What is the purpose of nonsense-mediated decay (NMD)?

• A. To enhance translation efficiency.
• B. To promote mRNA splicing.
• C. To export mRNA into cytoplasm.
• D. To detect and degrade mRNAs with premature stop codons, preventing truncated proteins.

Answer: (D)

Nonsense-mediated decay is a quality-control mechanism that targets mRNAs likely to produce incomplete, potentially harmful proteins. It works by detecting premature stop codons in transcripts and triggering their degradation so the ribosome doesn’t translate a truncated protein. The surveillance hinges on signals left by splicing—exon junction markers help identify when a stop codon appears too early, prompting decay pathways to remove the faulty mRNA. This function is distinct from boosting translation, promoting splicing, or exporting mRNA, which are separate steps in gene expression. In short, NMD protects the cell by eliminating transcripts that would yield undesirable truncated proteins.