Which of the following is considered a stop codon?

The concept of stop codons is crucial in the process of protein synthesis. Stop codons signal the termination of translation, which is the process where ribosomes synthesize proteins based on the sequence of messenger RNA (mRNA).

Among the options provided, UGA and UAG are both recognized as stop codons. These codons do not correspond to any amino acids and instead instruct the ribosome to end the synthesis of the polypeptide chain.

On the other hand, AUU and CUG are not stop codons; they correspond to specific amino acids (isoleucine and leucine, respectively) in the genetic code. The importance of identifying stop codons lies in their role in ensuring that proteins are synthesized correctly and to their full length, as they dictate where the ribosomal machinery should halt.

Therefore, UGA and UAG are the correct stop codons, integral to the translation process, indicating that the synthesis of a protein should be terminated at these sequences.