Nonsense-mediated mRNA decay (NMD) is a surveillance mechanism that degrades mRNAs containing:
- A Premature stop codons more than 50-55 nucleotides upstream of the last exon-exon junction ✓
- B Internal ribosome entry sites (IRES) in the 5' UTR
- C Poly-A tails shorter than 200 adenosine residues
- D 5' cap structures with aberrant methylation
Correct answer: A. Premature stop codons more than 50-55 nucleotides upstream of the last exon-exon junction
Explanation
NMD is triggered when a ribosome encounters a premature termination codon (PTC) located >50–55 nt upstream of an exon-junction complex (EJC) deposited post-splicing; the upstream frameshift junction signals abnormal termination to UPF1-UPF2-UPF3 complex, triggering mRNA degradation. This prevents production of truncated, potentially dominant-negative proteins. Normal stop codons in the last exon are downstream of all EJCs and escape NMD.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.