How many common amino acids are there?

The correct answer is 20, as there are 20 standard amino acids that are commonly found in proteins. These amino acids serve as the building blocks of proteins and play crucial roles in various biological processes. Each of these amino acids has a specific side chain or R group that distinguishes it from the others, determining its properties and function.

Amino acids are categorized into essential and non-essential groups, with essential amino acids being those that the body cannot synthesize and thus must be obtained from the diet. The standard 20 amino acids are critical for protein synthesis and are encoded by the genetic code.

While there are other amino acids that exist and may have biological significance, such as non-standard amino acids, the commonality pertains specifically to these 20 that are universally recognized in the genetic code.