Question

Nonsense codon-mRNAs must be decapped without first being deadenylated. Decapping = rate-limiting step for decay of nonsense codon-mRNA. This has nothing to do with the PolyA tail. Xrn1p or other 5' → 3' exonucleases are responsible for digestion of decapped mRNA.

A) Nonsense-Mediated Decay (NMD)
B) Exosomal Decay
C) Internal Decay
D) ARE-Mediated Decay

Answer 1

The process where mRNAs must be decapped without deadenylation and is the rate-limiting step for the decay of nonsense codon-mRNA is the Nonsense-Mediated Decay (NMD). It involves the removal of the protective 5' methylguanosine cap, facilitating the mRNA's degradation by exonucleases like Xrn1p. Option A is correct.

Step-by-step explanation:

The process described where mRNAs must be decapped without first being deadenylated is known as Nonsense-Mediated Decay (NMD). This is a quality control mechanism that degrades mRNAs containing nonsense codons, which are signals for premature termination of translation.

The decapping of the 5' end of the mRNA, which removes the methylguanosine cap, is critical as it precedes digestion by 5' → 3' exonucleases such as Xrn1p. This cap normally protects mRNA from degradation and is part of the post-transcriptional modifications, which also include the addition of a poly-A tail to the 3' end.

However, in NMD, the focus is on the removal of the cap rather than the poly-A tail, signaling for the mRNA to be degraded.