<prompt> 7-methyl guanosine is added as a cap on mRNA. This methyl group is donated by:S-Adenosyl methionine SAMMethenyl THFTetra hydro Folate THFVit B12  <response>: Capping is a post transcriptional modification. 7-methyl guanosine cap is added at the 5end of mRNA by Guanylyl Transferase. Methyl group is donated by SAM S-Adenosyl Methionine by enzyme Guanine-7-Methyl Transferase, which occurs in cytoplasm rest all post transcriptional modifications occur in nucleus. This cap facilitate initiation of translation and stabilize mRNA by preventing it from attack by 5 exonucleases. Cap is attached to 5 end of mRNA by an unusual 5 5 triphosphate linkage. 