How many different amino acids are used to build proteins in living organisms?

How many different amino acids are used to build proteins in living organisms?

• A. 4
• B. 20
• C. 64
• D. 21

Answer: (B)

Proteins are built from amino acids, and the standard set used across most living organisms is twenty. The genetic code translates codons—triplets of nucleotides in mRNA—into these amino acids. There are 64 possible codons, but they map to just twenty amino acids plus stop signals, with different codons sometimes coding for the same amino acid. While there are rare cases where additional, nonstandard amino acids like selenocysteine are incorporated in specific contexts, the typical building blocks for proteins are twenty.

Proteins are built from amino acids, and the standard set used across most living organisms is twenty. The genetic code translates codons—triplets of nucleotides in mRNA—into these amino acids. There are 64 possible codons, but they map to just twenty amino acids plus stop signals, with different codons sometimes coding for the same amino acid. While there are rare cases where additional, nonstandard amino acids like selenocysteine are incorporated in specific contexts, the typical building blocks for proteins are twenty.