7-methyl guanosine is added as a cap on mRNA. This methyl group is donated by:

Capping is a post transcriptional modification. 7-methyl guanosine cap is added at the 5'end 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.