this ppt talks about the process of transcription in prokaryotes.

1. S.Kaayathri Devi
I6UTIO
III B.Sc., Biotechnology
ANJAC

2. Central dogma of biology
REPLICATION
DNA DNA
TRANSCRIPTION POST
TRANSCRIPTIONAL
MODIFICATION
RNA
TRANSLATION POST TRANSLATIONAL
MODIFICATION
PROTEIN

4. INTRODUCTION
 Synthesis of RNA by copying DNA template is known
as transcription.
 Only selected portion of the genome (exons)is
transcribed .
 RNA polymerase requires no primers it is a DNA
dependent RNA polymerase

6. The process of transcription takes place in three major
steps. They are :
1. Initiation.
2.Elongation.
3.Termination.

7. INITIATIATION
 Transcription is initiated by RNA Polymerase
holoenzyme from a specific region called promotor
sequence in DNA .
 Principle enzyme – Bacterial RNA Polymerase
 RNA Polymerase alone is called core Polymerase .
 Core polymerase consists of α,β,β’and ω sub units.

9. The binding of core polymerase to promotor
sequence is facilated by σ factor.
Core polymerase + σ factor = RNA polymerase
holoenzyme
Promotor sequence E. Coli :
1. 5’-TTGACA-3’ at – 35 element
(pribnow box)
2. 5’ – TATAAT – 3’ at – 10 element
(TATAA box)

10.  Closed complex formation :
Binding of RNA polymerase holoenzyme
to promotor sequence.
 Open complex formation :
1. RNA polymerase holoenzyme separates
10-14 DNA basepairs extending from
-11 to +3 and this is called melting.
2. It leads to the formation of open
complex.
3. closed complex to open complex
isomerization

11. Teritiary complex formation :
1. If the enzyme synthesizes RNA <10
basepairs ,it does not elongate it
abortive initiation.
2. Because σ factor acts as a mimic of RNA
and blocks the RNA exit channel in open complex.
3. When RNA polymerase synthesizes
RNA > 10 base pairs , it ejects σ factor and RNA
elongates further and moves out through exit channel.
This is known as the teritiary complex

15. ELONGATION
 After the formation of teritiary complex the RNA
elongates further by the addition of dNTPs .
 The synthesized is proof read by hydrolytic editing.
Gre factor enhances the proof reading activity.
Pyrophospholytic editing is another proof reading
mechanism.

17. TERMINATION
 The process of termination can happen in two
different mechanisms. They are :
1. Rho – independent termination.
2. Rho – dependent termination.

18. Rho – independent termination
In this mechanism, transcription is terminated by
termination sequence in DNA.
Terminator sequence contains invert repeat causing
complementary pairing as transcript RNA form
hairpin structure.
Invert repeat is followed by a large number of T (~8
basepairs), in RNA “U” appears.
The load of hairpin structure isn’t tolerated by A=U
bps so RNA gets separated from RNA –DNA
heteroduplex.

20. Rho – dependent termination
In this mechanism,transcription is terminated by rho
protein (ρ).
Rho protein is a single strand ATPase protein.
Rho protein binds to the single stranded RNA after
termination.
In bacteria, transcription and translation occurs
simultaneously so Rho protein binds to the mRNA
after translation i.e.,when translation is completed but
transcription is “ON”.

22. 