Such processing is vital for the correct translation of eukaryotic genomes because the initial precursor mRNA produced by transcription often contains both exons (coding sequences) and introns (non-coding sequences); splicing removes the introns and links the exons directly, while the cap and tail facilitate the transport of the mRNA to a ribosome and protect it from molecular degradation.
The diphosphate 5' end then attacks the alpha phosphorus atom of a GTP molecule in order to add the guanine residue in a 5'5' triphosphate link.
The enzyme (guanine-N7-)-methyltransferase ("cap MTase") transfers a methyl group from S-adenosyl methionine to the guanine ring.
The cap protects the 5' end of the primary RNA transcript from attack by ribonucleases that have specificity to the 3'5' phosphodiester bonds.
For UGUA dependent processing sites, binding of the multi protein complex is done by Cleavage Factor I (CF I).
The resultant protein complex formed contains additional cleavage factors and the enzyme Polyadenylate Polymerase (PAP).
Poly(A) polymerase then adds about 200 adenine units to the new 3' end of the RNA molecule using ATP as a precursor.
This process is known as alternative splicing, and allows production of a large variety of proteins from a limited amount of DNA.