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