There are 22 amino acids that are commonly found in proteins, with the 23rd being selenocysteine which is found in certain proteins. The 22 are referred to as the standard amino acids.

Is there a 22nd amino acid?

Yes, there is a 22nd amino acid, selenocysteine. Selenocysteine is an amino acid that is encoded by the UGA codon, and is found in many proteins. It is a non-standard amino acid, meaning it is not found in the standard genetic code, and is not incorporated into proteins during regular translation. Selenocysteine is a sulfur-containing amino acid that is important for many cellular processes and is involved in redox reactions.

Are there 20 or 24 amino acids?

There are 20 amino acids that are commonly found in proteins. These are called the «standard» amino acids. In some cases, four additional amino acids may also be present in proteins, making a total of 24. These four additional amino acids are selenocysteine, pyrrolysine, hydroxyproline, and gamma-carboxyglutamate.