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Transcription
By
Dr. Ahmed Okasha
Transcription unit
Promotor region
It formed of certain sequence that are
responsible for initiation of
transcription
Prokaryotic promotor
Pribnow box formed of sequence
TATAAT
Second nucleotide sequence
TGTTGACA
Promotor
Sigma
Eukaryotic promotor
TATA box
CAAT box
Transcription region
Part of DNA transcribed to RNA
Termination region
Part of DNA help in termination of
transcription
Requirement for RNA synthesis
ATP, GTP, UTP, CTP (NTP = nucleotide
triphosphate)
RNA polymerase
Prokaryotic RNA polymerase
Holoenzyme of RNA polymerase formed of core
molecule and specific protein is sigma factor
δ
Core molecule formed of 2 α and 2 (β and β/)
Sigma factor (δ)
Help RNA polymerase recognize promotor
’
’
Core enzyme
’
’
Holoenzyme
Search for initiation site
Detect termination site
Interact with activator and
inhibitor
Unwinding DNA
Function of RNA polymerase Sigma factor (δ Search for initiation site
Eukaryotic RNA polymerase
1- RNA polymerase I synthesis
rRNA,
2- RNA polymerase II synthesis
mRNA,
3- RNA polymerase III synthesis tRNA, DNA 5___GCATAC __3 Coding or sense or + strand
RNA 5___GCAUAC __3 RNA synthesized
DNA 3___CGTATG __5 Template or antisense or – strand
is the strand Sense strand
whose sequence similar to
sequence of RNA synthesized
except for U and T
region before Upstream:
promoter indicated by –
region after Down stream
promoter indicated by +
Step of RNA synthesis
Initiation
Sigma factor (δ) help RNA
polymerase recognize promotor
β/ Subunit bind to DNA template
β Subunit bind to ATP, GTP, UTP,
CTP
Binding of RNA polymerase to DNA
lead to separation of DNA double
helix
RNA polymerase
Sigma subunit binds closed promotor
Initiation takes place
Open
promoter
5 3
DNA 3 5
Promoter Termination region
Initiation
2- Elongation
Elongation occur in 5 to 3
direction antiparallel to template
Nucleotide inserted according to
pairing rule A=U , G=C
No primer are required
RNA polymerase
Sigma subunit binds closed promoter
Initiation takes place, factor dissociate from core polymerase
Elongation occurs with
core RNA polymerase
Open
promoter
5 3
DNA 3 5
Promoter Termination region
3-Termination
Rho factor dependant
termination
Binding of Rho factor to RNA
polymerase
Slowing down of RNA polymerase
at termination sites
RNA polymerase
Sigma subunit binds closed promoter
Initiation takes place, factor dissociate
from core polymerase
Elongation occurs with
core RNA polymerase
Open
promoter
Termination
RNA
5 3
DNA 3 5
Promoter Termination region
RNA 5’ 5’
3-Termination
Rho factor independant -B
termination (means the palindromeFormation of
sequence that read from right to left in the
top as left to right in the bottom 5 – GAATTC – 3
3- CTTAAG – 5
it is self hair pin,Formation of complementary strand of RNA followed
by several U
Antibiotic inhibitor of
transcription
Rifamycin
Bind to β subunit of RNA
polymerase so prevent
transcription
Actinomycin D
Bind to DNA template so prevent
RNA polymerase on movement on
DNA
Difference between DNA polymerase
and RNA polymerase
RNA
polymerase
DNA polymerase
Synthesize RNA
Not need
No proof reading
Synthesize DNA
Need RNA primer
Has proof reading activity
RNA
polymerase
DNA polymerase
Low fidelity (no
nuclease activity)
Make unwinding
High fidelity
Cannot make
unwinding
Post transcription modification (processing) of RNA
Eukaryotic mRNA
methy guanosine7 Cap formed of -5
Function : 1- Protect mRNA from 5
exonuclease
Help in translation of mRNA to protein
end 3 Poly A tail at
Function : Protect mRNA from 3
exonuclease , also help in translation
Post transcription modification (processing) of
RNA
Splicing
Exon : is a coding sequence on mRNA
expressed to protein
Intron : is a noncoding sequence on
mRNA not expressed to protein
CGGAAM GGCAAU
Exon Intron Exon
Splicing is a removal of intron
and joining of exon by
splicesomes
Splicing made by help of other
protein as HnRNA, small
nuclear ribonucleoprotein
particle and other protein as U1,
U2, U4, U5, U6 Alternative splicing
It removal of intron and joining of specific exon
Function : 1- Protect from mutation
2- Production of specific protein
HnRNA + SmRNA ______ mature mRNA
Modification of tRNA
1-Removal of 5 extrasequence
2- Addition of CCA at 3 end
3- Methyaltion of some baes
4- Addition of anticodon loop
Difference between cytoplasmic mRNA
and nuclear analogue
Nuclear mRNA Cytoplasmic
mRNA
1- No Post
transcription
modification
2- Contain exon and
intron
3- No Cap and tail
1- Post transcription
modification
2- Contain exon only
3- Contain Cap and
tail
Difference between
transcription in prokaryote and
eukaryote Eukaryote Prokaryote
Present No Post transcription
modification
Long short T half
Separate Occur at same
time
Transcription and
translation
Monocistron polycistron RNA