Nonsense-mediated decay is an mRNA surveillance pathway that eliminates mRNAs with premature stop codons. How does the cell distinguish between normal mRNAs and those with a premature stop?

NMD is a cellular quality control mechanism that detects and degrades mRNAs containing premature stop codons to prevent the production of truncated, potentially harmful proteins. It ensures only properly processed mRNAs are translated, maintaining protein integrity.

Premature stop codons occur when a stop signal appears earlier than expected in the mRNA sequence, often due to mutations. The cell distinguishes these by their position relative to downstream exon-exon junctions marked by exon junction complexes (EJCs), which are key signals in identifying faulty mRNAs.

EJCs are protein complexes deposited near exon-exon junctions after splicing. During the pioneer round of translation, if a stop codon is detected upstream of an EJC, the mRNA is flagged as abnormal, triggering NMD. This spatial relationship helps the cell differentiate normal from premature stop codons.