Final answer:
The twenty-first and twenty-second amino acids are selenocysteine and pyrrolysine, uniquely incorporated into proteins and not encoded by the standard genetic code.
Step-by-step explanation:
The amino acids that are sometimes referred to as the twenty-first and twenty-second amino acids are selenocysteine and pyrrolysine. Selenocysteine, which is known as the 21st amino acid, was discovered in 1986 and is sometimes found in proteins across all domains of life. Pyrrolysine, identified as the 22nd amino acid, was discovered in 2002 and is less common, found in some methanogenic archaea and bacteria. Both of these amino acids are incorporated into proteins via unique mechanisms and are not encoded directly by the standard genetic code.