Standard Amino Acids in Biology

Steps:

• Recall that proteins are built from amino acids as monomers.
• Identify the standard set used in human and most eukaryotic proteins.
• Count the genetically encoded amino acids from the genetic code.
• Confirm the total is 20, including essentials like glycine and tryptophan.

Why A is correct:

• There are exactly 20 standard amino acids encoded by the 64-codon genetic code, forming the basis of protein synthesis.

Why the others are wrong:

• B: Incorrectly includes selenocysteine as standard, but it's rare and not universal.
• C: Understates by excluding non-essential ones like glutamine.
• D: Overcounts with post-translational modifications, not base amino acids.

Final answer: A