The document discusses post-transcriptional modifications in eukaryotic cells, specifically focusing on processes such as 7-methylguanylate capping, polyadenylation, and splicing. It covers the roles of small nuclear RNA (snRNA) and ribonucleoproteins (RNPs) in creating spliceosomes and highlights the impact of faulty splicing on protein production, exemplified by beta thalassemia. Additionally, it mentions RNA editing as a deviation from the central dogma, altering coding information in mRNA through chemical modifications.

1. Post Transcriptional Modification

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Post Transcriptional Modification
Post Transcriptional Modification
Prokaryotes Eukaryotes
mRNA not subjected to Post Transcriptional Modification
Translation started simultaneous with Transcription
tRNA and rRNA undergo Post Transcriptional Modification
Primary Transcript or Heteronuclear RNA ( hn RNA)
hnRNA underoges extensive post transcriptional modification
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 & Poly 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 Methionine
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 bases
• 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 RNP)
Uracil Rich RNA which acts as Enzyme ( Ribozyme ). U6 is essential
U7 is SnRNA involved in production of correct 3’ ends of histone mRNA that lacks Poly A tail
SnRNA + RNP = SNURPs ( Clinical Correlation :- SLE antibodies 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 correct 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 activity 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 cytosolMethylations
A mechanism of producing a diverse set of proteins from a limited set of genes. ways in different tissues
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 changedAlternative Polyadenylation Different Site is used for Polyadenylation
Application of Alt. Splicing
Generation of Membrane bound or Secretory IgG by alternative polyadenylation site
Production of several tissue specific isoforms of tropomyosin 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 more 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 chemical modification of nitrogenous bases present in the 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 Changed to Arginine) and Trypanosome mitochondrial DNA