This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Mutations & Codons, Subject name: Biochemistry
Which of the following is a Nonsense codon : 
Select and explain. 
A. UAG, B. AUG, C. AGG, D. UUA
A: UAG
Answer: A: UAG 
Explanation: Non-sense codons/Terminator Codons/Punctuator codons Codons which do not code for any paicular amino acid. Mark end of protein synthesis. These codons are UAA UAG UGA UGA is a stop codon; But in special circumstances, it stands for seleno-cysteine ("21st" amino acid). UAG is a stop codon; But in special circumstances, it stands for pyrrolysine (22nd amino acid)