transcription process synthesized ssRNA in prokaryotes, RNA polymerase and promoter of prokaryotes and eukaryotes

1. TRANSCRIPTION
Mrs. Praveen garg
VITS College, Satna

2. INTRODUCTION
• Transcription is the process by which the genetic
information occur in a double stranded DNA, is transfer
into a new molecule of messenger RNA (mRNA).
• Both prokaryotes and eukaryotes have the same process of
transcription, but eukaryotic have membrane bound
nucleus . It is major differences from prokaryotes.
• In the eukaryotes, transcription occurs in the nucleus of the
cell and the mRNA transcript must be transported to the
cytoplasm.
• In prokaryotes, which lack membrane-bound nuclei and
other organelles, transcription occurs in the cytoplasm of
the cell.

3. RNA POLYMERASE (E.coli)

4. • RNA Polymerase is the enzyme that produces the mRNA
molecule.
• Prokaryotes use the same RNA polymerase to transcribe
all of their genes.
• In E. coli, the RNA polymerase is composed of five
polypeptide subunits - two α, one β, one β’ and one ω and
σ subunit (α2ββ’ωσ).
• This complete enzyme is called as the holoenzyme.
• The σ subunit may dissociate from the other subunits to
form known as the core enzyme.

5. EUKAYOTIC RNA POLYMEARASE
ENZYMES LOCATION TRANSCRIBED
RNA POL I NUCLEUS Large rRNA
RNA POL II NUCLEOPLASM mRNA, snRNA
RNA POL III NUCLEOPLASM Smaller rRNA, tRNA,

6. EUKARYOTIC GENE EXXPRESSION

7. PROKARYOTIC PROMOTER
• A promoter is a regulatory region of DNA located
upstream (towards the 5' region) of a gene, providing a
control point for regulated gene transcription.
• The promoter contains specific DNA sequences that are
recognized by proteins known as transcription factors.
Pribnow box

8. • In prokaryotes, the promoter consists of two short sequences at
-10 and -35 positions upstream from the transcription start site.
• The sequence at -10 is called the Pribnow box, or the -10
element, and consists of the six nucleotides TATAAT.
• The Pribnow box is essential to start transcription in
prokaryotes.
• The other sequence at -35 (the -35 element) consists of the six
nucleotides TTGACG. Its presence allows a very high
transcription rate.
• The site on the DNA from which the first RNA nucleotide is
transcribed is called initiation site. Nucleotides that come
before the initiation site are marked with negative numbers,
said to be upstream.
• Nucleotides that come after the initiation site are marked with
positive numbers, said to be downstream.

9. Eukaryotic promoters
• Eukaryotic promoters are extremely diverse and are
difficult to characterize.
• They typically lies upstream of the gene and can have
regulatory elements several kilobases away from the
transcriptional start site.
• Many eukaryotic promoters, contain a TATA box
(sequence TATAAA), which in turn binds a TATA binding
protein which assists in the formation of the RNA
polymerase transcriptional complex.
• The TATA box typically lies very close to the
transcriptional start site (often within 50 bases)

11. MECHANISM OF TRANSCRIPTION
Transcription process complete in mainly three steps in
both prokaryotes and eukaryotes.
 INITIATION
 ELONGATION
 TERMINATION

12. INITIATION
• Transcription in prokaryotes (and in eukaryotes) requires the
DNA double helix to partially unwind in the region of mRNA
synthesis.
• During the process of initiation, RNA polymerase enzyme bind
with the promoter region and unwind the dsDNA. This region is
called as transcription bubble.
• The –10 sequence are important recognition site that interact
with sigma factor of RNA polymerase.
• The –35 sequence has the consensus TTGACA and is important
in DNA unwinding during transcriptional initiation.
• Once the transcription bubble has formed, the polymerase can
start transcribing.
• RNA polymerase does not need a primer to begin transcription;
having bound to the promoter site, the RNA polymerase begins
transcription directly.

14. ELONGATION
• After synthesis of RNA more than 10 bp long, the σ-factor
is ejected.
• The enzyme move along 5’-3’ direction continuously
synthesizing ssRNA by adding complementary nucleotide
of template DNA.
• The synthesized RNA is proof reads by Hydrolytic editing.
• For this the polymerase back track by one or more
nucleotide and cleave the RNA removing the error and
synthesize the correct one.
• The Gre factor enhance this proof reading process.
• mRNA chain synthesized from 5’ to 3’ direction.

15. Transcription bubble

16. TERMINATION
Termination process occur in two stages:
Rho independent termination:
• Rho-independent termination is a
mechanism in prokaryotes that
causes RNA transcription to stop and
release the newly made RNA.
• Termination signal present in the
sequence of the RNA transcript itself.
• Hairpin loop forms followed by
stretch of the U residues.
• Stem loop formation cause RNA
polymerase to pause.
• A:U base pair is more unstable, weak
and dissociate from the complex.

18. Rho dependent:
• In this mechanism, transcription is terminated by rho (ρ)
protein.
• A ρ factor (Rho factor) is a prokaryotic protein involved in
the termination of transcription.
• It binds to the transcription terminator pause site.
• The rho protein bind the single stranded RNA as it exit from
polymerase enzyme complex and hydrolyse the RNA from
enzyme complex.
• The rho protein does not bind to those RNA whose protein is
being translated.
• It bind to RNA after translation.
• In bacteria transcription and translation occur
simultaneously so the rho protein bind the RNA after
translation has completed but transcription is still ON.

19. Rho protein

20. THANK YOU