The primary transcripts ($hn$-$RNA$) 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 $5^{\prime}$-end of $hnRNA$. In tailing, adenylate residues ($200$-$300$) are added at $3^{\prime}$-end in a template independent manner. It is the fully processed $hnRNA$, now called $mRNA$, that is transported out of the nucleus for translation.