 4 possible nucleotides, so 4^3 = 64 possible codons. Of these, three act as stop signals to terminate translation, leaving 61 codons that specify amino acids. However, those 61 codons don’t map one-to-one to 61 different amino acids; several codons encode the same amino acid. This redundancy, or degeneracy, means a single amino acid can be represented by multiple codons, and the end result is that there are twenty distinct amino acids specified by the genetic code. The start codon, typically AUG, also marks the beginning of translation and encodes methionine in initiation.