Most eukaryotic genes are interrupted by non-coding or non-translatable sequences known as introns. The coding or translatable sequences are known as exons.Such genes are called split genes/interrupted genes. Both intron and exon sequences are transcribed to produce a primary transcript/precursor RNA/pre RNA.
The precursor RNA for mRNA is known as heterogenous RNA/hn RNA. Following the transcription, the intron sequences are removed and exons are joined to form a mature or functional RNA. This process is known as RNA splicing. For this reason, the RNA transcript in nucleus differ from the mRNA found in the cytoplasm for translation For each split gene only about 25% of hn RNA takes part in splicing leading to mRNA formation. The rest undergoes degradation.
The hn RNA gets modified by the addition of 7-mcthyl Guanosine residue (m7G) to the 5′-end even before the transcription is completed. This is called s’-capping. The 5′-cap of myG helps in recognition of ribosome. The capping is initiated by addition of GTP in s’-end in reverse orientation of 5′-5′ phosphodiester (not the usual s’-3′ diester) and then a methyl group is transferred to X-7 by methyl transferase enzyme. However, some eukaryotic mRNA like those for histone proteins, lack 5′-cap.
The 3′-end of most eukaryotic mRNA has a long stretch "A" residues added after transcription. This stretch of "A" residues is called Poly-A-tail and the process of its addition is called Polyadenylation. Poly-A polymerase enzyme adds a poly-A tail of about 200 Adenine nucleotide to the 3′-end of the primary transcript.
Some of-thc hn RNAs are spliced in large complexes called spliccosomcs. Spliceosomes arc complexes of proteins and five types of small nuclear RNAs (S" RNA). These small nuclear RNAs are Ui, U2,U4, U5 and U6. Some HN RNA molecules are also auto spliced without the involvement of spliceosome.