Answer: In eukaryotes the primary transcript, hn-RNA (heterogeneous nuclear RNA), contains both exons and introns. It is processed into functional m-RNA by three steps:
- Capping: an unusual nucleotide, methyl guanosine triphosphate (7-mGTP), is added to the 5′ end of hn-RNA. The cap protects the RNA from nucleases and helps the ribosome to bind during translation.
- Tailing (polyadenylation): about 200–300 adenylate residues (poly-A tail) are added to the 3′ end in a template-independent manner. This stabilises the m-RNA and helps its export from the nucleus.
- Splicing: the non-coding introns are removed by spliceosomes (snRNPs) and the coding exons are joined together in a definite order.
The mature m-RNA then moves out of the nucleus into the cytoplasm for translation.
