The primary transcripts (hnRNA) contain both the exons and the introns and are non-functional. Hence, it is subjected to a process called splicing where the introns are removed and exons are joined in a defined order. Intron is the portion of gene which is transcribed but not translated. In prokaryotes hnRNA is absent so splicing in not required. hnRNA undergoes additional processing called as capping and tailing.
In capping an unusual nucleotide (methyl guanosine triphosphate) is added to the 5′-end of hnRNA. In tailing, adenylate residues (200-300) are added at 3′-end in a template independent manner. It is the fully processed hnRNA, now called mRNA, that is transported out of the nucleus for translation.