"Modifying" pre mRNA - (Jan/03/2019 )

Hi,

          Where is mRNA "modified" after being transcribed by the RNA polymerase? More specifically, where are the introns spliced from the rest of the sequence?

If you mean “where in the cell…,” splicing takes place in the nucleus.  If you mean “where in the RNA strand…,” there are specific patterns of bases (G, A, U, or C) that are recognized by the “spliceosome” at the start, close to the end, and at the end of the sequence (intron) that is to be removed, and the remaining exons are to be re-joined.  There is a consensus sequence for these recognized base sequences but, of course, just to complicate the explanation, the actual sites are variable and difficult to predict. For more details, the Wikipedia article is not a bad place to start: https://en.wikipedia.org/wiki/RNA_splicing. It includes a diagram that you might find helpful. Predicting splice sites is an “art” that has inspired a number of softwares but apparently none are perfect, as some aspects of recognition of splice sites are still unknown. The imperfection of splicing seems to be part of its function, as mRNA splice variants do occur and are important in Biology.

Thank you. I asked for the first answer. I didn't know it happened in the nucleus.