Genetic code fundamentals In molecular biology, which codon(s) are the stop (nonsense) signals that terminate translation on the ribosome?

Introduction / Context:
Protein synthesis stops when the ribosome encounters a stop signal in the mRNA. These signals are called nonsense or termination codons. Knowing exactly which triplets encode “stop” is a foundational genetics concept used in cloning, mutagenesis, and interpreting sequencing results.

Given Data / Assumptions:

• The standard genetic code is assumed (bacterial/eukaryotic nuclear code).
• We are identifying the triplet codons that do not encode amino acids but instead instruct termination.
• tRNA molecules do not recognize these stops; release factors do.

Concept / Approach:

The three termination codons are UAG (amber), UAA (ochre), and UGA (opal/umber). When any of these appears in the A site of the ribosome, protein release factors bind and promote hydrolysis of the peptidyl-tRNA bond, freeing the nascent polypeptide.

Step-by-Step Solution:

Verification / Alternative check:

Any codon table in molecular biology texts confirms that UAG, UAA, and UGA terminate translation. Specialized codes (e.g., mitochondrial) may reassign one stop (often UGA) to tryptophan, but the standard code retains these three stops.

Why Other Options Are Wrong:

• UAG alone, UAA alone, UGA alone: Each is a stop, but the question asks for the complete set.
• None of these: Incorrect because the standard code clearly defines three stops.

Common Pitfalls:

• Assuming start codon AUG also acts as stop—start and stop signals are different.
• Forgetting the historical names (amber/ochre/opal), which are often tested.

Final Answer:

all of these