How many amino acids are there?

How many amino acids are there?

• A. 19
• B. 21
• C. 22
• D. 20

Answer: (D)

Proteins are built from a standard set of twenty amino acids. The genetic code specifies these twenty building blocks using codons in mRNA, and then ribosomes link them together in chains to form proteins. There are 64 possible codons, but many codons encode the same amino acid (redundancy), and some codons signal the end of a protein (stop signals). In addition to these, a couple of rare exceptions exist in certain organisms—selenocysteine and pyrrolysine—but they are not part of the common, canonical set used in most biology contexts. So, the typical count you’ll see and use is twenty.

Proteins are built from a standard set of twenty amino acids. The genetic code specifies these twenty building blocks using codons in mRNA, and then ribosomes link them together in chains to form proteins. There are 64 possible codons, but many codons encode the same amino acid (redundancy), and some codons signal the end of a protein (stop signals). In addition to these, a couple of rare exceptions exist in certain organisms—selenocysteine and pyrrolysine—but they are not part of the common, canonical set used in most biology contexts. So, the typical count you’ll see and use is twenty.