Nonsense-mediated decay (NMD) primarily serves to degrade transcripts that contain what feature?

• A. mRNAs lacking a termination codon.
• B. mRNAs with premature stop codons.
• C. mRNAs with polyadenylation signals at the 3' end.
• D. Transcripts with extended 3' UTRs.

Answer: (B)

Nonsense-mediated decay is a quality-control pathway that prevents production of potentially harmful truncated proteins by recognizing mRNAs that end too soon. The telltale signal is a premature stop codon that appears long before the normal termination site, often within a transcript that has been spliced and bears downstream exon junction complexes. When translation reaches this early stop, NMD factors detect the abnormal termination and recruit decay machinery, so the mRNA is degraded before a full-length protein is made. Normal transcripts that terminate at the correct place don’t present those downstream signals and thus are spared. The other features listed aren’t the primary trigger: lack of a stop codon would invoke a different surveillance mechanism, normal polyadenylation signals are standard processing, and while a very long 3′ UTR can influence NMD in some contexts, it’s the premature termination event that best defines NMD targets.