Transcription, Promoter, Promoter Structure, RNA polymerase, Sigma factor, Steps of transcription, Trasncriptional Initiation, Transcription Elongation, Transcriptional Termination, Mechanisms of termination, Rho dependent termination , Rho indepenent termination, Transcription in prokaryotes, Transcription in Eukaryotes.

1. TRANSCRIPTION
BY HASNAT TARIQ

2. DNA TRANSCRIPTION
• It is a process in which RNA is synthesized by using DNA as a
template.
• DNA (Double Stranded) RNA ( Single Stranded)
• RNA Polymerase is the key enzyme in transcription process.
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3. RNA Polymerase Structure in Prokaryotes
• In 1969, the polypeptides that make up the E. coli RNA polymerase had
been identified by SDS polyacrylamide gel electrophoresis (SDS-PAGE).
• Two very large subunits: beta (β) and beta-prime (β’), with molecular
masses of 150 and 160 kD.
• The other RNA polymerase subunits visible on this gel are called sigma
(s) and alpha (a), with molecular masses of 70 and 40 kD.
• Another subunit, omega (ω), with a molecular mass of 10 kDa is not
detectable here, but was clearly visible in urea gel electrophoresis
experiments.
• In 1969, the polypeptides that make up the E. coli RNA polymerase had
been identified by SDS polyacrylamide gel electrophoresis (SDS-PAGE).
• Two very large subunits: beta (β) and beta-prime (β’), with molecular
masses of 150 and 160 kD.
• The other RNA polymerase subunits visible on this gel are called sigma
(s) and alpha (a), with molecular masses of 70 and 40 kD.
• Another subunit, omega (ω), with a molecular mass of 10 kDa is not
detectable here, but was clearly visible in urea gel electrophoresis
experiments.
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4. Continue..
• β': The β' subunit is the largest subunit, and is encoded by the
rpoC gene.
• β: The β subunit is the second-largest subunit, and is encoded by
the rpoB gene
• αI and αII: The α subunit is the third-largest subunit .
• ω: The ω subunit is the smallest subunit.
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5. Sigma (σ) as a Specificity Factor
• Sigma factors are subunits of all bacterial RNA polymerases.
• They are responsible for determining the specificity of promoter DNA
binding and control how efficiently RNA synthesis (transcription) is
initiated.
• The first sigma factor discovered was the sigma70 (σ70) of the highly
studied bacterium Escherichia coli.
• A core polymerase (with subunit structure α2ββ′) can transcribe DNA into
RNA inefficiently and nonspecifically. With sigma factor, it can bind to core
forming a holoenzyme (α2ββ′σ) that is capable of specific engagement with
duplex DNA at the beginning of genes (promoters) as well as efficient
initiation of transcription.
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6. Promoters
• The polymerase binding sites are called promoters.
• Present at the upstream of the coding region of DNA.
• Transcription that begins at promoters in vitro is specific and
mimics the initiation that would occur in vivo. Thus, operates by
directing the polymerase to initiate at specific promoter
sequences.
• Sigma stimulates tight binding between RNA polymerase and
promoter.
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7. Binding of RNA Polymerase to Promoters
• The holoenzyme finds two kinds of binding sites, tight binding sites and
loose ones. On the other hand, the core polymerase is capable of binding
only loosely to the DNA.
a) Promoter search
The holoenzyme binds and rebinds loosely to the DNA, searching for
a promoter.
b) Closed promoter complex formation
The holoenzyme as found a promoter and has bound loosely, forming
a closed promoter complex.
c) Open promoter complex formation
The holoenzyme has bound tightly, melting a local region of DNA and
forming an open promoter complex.
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8. Prokaryotic
Promoters
• Promoters in prokaryotic organisms
are two short DNA sequences
located at the -10 (10bp 5' or
upstream) and -35 positions from
the transcription start site.
• The Pribnow box (TATAAT) is
located at the -10 position and is
essential for transcription
initiation. The -35 position, simply
titled the -35 element, typically
consists of the sequence TTGACA
and this element controls the rate
of transcription.
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9. Eukaryotic Promoters
• Eukaryotic Promoters are complex and diverse.
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10. Enhancers
• Enhancer DNA sequences bind
transcription factors with special protein
called enhancer-binding proteins which
increase the rate of transcription.
• Enhancer sequences may be at a
distance of kilobases away from the
gene they influence.
• An enhancer complex may interact with
promoter complexes by bringing the
sites into direct contact, may be by
formng a loop-like structure.
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11. Steps in Transcription
1. Initiation
2. Elongation
3. Termination
1. Initiation
2. Elongation
3. Termination
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12. 1. Initiation
Stages of transcription initiation
(a) RNA polymerase binds to DNA in a closed
promoter
(b) The s-factor stimulates the polymerase to
convert the closed promoter complex to an open
promoter complex.
(c) The polymerase incorporates the first 9 or 10
nt into the nascent RNA. Some abortive
transcripts are pictured at left.
(d) The polymerase clears the promoter and
begins the elongation phase. The s-factor may be
lost at this point or later, during elongation.
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13. 2. Elongation
• After initiation of transcription is accomplished, the core
continues to elongate the RNA, adding one nucleotide after
another to the growing RNA chain.
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14. Core Polymerase Functions in Elongation
• The core polymerase contains the RNA synthesizing machinery, so
the core is the central player in elongation.
• Beta (β) and beta-prime (β’) subunits are involved in
phosphodiester bond formation, that these subunits also
participate in DNA binding.
• (α ) subunit has several activities, including assembly of the core
polymerase.
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15. 3. Termination
• RNA synthesis will continue along the DNA template
strand until the polymerase encounters a signal that tells
it to stop, or terminate, transcription.
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16. Termination in Eukaryotes 16

17. Termination in
Prokaryotes
• In prokaryotes, there are
two different mechanisms
of termination.
a) rho-independent
b) rho-dependent.
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18. Rho-independent
Termination
• The rho-independent terminator is the
more simple of the two systems and as a
result is also called simple termination.
• The rho-independent signal is found on
the DNA template strand and consists of
a region that contains a section that is
then repeated a few base pairs away in
the inverted sequence.
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19. Rho-dependent
Termination
• The rho-dependent terminator received its
name because it is dependent on a specific
protein called a rho factor. The rho factor
is thought to bind to the end of the RNA
chain and slide along the strand towards
the open complex bubble. When the factor
catches the polymerase, it causes the
termination of transcription. The
mechanism of this termination is unclear,
but the rho factor could in some way pull
the polymerase complex off of the DNA
strand.
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20. THANK YOU
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