The document discusses post-transcriptional modifications in molecular biology, highlighting its significance in transforming nascent RNA into mature functional RNAs, specifically focusing on mRNA. It details the types of modifications that occur in eukaryotes, such as 5’ capping, polyadenylation, splicing, and alternative RNA processing. The document also emphasizes the roles and mechanisms of these modifications in ensuring mRNA stability and initiation of translation.

1. POST-TRANSCRIPTIONAL
MODIFICATIONS
MOLECULAR BIOLOGY-II (BBC 207)
SRISHTI
S.Id-2019008455
ROLL NO-1907143010
BSc. Hons BIOCHEMISTRY (TERM-04)
SBSR, SHARDA UNIVERSITY

2. Post Transcriptional Modification
DNA RNA PROTEINS
MODIFIED RNA

3. Post Transcriptional Modification
• Post transcriptional modification or Co-transcriptional modification
• Chemical modification of RNA primary transcript to produce mature
functional RNA.
• Nascent RNA, also known as primary transcript, needs to be modified to become
functional tRNAs, rRNAs, and mRNA.
• Primary transcripts of mRNA are called as heterogenous nuclear RNA (hnRNA).

4. Post Transcriptional Modification
PROKARYOTES
• mRNA not subjected to Post
Transcriptional Modification
• Translation starts simultaneously
with Transcription
EUKARYOTES
• Primary Transcript or
Heterogenous RNA (hnRNA)
• hnRNA undergoes extensive post
transcriptional modification

5. Types of Post Transcriptional Modification
1. 7 methylguanosine capping at 5’ end
2. Addition of poly A tail at 3’ end
3. Methylations
4. Splicing
5. Alternative RNA processing
6. RNA editing

6. Post Transcriptional
Modification-(5’methyl
cap)
In Nucleus
• Guanosine Triphosphate attached to 5’end
by 5’-5’ unusual linkage by enzyme
Guanosyltransferase
In Cytosol
• Methylation with methyl group from S
Adenosyl Methionine using guanine 7
methyltransferase
Functions
• Initiation of translation
• Stabilize the mRNA
• Prevent attack of 5’->3’ exonuclease
NOTE: Not seen in tRNA

7. Post Transcriptional
Modification-(Poly A
Tail at 3’end)
In Nucleus
• Poly A Tail added at 3’end by
polyadenylate polymerase
• The length of Poly A tail can be
up to 200 adenine bases
Functions
• Poly A Tail & its binding protein PAB-1
is required for efficient initiation
of protein synthesis
• Stabilize the mRNA
• Prevent attack of 5’->3’
exonuclease

8. Post Transcriptional Modification-(Splicing)
Joining together of EXONS (after Removal of Introns)
Binding of SnRNP brings neighboring exons into correct alignment of
splicing

10. Post Transcriptional Modification-(Splicing)
Self Splicing – Certain hnRNA has splicing activity because of Ribozyme Activity
Spliceosomes
Primary Transcript + 5snRNA (U1/ U2/ U4/ U5/ U6) & more than 60 proteins (Ribonucleoproteins or RNPs)
Small Nuclear RNA (Sn RNA)
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
SnRNP Complex (SNURPs)
SnRNA + RNP = SNURPs (Clinical Correlation :- SLE antibodies against SNURPs)

11. Methylation & Alternative Splicing
Methylations
• Methylation of N7 of Adenine and 2’ hydroxyl group of ribose in cytosol
Alternative Splicing
• A mechanism of producing a diverse set of proteins from a limited set of genes
varies in different tissues.
• The pre mRNA from same gene is spliced in two or more alternative ways in
different tissues

12. 3
2 AAUAA AAUAA 3
1 AAUAA
1 AAU
Selective Splicing –selective inclusion or exclusion of proteins

13. 3
2 AAUAA AAUAA 2
1 3
1 AAUAA
Alternative polyadenylation –different site is used
for polyadenylation

14. 3
2 AAUAA AAUAA 2
1 3
1 AAUAA
Alternative 5’ donor –5’ donor site of certain exons is
changed
AAUAA

15. 3
2 AAUAA AAUAA 2
1 3
1 AAUAA
Alternative 3’ acceptor –3’ acceptor sites of certain exons
is changed
AAUAA

17. RNA Editing
• Coding information is changed at mRNA level by chemical
modification of bases present in the codes
• Hence is an exception Of Central Dogma