ID #1180

I understand that the 5 prime cap structure lets ribosomes know where to start protein synthesis, but I'm not sure how they provide stability to mRNA and protect from RNases.

If you're a 5'-3' exonuclease specific for RNA, you're looking for a phosphate (or more likely, a triphosphate) group at the 5' end of a "normal" looking RNA strand. If you encounter a processed eukaryotic mRNA, and "feel" the 5' end, you don't "feel" a free triphosphate group, do you? In fact, it probably "looks" more like the 3' end of a molecule to you, although there's something not quite right about that either! So if the nuclease encounters this cap structure, it probably won't do a very good job of cutting away that first base, which ultimately protects the entire molecule.

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