Post-transcriptional Modifications  

The primary transcripts are non-functional, containing both the coding region, exon and non-coding region, intron in RNA and are called heterogeneous RNA or hnRNA.
 In eukaryotes, three types of RNA and are polymerases are found in the nucleus

(i) RNA polymerase l transcribes rRNAs (28 S and 5.8 S).

(ii) RNA polymerase ll transcribes the precursor of mRNA (called heterogeneous nuclear RNA or hnRNA)

(iii)  RNA polymerase lll transcribes tRNA, 5 S rRNA and snRNAs (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 form mRNA by the process called splicing.