Final answer:
Amino acids not among the standard 20, such as hydroxylsine and thyroxine, are incorporated into proteins through post-translational modification.
Step-by-step explanation:
Amino acids, such as hydroxylysine and thyroxine, that are not among the 20 standard amino acids coded for by DNA, are usually formed by modification after a protein has been synthesized. Humans synthesize about half of the 20 standard amino acids; the rest are essential and must be ingested. Selenocysteine and pyrrolysine are rare amino acids also found in proteins. For example, an amino acid derivative like hydroxyproline is created by the modification of proline after it has been incorporated into a protein. This process is known as post-translational modification. The commonly known set of 20 amino acids is used to construct proteins in all living species. Among these, only about half can be synthesized by humans and the rest are essential amino acids that must be obtained through diet.
Moreover, two additional amino acids, selenocysteine and pyrrolysine, have been discovered as being incorporated into proteins in limited quantities.