I am slightly confused as to what part of the genome codes for non-coding RNAs. Is it the introns? This would make sense to me as to why they are not transcribed as the introns are not transcribed. Or is non-coding RNA actually exons which are simply not transcribed? If so, what are the introns actually for? I cannot seem to find a succinct answer to this. Wikipedia, for example, seems rather vague:
While introns do not encode protein products, they are integral to gene expression regulation. Some introns themselves encode functional RNAs through further processing after splicing to generate noncoding RNA molecules. Alternative splicing is widely used to generate multiple proteins from a single gene. Furthermore, some introns play essential roles in a wide range of gene expression regulatory functions such as non-sense mediated decay and mRNA export.
In either case, does anyone also have any information as to why the ncRNA is not translated? What is the mechanism that prevents it from being translated? My guess is that, for some ncRNAs, it is a matter of them not being able to leave the nucleus as I understand many ncRNAs deal with post-transcriptional gene silencing, although tRNA and rRNA clearly do leave the nucleus...
I would be very grateful for any help about introns/exons/ncRNAs. Thank you :)