Final answer:
In some archaea and bacteria, a special tRNA recognizes the UAG stop codon and incorporates the amino acid pyrrolysine instead of terminating translation.
Step-by-step explanation:
The special tRNA recognizes the UAG stop codon but incorporates the amino acid pyrrolysine.
In standard genetic coding, the three stop codons (UAA, UAG, and UGA) signal the end of protein synthesis and do not normally code for an amino acid. However, in some organisms, such as certain archaea and bacteria, exceptions to this rule occur where these codons can be reassigned to incorporate specific amino acids. One such case is with the UAG codon, which can be used to insert the amino acid pyrrolysine. The special tRNA with an anticodon of CUA pairs with this UAG stop codon to facilitate the addition of pyrrolysine into the growing polypeptide chain. The incorporation of pyrrolysine also requires the presence of the gene pylS which allows for this unusual translation event. This phenomena showcase the plasticity of the genetic code and how certain organisms have evolved mechanisms to include non-standard amino acids in their proteins.
For some organisms, the UAG stop codon can be recognized by a special tRNA that incorporates the amino acid pyrrolysine (Option 4) into the growing polypeptide chain. Pyrrolysine is not one of the standard 20 amino acids found in proteins, but it is used by certain archaea and bacteria. This incorporation of pyrrolysine requires a unique tRNA with a CUA anticodon and the pyls gene.