Post transcriptional modification

1. What is post transcriptional Modification of RNA
2. How is Post Transcriptional Modification of RNA different in Prokaryotes and Eukaryotes
3. What are the Various Types of Post Transcriptional Modification of RNA
4. What is the Mechanism of 5' Capping of RNA
5. What is the Mechanism of 3' Polyadenylation of RNA
6. What is the Function of 5' Capping of RNA
7. What is Function of 3' Polyadenylation of RNA
8. What is Splicing ?
9. What is the Mechanism of Splicing ?
10. What is Spliceosomes ?
11. What is Sn RNA or Small Nuclear RNA ?
12. What is SnRNP Complex or SNURPs ?
13. Beta Thalessemia because of faulty Splicing ?
14. What is Methylation post transcriptional modification ?
15. What is Alternative Splicing?
16. What is Selective Splicing ?
17. What is Alternative polyadenylation ?
18. What is Alternative 5' donor Splicing ?
19. What is Alternative 3' Donor Splicing ?
20. What is the role of Alternative Splicing ?
21. What is RNA Editing ?
22. How is RNA Editing an Exception to Central Dogma ?
23. Example of Apolipoprotein B Gene for RNA Editing
24. Other Examples of RNA Editing

1. Post Transcriptional Modification

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Post Transcriptional Modification
Post Transcriptional Modification
Prokaryotes
Eukaryotes
mRNA not subjected to Post Transcriptional Modifica tion Translation started simultaneous with Transcription tRNA and rRNA undergo Post Transcriptional Modificati on
Primary Transcript or Heteronuclear RNA ( hn RNA) hnRNA underoges extensive post transcriptional modifi cation
Kind of Post Transcriptional Modification
•7 methylguanosine capping at 5’ end •Addition of poly A tail at 3’ end •Splicing •Methylations •Alternative RNA Processing
DNA
RNA
Proteins
Transcription
Translation
Post Transcriptional
Modification

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Post Transcriptional Modification (Methylation & Po ly A )
7 methyl guanosine 5’ Capping
•In Nucleus
•Guanosine Triphosphate attached to 5’ End by 5’-5’ unusual
linkage by enzyme Guanylyltransferase
•In Cytosol
•Methylation with methyl group from S Adenosyl Methio nine
using guanine 7 methyltransferase
•Functions
•Initiation of Translation
•Stabilize the mRNA
•Prevents attack of 5’->3’ exonuclease
Addition of Poly A Tail at 3’ End
•In Nucleus
•Poly A Tail added at 3’ by
polyadenylate polymerase
•The Length of Poly A Tail can be upto 200 adenine bas es
•Functions
•Poly A Tail and its binding protein PAB-1
is required for efficient
initiation of protein synthesis
•Stabilize the mRNA
•Prevents attack of 3’ to 5’ exonuclease

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Primary Transcript + 5 snRNA (U1 / U2/ U4 / U5 / U6) and more than 60 Proteins (Ribonucleoproteins or RN P) Uracil Rich RNA which acts as Enzyme ( Ribozyme ). U6 is essential U7 is SnRNA involved in production of correct 3’ end s of histone mRNA that lacks Poly A tail SnRNA + RNP = SNURPs ( Clinical Correlation :- SLE ant ibodies against SNURPs)
Spliceosomes
Small Nuclear RNA
(Sn RNA)
SnRNP Complex
(SNURPs)
Joining Together of EXONS (After Removal of Introns )
Binding of SnRNP brings neighboring exons into corre ct alignment for splicing Nucleophilic attack at 5’ end of intron Lariat Formation and Cut at 3’ End of Intron Ligation of 3’ endo of Exon 1 to 5’ End of Exon 2
Self Splicing – Certain hrRNA has Self Spicing activi ty because of Ribozyme Activity
Faulty Splicing Can lead to aberrant proteins .
Example – Incorrect splicing of Beta Globin mRNA is responsible for some cases of Beta Thalassemia
Post Transcriptional Modification
-
Splicing

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Methylation & Alternative Splicing
Methylation of N7 of adenine and 2’ Hydroxyl group of ribose in cytosol
Methylations
A mechanism of producing a diverse set of proteins from a limited set of genes. ways in different ti ssues
Selective Splicing -
Selective Inclusion or exclusion of exons
Alternative 5’ Donor
5’ Donor Site of certain exons is changed
Alternative 3’ Acceptor
3’ Acceptor site of certain exons is changed
Alternative Polyadenylation
Different Site is used for Polyadenylation
Application of Alt. Splicing
Generation of Membrane bound or Secretory IgG by alt ernative polyadenylation site Production of several tissue specific isoforms of t ropomyosin from single mRNA Transcript
1
2
3
AAUAA
AAUAA
1
3
AAUAA
AAUAA
1
2
3
AAUAA
AAUAA
1
2
3
AAUAA
AAUAA
1
2
3
AAUAA
The Pre mRNA from same gene is spliced in two or mo re alternative ways in different tissues
Alternative Splicing

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RNA Editing
DNA
RNA
Proteins
The Central Dogma
RNA Editing
Coding information is changed at mRNA level by chem ical modification of nitrogenous bases present in t he codes Hence is an exception to Central Dogma
Apolipoprotein B Gene
Codon #1
Codon #4564
Codon #2153
CAA
Liver
Intestine
Unedited
mRNA
Apoprotein B – 100
•4563 Amino Acids (512kDa)
•Transport Cholesterol in
blood
CAA (Gln)
(Cytidine Deaminase)
UAA (STOP)
Apoprotein B – 48
•2152 Amino Acids ( 242kDa)
•Absorption of Lipid from
intestine
Other Examples – Glutamate Receptor (Glutamine Chang ed to Arginine) and Trypanosome mitochondrial DNA