Give an account of post transcriptional modifications of a eukaryotic mRNA.

Explanation

Post-transcriptional Modifications
The primary trasncripts are non-functional, containing both the coding region, exon and non-coding region, intron in RNA and are called heterogenous RNA or hnRNA. In eukaryotes, three types of RNA polymerases are found in the nucleus
(i)RNA polymerase I transcribes rRNAs (28 S and 5.8 S).
(ii) RNA polymerase II transcribes the precursor of mRNA (called heterogeneous nuclear RNA or hnRNA).
(iii) RNA polymerase III transcribes tRNA, 5 S rRNAs (small nuclear RNAs). The hnRNA undergoes two additional processes called capping and tailing
In capping, an unusual nucleotide, methyl guanosine triphosphate is added to the 5'- end of hnRNA.
In tailing, adenylate residues (about 200-300) are added at 3'-end in a template independent manner.
Now the hnRNA undergoes a process where the introns are removed and exons are joined to from mRNA by the process called spllcing.

Note In prokaryotes, mRNA does not require any processing.