Post Transcriptional Modification Of Rna

Small Nuclear RNAs (snRNAs), a subset of the small RNAs, are significantly involved in rRNA and mRNA processing and gene regulation. Of the several snRNAs, U1, U2, U4, U5, and U6 are involved in intron removal and the processing of mRNA precursors into mRNA

• Initiation: RNA Polymerase Binds to DNA. DNA is transcribed by an enzyme called RNA polymerase. Specific nucleotide sequences tell RNA polymerase where to begin and where to end. RNA polymerase attaches to the DNA at a specific area called the promoter region.
• Elongation: Certain proteins called transcription factors unwind the DNA strand and allow RNA polymerase to transcribe only a single strand of DNA into a single stranded RNA polymer called messenger RNA (mRNA). The strand that serves as the template is called the antisense strand. The strand that is not transcribed is called the sense strand.
• Termination: RNA polymerase moves along the DNA until it reaches a terminator sequence. At that point, RNA polymerase releases the mRNA polymer and detaches from the DNA.

• RNA splicing: pre-mRNAs typically include introns.Exons:are the continuous coding regions. Introns: are the  noncoding intervening sequences. They may serve to separate functional domains (exons) of coding information in a form that permits genetic rearrangement by recombination to occur more rapidly than if all coding regions for a given genetic function were contiguous. Introns are removed by RNA processing in which the intron is looped out and cut away from the exons by snRNPs, and the exons are spliced together to produce the translatable mRNA.

B i.e. Polylysine

A i.e. RNA splicing

A i.e. Intron

B i.e. snRNA

–   Self splicing introns function as ribozyme and do not require external protein enzymes or high energy cofactors (eg ATP). Intron forms lariat in group II but not in group I self splicing introns.

–  Splicing is process of removing introns (i.e. the segment of gene that is not represented in mature m-RNA) from primary transcript and joining (or ligating together) exons (RNA sequences that code protein)Q. It is mediated by sn RNP or snrups formed from sn RNAQ.

Small nuclear RNAs (sn RNAs) are involved in RNA splicingQ; small nucleolar RNAs (sno RNAs) are involved in r RNA modificationsQ; micro RNAs (mi RNAs) & small temporal RNAs (st RNAs) are involved in inhibition (regulation) of gene expression (gene silencing)Q; small interfering RNAs (si RNAs) are involved in RNA interference (RNAi)Q; and B2 RNA binds to pol II and block transcription of many genes during heat shock

– Mammalian genomes seem to encode more non coding RNAs (nc RNAs) than coding m RNAs. nC RNAs are RNAS that do not encode proteins including r RNAS, t RNAs, mi RNAs, si RNAs, st RNAs and sn RNAs.

m-RNA processing

Prokaryotic mRNA is functional immediately upon synthesis, i.e. prokaryotic primary transcript of mRNA is functional. Thus it does not require post-transcriptional modification. In Eukaryotes the primary tran­script of mRNA is the hn RNA (hetrogeneous nuclear RNA). After transcription hnRNA is extensively modi­fied to form functional mRNA. These modifications are as follows.

1) The 5′-capping :- This is the first processing reaction. 5¹-end of mRNA is capped with 7-methylguansosine. This cap helps in initiation of translation (protein synthesis) and stabilizes the structure of mRNA by protecting from 5′-exonuclease.

2) Addition of poly ‘A’ tail :- As the name suggests, multiple ‘A’ (adenylate) residues are added at 3’end.This poly-A tail is not transcribed from DNA, but rather added after transcription. These tails helps to stabilize the mRNA (by protecting from 3′-exonuclease), facilitate exit from the nucleus, and aid in translation. After mRNA enters the cytosol, the poly-A tail is gradually shortened. Some mRNAs do not have poly-A tail, e.g. mRNAs of histones and some interferons.

3) Removal of introns (splicing) :- Eukaryotic genes contain some coding sequences which code for protein and some intervening non-coding sequences which do not code for protein. The coding sequences are called `exons’ and intervening non-coding sequences are called `introns’. The process by which introns are excised and exons are linked to form functional mRNA is called splicing. Thus mature mRNA does not contain introns.

• Splicesome Splicesome is an assembly made up of small nuclear RNA (snRNA), some proteins and hnRNA. snRNA combines with proteins to form small nuclear ribnonucleoprotein particles (snRNPs or snurps) that mediate splicing. It is snRNA component of snurps that catalyzes splicing. Snurps are U4, U5 and U6.
• Only about 1-5% of human DNA has coding sequence (exons). Remaining is non-coding (introns).

4) 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. For example, difference isoforms of tropomyosin in different tissues in due to alternate splicing.

Defective splicing (defect in snurps) is the most common mutation causing thalassemia.

• Molecular defect in pathogensis of thalassemia:‑

A) β-Thalassemia

• Most common type of genetic abnormality in β-thalassemia is point mutation i.e., nonsense.
• Some may also occur due to deletion or insertion i.e., frame shift mutations.
• Defect may occur at different steps of β-chain synthesis:

i) Splicing mutations

• Mutations leading to aberrant splicing are the most common cause of P-thalassemia.

ii) Chain terminator mutations

• This cause premature termination of mRNA translation.

iii) Promoter region mutations

• This results in transcription defect.

B) α-Thalassemia

• The most common cause of reduced a-chain synthesis is deletion of a-globin genes.
• Rarely nonsense mutation may also cause a-thalassemia.

 
In molecular biology and genetics, splicing is a modification of an RNA after transcription, in which introns are removed and exons are joined.

This is needed for the typical eukaryotic messenger RNA before it can be used to produce a correct protein through translation.

For many eukaryotic introns, splicing is done in a series of reactions which are catalyzed by the spliceosome a complex of small nuclear ribonucleoproteins (snRNAs), but there are also self-splicing introns.