How many amino acids are known to combine to form proteins?

Proteins are composed of building blocks called amino acids, and there are a total of 22 standard amino acids that are known to be used in protein synthesis. These include the 20 naturally occurring amino acids that are specified by the genetic code, as well as the two additional amino acids that are incorporated into proteins through post-translational modifications: selenocysteine and pyrrolysine. Selenocysteine is considered the 21st amino acid, while pyrrolysine, found in some archaea and bacteria, is the 22nd.

Understanding the composition of amino acids is crucial in the study of biochemistry and molecular biology, as the specific sequence and arrangement of these amino acids dictate the function and structure of the resulting proteins. Hence, the correct answer reflects the inclusive count of all the amino acids utilized in protein formation, highlighting the complexity of genetic coding and protein synthesis within living organisms.