Post-transcriptional modifications mainly include the processing of heterogeneous RNA, which is generated as the product of transcription and converted to its mature form. This includes the 5’ capping, splicing and 3’ polyadenylation.  In the 5’ processing step, the 5’ end of the primary transcript is capped with 7-methylguanosine and requires the aid of enzymes such as phosphatase and guanosyl transferase. This capping protects the primary transcript from the attack of Ribonuclease enzymes, which exhibit specificity for the 3’-5’ phosphodiester bonds. In splicing, the intervening non-coding sequences (introns) are spliced out and the exons (coding sequences) are joined together to provide a continuous stretch of coding nucleotide sequence. In many cases, this process occurs concomitantly with transcription itself and makes use of a complex Spliceosome Assembly and small nuclear RNA to bring about splicing. The process by which a given pre-mRNA transcript is spliced differentially and hence is responsible for giving rise to different protein products is known as ‘Alternative Splicing’. The 3’ processing involves the cleavage of the 3’-end and addition of approximately 250 adenine residues, rendering a poly A tail at the 3’-end of the pre-mRNA. Polyadenylation usually occurs at a site where a polyadenylation signal sequence (5'- AAUAAA-3') is recognized near the pre-mRNA. This tail addition is also responsible for protecting the RNA transcript from the attack of Ribonuclease enzyme. The RNA thus generated after these steps of processing is termed as the mature RNA.

Pre-mRNA is synthesized from genomic DNA by the process of transcription. The gene to be transcribed is bound by transcription initiation factors and then by RNA Polymerase, which transcribes the gene in the 5’ to 3’ direction. The DNA strand that gets transcribed is known as the template strand. Once the termination sequences are reached, the enzyme and the newly formed mRNA transcript are released. The genomic DNA that gets transcribed into mRNA contains exons, the coding sequences, as well as introns which are intervening, non-coding sequences. This pre-mRNA has certain recognition sites within its intron sequence that allows the spliceosome assembly to recognize and bind to it. There is a conformation change that takes place upon binding of the protein-RNA complex. The remaining snRNPs bind following the conformation change of the pre-mRNA and there is cleavage at the GU site on the 5’ end of the intron. It attaches to the branch site adenine nucleotide near its 3’ end to form the lariat structure. The assembly cleaves the 3’ end of the intron sequence containing the AG recognition element. The free 3’ hydroxyl group of the first exon attacks the 5’ end of the second exon such that they are joined to give the mature mRNA.