How do cells determine RNA types?
Well remember that an mRNA, the RNA that is destined for the ribosome, has already been processed by the time it gets there. This is done by the splicing machinery, among other things (capping enzymes, for example). But the splicing machinery isn't smart in itself. Rather, the gene has the elements, just like it does to select the RNA Pol it needs, to direct the sort of modification the RNA needs to obtain. The image below depicts two types of RNA transcribed by RNA Pol II and how the processing elements let the modification machinery know how to handle them:



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