Post transcriptional changes

this explains the modification of RNA in eukaryotes

1. Asst. Prof. R. M. Mahindrakar
Botany Department
J.S.S. Arts, Science and Commerce College Gokak

2. Post Transcriptional Changes

3. mRNA Processing
1.Capping (addition of a 5’ 7-methyl guanosinecap)
2.Polyadenylation(addition of a poly-A tail at the 3’)
3.Splicing to remove intervening sequences (introns)

4. Process of Capping:

5. Cap Functions
Cap provides:
1. Protection from some ribonucleasesdegradation
2. Stabilizes mRNA
3. Enhanced translation and splicing
4. Enhanced transport from nucleus to cytoplasm

6. Addition of Poly –A Tail

8. Functions:
The Poly A tail also provides stability
It facilitate the exit of mRNA from the nucleus.
After the mRNA enters the cytosol, the poly A tail is
gradually shortened

9. Splicing

10. Splicing

15. EXON SHUFFLING
Some genes have exonsthat can be exchanged by a process
called exonshuffling.
For example a gene with four exonsmight be spliced
differently in two different cell types
In Cell type 1 : exons2 and all intronsare spliced resulting
into a protein with 1,3 and 4 exons.
In Cell type 2 : exons3 and all intronsare spliced resulting
into a protein with 1,2 and 4 exons.
Thus different protein types will be produced by two cell
types
This phenomenon is known as Exonshuffling.

17. Processing of tRNA
Removal of leader sequence & trailer
Replacement of nucleotide
Modification of certain bases:
Splicing of an intron

19. Processing of ribosomal RNA
Processing of 45s molecules occurs inside nucleolus
45s molecules tightly associated protein forming
(RNPs)
First cleavage: It occurs at site I & remove 5’ terminal
leader sequence, produces 41s intermediate & 18s
Second cleavage: occurs in 41s intermediate at site 3’
generates 32s intermediate
Final cleavage: separation of 32s intermediate into 28s,
5.8s
Processed rRNA28s, 5.8s & 18s