molecular biology

1. Post transcription
modification
Prepared by :
Rawya Alnaqeeb

2. In this Lecture , we take a
closer look at how eukaryotic
cells convert the initial
primary transcript
synthesized by RNA
polymerase II into a
functional mRNA.

3. Three major events occur during
the process:
 5, capping, 3, cleavage.
 polyadenylation.
 RNA splicing.
Processing occurs in the nucleus as the
nascent mRNA precursor is being
transcribed, and the functional mRNA
produced is transported to the cytoplasm
by mechanisms discussed later

5.  The nascent RNA ,also known as primary
transcript , needs to be modified to become
functional tRNAs, rRNAs , and mRNA.
 Primary transcription of mRNA are called as
heterogenous nuclear RNA(hnRNA) befor
splicing occurs .
 hnRNA are larger than matured mRNA
 hnRNA proteines to help keep the hnRNA
in single stranded form and to assist in the
various RNA processing reactions .
Post transcription modification

6. mRNA procssing
 Capping addition of 5, 7– mthyl gunosine.
 Polyadenylation addition of apoly
A tail at the 3,
 RNA splicing remove intervening
sequences (introns).

8. 5, capping ;-
The 5, Cap Is Added to Nascent RNAs Shortly
After Initiation by RNA Polymerase II
After nascent RNA molecules
produced by RNA polymerase II reach
a length of 25–30 nucleotides, 7-
methylguanosine and the other
components of the 5 cap found on
eukaryotic.

9. Formation of cap require three steps :-
1. 5, terminal phosphate group removes by
phosphates enzyme leaving diphosphate .
2. GTP is added by releasing pyrophsphate .
3. 7 th N of guanine is methylated by methyl
transferase enzyme .
Methyl group doner is S.adenosy methionine.

10. Function of cap :-
Cap provides
Protection from some ribonucleases
degradation .
Stabilizese mRNA .
Enhanced translation and splicing.
Enhanced transport mRNA for nucleus
to cytoplasme.

11. Polyadenylation ( poly A):-
 Additon of apoly a tail at the 3,, most
eukaryotic mRNA have a string of 40 to
200 adenine residues , making up the
poly (A) tail
 Adenine nucleotides are added by
enzyme adenylate transferase

12. RNA splicing.
The structural genes are composed of coding
and non coding regions .
 Coding sequences called expressed
sequences or Exons
 Noncoding sequences called
intervening sequences or Intron

14. Mature mRNA Structure
• 5, cap
• 5, Untranslated region
• Start codon
• Coding sequence
• Stop coden
• 3,
Untranslated region
• Poly A tail

15. tRNA:-
 Transfer RNA Soluble RNA Supernatant
RNA adaptor RNA.
 Smallest among RNA (75- 80 ) nucleotides .
 Recognized codon on mRNA .
 Carry amino acid to the site of protein synthesis.
 t RNA is transcribed by RNA polymerase 111.
 t RNA genes also occur in repeated copies
through the genome and may contain introns
anti cod .

16. D = dihydrouridine Ψ = pseudouridine

17. Base modification
 5, cleavage
 3, cleavage
 CCA addition
Processing of tRNA :-
1. Removal of leader sequence
2. Replacement of nucleotide
3. Modification of certain bases :
A. Replacement of U residues at the 3,
end of pre-tRNA with a CCA sequence

18. B. Addition of methyl and isopentenyl
groups to the heterocyclic ring of purine
bases
C. Methylation of the 2, -OH group in the
ribose of any residue ; and conversion of
specific uridines to Dihydrouridine ( D ) ,
Pseudouridine (Ψ)