Post Transcriptional Modification Of Rna

• The primary transcripts of both prokaryotic and eukaryotic tRNA and rRNA are post-transcriptionally modified by cleavage of the original transcripts by ribonucleases.
• In Prokaryotes mRNA are is not subjected to post-transcriptional processing.
• In eukaryotes, The collection of all the primary transcripts synthesized in the nucleus by RNA polymerase II is known as heterogeneous nuclear RNA (hnRNA).
• The primary transcripts are extensively modified in the nucleus after transcription.

These modifications usually include:

A) 5′ “Capping”: 

• The first of the processing reactions for pre-mRNA, The cap is a 7-methylguanosine attached “backward” to the 5′-terminal end of the mRNA, forming an unusual 5’→5′ triphosphate linkage.
• Methylation of this terminal guanine occurs in the cytosol and is catalyzed by guanine-7-methyltransferase.

B) Addition of a poly-A tail:

• Poly A tail is added to the 3′ end of the hnRNA
• Polyadenylate Polymerase is the enzyme
• Takes place in the nucleus
• Poly  (A)  tail  translates  into  Polylysine
• Length of Poly-A tail is up to 200 Adenine bases
• These tails help to stabilize the mRNA (by protecting from 3r-exonuclease), facilitate exit from the nucleus, and aid in translation.
• Some rRNAs do not have a poly-A tail,  e.g. mRNAs of histones and some interferons.

C) Removal of introns  (splicing):-

• Intron: an Intervening sequence that does not code for the amino acid
• Introns  are  exised  by RNA splicing by Sn-RNAs/Snurp.
• Exon: Amino acid coding sequence Molecular machinery that carries out splicing is called Spliceosome
• The process by which introns are excised and exons are linked to form functional mRNA are called splicing, Thus mature mRNA does not contain introns.
• snRNA  combines  with  proteins to  form  small  nuclear  ribonucleoprotein  particles  (snRNPs  or  snurps)
• It  is the snRNA  component  of snurps  that  catalyzes  splicing
• Snurps are U, U4, Us and Uu. Defective splicing (splicing mutation) is the most common cause of β-thalassemia.
• only about 1.5% of human DNA has coding sequence (exons), remaining is non-coding(introns).
• Two important gene-silencing RNAs are :- (i) Micro RNAs, and (ii) interfering RNAs (siRNAs).

D) Alternate splicing :-

• The hn-RNA molecules from some genes can be spliced in alternative way in different tissues.
• Thus two or more different mRNA (and therefore 2 or more proteins) can be synthesized from same hnRNA.