7-Methyl-guanosine cap

The translation of eukaryotic mRNA is similar to
that of prokaryotic mRNA, with two distinct
differences: (1) transcription and translation
occur at different locations in the eukaryotic
cell: transcription occurs in the cell nucleus,
and translation in the cytoplasm; (2) the 5! and
3! ends of eukaryotic mRNA have special structures.
The structure at the 5! end is called a cap.
Through the action of guanosine-7-methyltransferase,
guanosine is bound by a triphosphate
bridge to the first and second ribose
groups of the precursor mRNA chain. The guanosine
is methylated in position 7, as are the
two initial ribose residues at the beginning of
the RNA chain. Except for the mRNAs transcribed
by DNA viruses, eukaryotic mRNA usually
contains a single protein-coding sequence
(monocistronic messenger).