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?

Video transcript

Welcome back everyone. Here's our next question which of the following methods enables cells to discriminate between normal M. RNA and those with a premature stop. So we're looking for a specific method when we have an M. R. N. A sequence that has a stop code on before translation is supposed to stop before the complete transcript of the entire gene. So let's think about this and look at our answer choices. Well, when we think about a particular method that is there to help discriminate a premature stop code on that process is called the nonsense mediated decay pathway. That terminology from the fact that a stop code on is also a nonsense code on since it doesn't code for an amino acid and what's involved in that pathway is choice be the deposition of the exon junction complex or E. J. C. So let's recall how that occurs. Just going to put up a definition here. So as we said, the E. J. C deposition is key to the nonsense mediated decay pathway which is the degradation of M. RNA. Is with a premature stop code on. So the E. J. C. Is deposited on M. RNA during splicing at the exon exon junctions and during translation when the ribosome reaches the E J. C, it displaces it and then translation continues until the stop code on is reached. But if the stop code on has reached prematurely. So if there's a stop code on in the wrong place before the end of that Exxon that needs to be translated, the E. J. C. Will remain bound and in that case the E. J. C. Remaining on M. RNA targets the M. R. And that particular M. RNA molecule for degradation. So again if our M. RNA molecules correct the E. J. C. Is there marking the end of the sequence the M. RNA sequence that marks an entire Exxon to be translated? The ribs then moves along, gets the J. C. Knocks it off, hits the stop code on new polyp peptide sequence or chain goes on its way. If a stop code ons in the wrong place before you've gotten to the end of that M. RNA sequence, the E. J. C. Remains on the M. R. N. A. Sequence and M. RNA strand with the E. J. C. Still attached when it's been released from the ribosomes is targeted for degradation and that eliminates and prevents translation of M. RNA with a premature stop code on. So let's look at another answer choices to understand why they're not correct. Again, we're looking specifically for the method that enables cells to discriminate between normal M. R. And S. And those with a premature stop choice. A says ribosomes stopping at the stop code on. Well that's important in the translation process but doesn't discriminate between a premature stop or a stop in the right place. The ribosomes will stop when they hit that code on. That's why that's not the correct answer choice. C. Says the proof reading activity of RNA polymerase too. Well this is important again in uh maintaining the overall accuracy of the process. RNA polymerase too will detect mismatched bases but it's not barney isn't proofread as strictly as D. N. A. Is. So it won't and it won't distinguish. It's not the thing that lets the cells discriminate between normal M. RNA and a premature stop. So if the RNA polymerase misses that mistake, this method will then allow the cell to discriminate if there's a premature stop code on. But choice is not our answer. Then choice D. Is splicing efficiency. While splicing efficiency is and splicing efficiency is the percent of that pre M. RNA that gets spliced into mature M. R. N. A. So that's obviously important in regulation of gene expression. But again doesn't distinguish M. RNA is with a premature stop, so not our correct answer. So again, which of the following methods enable cells to discriminate between normal M. R. N. A. S. And those with a premature stop choice be deposition of exon junction complex or E. J. C. See you in the next video