Primary Transcript is often larger than the functional RNA. This primary transcript is called heterogeneous nuclear RNA or hn RNA and is converted into functional mRNA after post-transcriptional processing which involves 3 steps.

(i) Modification of 5’end by capping: Capping at 5' end occurs rapidly after the start of transcription. An unusual nucleotide i.e. methyl guanosine triphosphate is added to the 5’-end of hnRNA. It is catalysed by guanyl transferase. Cap is essential for Formation mRNA-ribosome complex. Translation is not possible if cap is lacking because cap is identified by 185-rRNA of ribosome unit. 

(ii) Tailing and Splicing: tailing is the addition a poly(A) tail is added to an RNA at the end of transcription .The poly(A) tail consists of multiple adenosine monophosphates; on newly formed hn RNA with the help of poly (A) polymerase. Splicing is the process of removal of introns and joining of exons in a defined order. Introns are removed by small nuclear RNA (SnRNA) and protein complex called small nuclear riboneucleo proteins or snRNP (Snurps).

The fully processed hn RNA is now called mRNA and it is transported out of the nucleus for translation.