Final answer:
Selenocysteine and pyrrolysine are unique amino acids encoded by UGA and UAG codons, which are typically stop codons, due to special mRNA and tRNA structures that enable the incorporation of these amino acids during protein synthesis.
Step-by-step explanation:
Selenocysteine and pyrrolysine are encoded by codons that usually function as stop codons. The genetic code is nearly universal, and these two amino acids present interesting exceptions that underline the complexity and adaptability of life's molecular machinery. Selenocysteine, referred to as the 21st amino acid, is signalled by the UGA codon, which typically signals the termination of protein synthesis, but when a specific stem-loop structure known as the SECIS element is present, it allows for the insertion of selenocysteine instead. Similarly, pyrrolysine, the 22nd amino acid, utilizes the UAG codon, another stop signal, which is redirected to encode pyrrolysine when certain gene products from the pyls gene are present and a unique transfer RNA with a CUA anticodon interacts with the ribosome.