Post transcriptional Modification of mRNA.

1. A Presentation
On
Post-Transcriptional Modification of mRNA
Submitted To
Rahima Nusrat Remme
Associate Professor
Agrotechnology Discipline
Khulna University
Submitted By
Rahul Das
Student ID-200809
3rd year, 1st term
Agrotechnology Discipline,
Khulna University
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2. Post-Transcriptional Modification
After transcription gene expression can be controlled
with the following mechanisms-
1. RNA editing
2. RNA interference
3.Alternative Splicing
4.mRNA degradation
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3. RNA Editing
 Some mRNAs are edited before translation modifying
their expression. It can occurs by two distinct
mechanism :
 1. Substitution editing
 2. Insertion/Deletion editing
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4. Substitution Editing
 Chemical alteration of individual nucleotides by
enzymes that recognizes a specific target sequence of
nucleotides. Ex-
 1.Cytidine deaminase converts a cytosine ( C) in the
RNA to Uracil (U)
 2.Adenisine deaminase converts an adenosine(A) to
inosine(I) which ribosome translates as guanosine (G)
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5. Substitution Editing
Apolipoprotein B-100
DNA Pre-Massenger RNA mRNA
29 Exons
28 Introns
4564 Codons
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6. Substitution Editing
In The Liver In The Intestine
Normal Transcription and
Translation
Codon 2153 is edited
4563 amino acid long protein CAA becomes UAA
Apolipoprotein B-100 Apolipoprotein B-48
Transports cholesterol and other
lipids in the blood
Lipid absorption in intestinal lumen
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7. Insertion/Deletion Editing
 Insertion or deletion of nucleotides in the RNA. These
alteration are made by guide RNA molecules which are called
gRNAs.
 gRNAs are small and complementary to the portion of the
edited mRNA.
 Base pairing of gRNA with unedited mRNA gives mismatched
regions which are recognized by the editing machinery
 Machinery include an Endonuclease, a Terminal Uridylyl
transfarase(TUTase) and a RNA ligase.
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8. Insertion/Deletion Editing
Endonuclease
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9. Insertion/Deletion Editing
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10. RNA Interference
 C. Melio and A. Fire discovered that dsRNA targets
mRNA and has a silencing function.
dsRNA
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11. Types and Functions of RNAi
siRNA miRNA
Short interfering RNA Micro RNA
Exogenous dsRNA Non coding RNA that forms hairpin
When a double stranded siRNA enters into the system, an enzyme called
DICER cleaves siRNA into smaller segments.
Then a protein from agronaut family binds with siRNA and discards the sense
Strand.
This protein with anti-sense strand forms RISC(RNA induced Silencing
Complex)
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12. RNAi Functions
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13. Alternative Splicing
 Same mRNA transcript can be spliced in different ways
that can yields different proteins from the same gene.
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14. mRNA Degradation
 RNA degradation occures in the nucleus and cytoplasm.
 Incorrectly spliced mRNAs are degraded in the nucleus
by an enzyme called exosome.
 Functional mRNAs are degraded after translation in
cytoplasm.
 Non-functional mRNAs are degraded in cytoplasm by
Nonsense mediated decay.
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15. mRNA degradation-Functional mRNAs
 Functional mRNAs have 5’ cap and a poly-A tail
 Degradation happens by removing the poly-A tail which
is called deadenylation.
 Then the mRNA is degraded by exosome- 3’to 5’
exonuclease activity.
 mRNA degradation happens in granular structure called
P-bodies.
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16. mRNA Degradation- Nonsense-mediated Decay
 Creation of a premature termination codon(PTC)
activates the NMD pathways.
 PTCs are result of genetic or somatic mutation or
mistakes during translation and transcription.
 This type of degradation is controlled by Exon junction
Complex(EJC)
 A translation termination complex made up of UPF1,
SMG1, eRF1,eRF3 degrades the mRNA and terminates
further transcription.
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