DNA And Gene Expression Practice Test

What is nonsense-mediated decay and when does it occur?

A quality-control mechanism that degrades mRNAs with premature stop codons to prevent truncated, potentially harmful proteins. A process that enhances translation efficiency of all mRNAs. A pathway that repairs DNA double-strand breaks. A mechanism that degrades rRNA.

Correct Answer:

A quality-control mechanism that degrades mRNAs with premature stop codons to prevent truncated, potentially harmful proteins.

Explanation:

Nonsense-mediated decay is a cellular quality-control mechanism that degrades messenger RNAs containing premature stop codons to prevent production of truncated, potentially harmful proteins. It typically detects when a stop codon occurs earlier than intended, especially when there are exon junction complexes left behind after RNA splicing. When the ribosome encounters this premature termination, NMD factors recruit decay pathways that remove the message from the cell, often by decapping, deadenylation, or targeted cleavage. This disposal stops the faulty mRNA from being translated into a shortened, possibly harmful protein. It isn’t about boosting translation, repairing DNA, or degrading ribosomal RNA.

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