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?

In 1998, future Nobel laureates Andrew Fire and Craig Mello, and colleagues, published an article in Nature entitled, 'Potent and Specific Genetic Interference by Double-Stranded RNA in Caenorhabditis elegans.' Explain how RNAi is both 'potent and specific.'
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Key Concepts
RNA Interference (RNAi) Mechanism
Specificity of RNAi
Potency of RNAi
AU-rich elements (AREs) are cis-elements in mRNAs that regulate stability and decay. How is it possible that a single mRNA sequence element can serve to stabilize an mRNA in some cases and lead to its decay in other scenarios?
What are processing bodies (P bodies), and what role do they play in mRNA regulation?
Present an overview of RNA interference (RNAi). How does the silencing process begin, and what major components participate?
RNAi may be directed by small interfering RNAs (siRNAs) or microRNAs (miRNAs); how are these similar, and how are they different?
miRNAs target endogenous mRNAs in a sequence-specific manner. Explain, conceptually, how one might identify potential mRNA targets for a given miRNA if you only know the sequence of the miRNA and the sequence of all mRNAs in a cell or tissue of interest.
