Understanding Nonsense Codons in Genetics

In the universal genetic code, codons are sequences of three nucleotides that specify which amino acid to add next during protein synthesis (translation). However, not all codons code for amino acids. Some codons signal the termination or end of the protein synthesis process. These special codons are called stop codons or nonsense codons.

What are Stop Codons?

Stop codons do not code for any amino acid. When the ribosome encounters a stop codon on the messenger RNA (mRNA) molecule, it signals the release of the polypeptide chain, effectively ending the translation process. There are typically three universal stop codons:

• UAA (Uracil-Adenine-Adenine)
• UAG (Uracil-Adenine-Guanine)
• UGA (Uracil-Guanine-Adenine)

These are also sometimes referred to as nonsense codons because they introduce a "nonsense" sequence from the perspective of amino acid coding, as they don't specify an amino acid.

Analyzing the Given Options

Let's examine the given options and compare them to the list of universal nonsense codons:

• UAG: This sequence is one of the three universal stop codons (UAA, UAG, UGA). Therefore, UAG is a nonsense codon.
• UGG: This codon codes for the amino acid Tryptophan (Trp). It is a sense codon, not a nonsense codon.
• UGC: This codon codes for the amino acid Cysteine (Cys). It is a sense codon, not a nonsense codon.
• UAC: This codon codes for the amino acid Tyrosine (Tyr). It is a sense codon, not a nonsense codon.

Based on this comparison, only UAG from the given options is a nonsense codon (stop codon) in the universal genetic code.

Conclusion

Nonsense codons are crucial signals for ending protein synthesis. The universal genetic code includes UAA, UAG, and UGA as nonsense codons. Among the provided choices, UAG fits this description.