Transcription is the process in which a gene's DNA sequence is copied (transcribed) to make an RNA molecule. RNA polymerase is the main transcription enzyme. Transcription begins when RNA polymerase binds to a promoter sequence near the beginning of a gene (directly or through helper proteins). RNA polymerase uses one of the DNA strands (the template strand) as a template to make a new, complementary RNA molecule. Transcription ends in a process called termination. Termination depends on sequences in the RNA, which signal that the transcript is finished.

1. RNA Polymerases
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2. Key points
• Transcription is the process in which a gene's DNA
sequence is copied (transcribed) to make an RNA molecule.
• RNA polymerase is the main transcription enzyme.
• Transcription begins when RNA polymerase binds to
a promoter sequence near the beginning of a gene (directly
or through helper proteins).
• RNA polymerase uses one of the DNA strands (the template
strand) as a template to make a new, complementary RNA
molecule.
• Transcription ends in a process called termination.
Termination depends on sequences in the RNA, which
signal that the transcript is finished.
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3. Introduction
• Ribonucleic acid polymerase is an enzyme which is
responsible for the production of all kinds of RNAs
such as messenger RNA, ribosomal RNA and transfer
RNA.
• It is the largest enzyme of 490,000 MW and consists of
six subunits made up of polypeptide chains: 2 alpha,
beta, beta dash, omega and sigma subunits.
• It catalyzes the process of transcription. It copies the
DNA sequence into RNA sequence.
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4. Introduction
• The complete RNA polymerase enzyme is termed
holoenzyme and can be represented as α 2ββ´ωσ in
which attachment of sigma (σ) subunit (or factor) is
not very firm, but resultant core enzyme (α2ββ´) does
not lose its catalytic activity of transcription.
• RNA polymerase found in prokaryotes, eukaryotes,
archea and in some viruses is almost similar, this
shows that they had evolved from a common ancestor.
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5. RNA Polymerase
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6. Functions of RNA polymerase
• Involved in the production of molecules which have a wide
range of roles
• Involved in post-transcriptional modification of RNAs to
make them functional and facilitates their export from the
nucleus towards their ultimate site of action.
• The beta and beta dash subunits of RNA polymerase form
the catalytic center of RNA-P and helps in unwinding of
DNA molecule for transcription.
• The sigma (σ) factor helps in the recognition of start signals
on DNA molecule and directs RNA polymerase in selecting
the initiation sites (promoter).
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7. Prokaryotic RNA polymerase
• A single RNA polymerase enzyme is involved in the
production of all kinds of RNAs in E.coli and other
prokaryotes.
• It is the largest enzyme of 490,000 MW in prokaryotes.
• The RNA polymerase of prokaryotes is made up of six
subunits which include a sigma factor which is dissociated
from enzyme complex after initiation of transcription.
• prokaryotes use the same RNA polymerase as a catalyst in
polymerization of coding as well as non- coding RNA
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8. Prokaryotic RNA polymerase
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9. Eukaryotic RNA Polymerase
• In eukaryotes, there are three major classes of RNA
polymerases which are designated as I, II and III and
are found in the nucleus.
• The three polymerases have different properties and
can be distinguished by the ions required for their
activity, the optimal ion strength and their sensitivity
to inhibition by various antibiotics (e.g., α -amanitin).
• Each of these enzymes is a large protein ( ~ 500,00
daltons), with two large and several ( 8 to 10) smaller
subunits.
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10. Table 1. Locations, Products, and Sensitivities of the Three Eukaryotic RNA Polymerases
RNA
Polymerase
Cellular
Compartment
Product of Transcription α-Amanitin Sensitivity
I Nucleolus All rRNAs except 5S rRNA Insensitive
II N21ucleus All protein-coding nuclear pre-
mRNAs
Extremely sensitive
III Nucleus 5S rRNA, tRNAs, and small
nuclear RNAs
Moderately sensitive
Types of Eukaryotic RNA Polymerases
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11. Types of Eukaryotic RNA Polymerases
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12. RNA polymerase I
• It is located in the nucleolus, it is a specialized nuclear
substructure in which transcription and assembling of
ribosomal RNA into ribosomes takes place.
• The ribosomal RNA molecules have a cellular role but
are not translated into protein so they are considered
structural RNAs.
• The rRNAs are components of the ribosome and are
essential to the process of translation.
• RNA polymerase I synthesizes all of the rRNAs except
for the 5S rRNA molecule.
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13. RNA polymerase II
• It is found in nucleus and it synthesizes all protein-coding
nuclear pre- mRNAs and is extremely sensitive.
• Eukaryotic pre-mRNAs undergo extensive processing after
transcription but before translation (as shown in the next
slide).
• RNA polymerase II is involved in the transcription of eukaryotic
genes.
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14. Eukaryotic mRNA contains introns that must be spliced out. A 5′ cap and 3′ poly-A
tail are also added.
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15. RNA polymerase III
• It is also located in the nucleus of the cell. It
transcribes small nuclear pre- RNAs and variety of
structural RNAs which includes the 5 S pre-rRNA and
pre-tRNA .
• The function of small nuclear RNA is in splicing and the
functions of rRNA and tRNA is to synthesize prorteins
and translate the mRNA to form the polypeptide chain,
respectively.
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16. Comparison between DNA and RNA
Polymerase
• The roles of DNA and RNA polymerases is almost similar i.e.
to catalyze the polymerization reactions taking place in
nucleus, however there are two differences between them
in their activity.
• There is no need of primer to start the polymerization in
case of RNA polymerase enzyme.
• On the other hand DNA polymerase requires primer to
begin transcription.
• RNA polymerase enzyme is capable of reaction from the
middle part of DNA strand.
• It also reads the stop signals through which it dissociate
itself from the template.
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17. RNA polymerase comparison in
prokaryotes and eukaryotes
• Transcriptions in Eukaryotes and Prokaryotes have some
similarities as well as some differences. The mechanisms of
transcription are diverged in both. There are some
characteristics of transcription which are common in both
eukaryotes and prokaryotes which are as follows:
• From E. coli to human the core is conserved
• Active site is also conserved and magnesium ions are
utilized by both polymerases to facilitate the process of
transcription
• Absorptive transcripts are found in both
• Both binds to promoters and have elements to speed up
transcription.
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18. RNA polymerase comparison in
prokaryotes and eukaryotes
• There are many differences between prokaryotic and eukaryotic
polymerases which are as follows:
• Eukaryotes have three RNA polymerases which are RNA
polymerase I, RNA polymerase II and RNA polymerase III while
prokaryotes have only one RNA polymerase.
• The molecular weight difference in RNA polymerase of
prokaryotes and eukaryotes is 100 kDa.
• There are six subunits of RNA polymerase in prokaryotes (2
alphas, beta, beta dash, an omega and a sigma) while RNA
polymerase have twelve subunits in eukaryotes (Rpb1- Rpb-12).
• The bridge helix in prokaryotes is in bended form while it is
straight in eukaryotes.
• ` 1
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19. Comparison between DNA and RNA
Polymerase
• The roles of DNA and RNA polymerases is almost similar i.e.
to catalyze the polymerization reactions taking place in
nucleus, however there are two differences between them
in their activity.
• There is no need of primer to start the polymerization in
case of RNA polymerase enzyme.
• On the other hand DNA polymerase requires primer to
begin transcription.
• RNA polymerase enzyme is capable of reaction from the
middle part of DNA strand.
• It also reads the stop signals through which it dissociate
itself from the template.
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20. DNA Polymerase
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21. RNA Polymerase
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22. Process of Transcription
• The binding of RNA polymerase to DNA starts the
transcription. The specific part of DNA at which
transcription starts is called promoter region.
• This binding takes place in the presence of other proteins
such as sigma factor which works in prokaryotes and in
eukaryotes various other factors helps in binding.
• A set of protein is necessary for all transcriptional activity of
eukaryotes. This set is called general transcription factors it
includes transcription initiation factor II A, factor II B, factor
II D, factor II E, factor II F and factor II H.
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23. Process of Transcription
• These are supplemented by specific signaling molecules that modulate gene
expression through stretches of non-coding DNA located upstream.
• Before a stretch of ten nucleotides is polymerized initiation is often aborted
multiple times.
• After this step, the polymerase loses most of the initiation factors and moves
beyond the promoter.
• After this step the unwinding of DNA takes place in which sort of bubble is
formed where active transcription occurs. This process is also called melting.
• The formed ‘bubble’ move along the DNA strand as the RNA polymer
elongates.
• When the process of transcription is complete, the process is stopped and
RNA strand is processed.
• The process of transcription is terminated by RNA polymerase III when there
is a stretch of Thymine bases on the non-template strand of DNA.
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24. Eukaryotic Transcription Mechanism
Following are the steps of eukaryotic transcription
mechanism:
A. Initiation of Eukaryotic Transcription
B. Elongation of RNA Chain in Eukaryotes
C. Termination of Eukaryotic Transcription
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25. Initiation of Eukaryotic Transcription
• The eukaryotic transcripts are regulatory DNA sequences (such as
promoters, enhancers and silencers) of the genes encoded by each of the
three different RNA polymerases.
• Different aspects of writing are also involved in the design of the writing
space required for the initiation of writing.
• Generally, each RNA polymerase is believed to have its own set of
transcripts, however, TF II D or a portion thereof (e.g., TBP = TATA protein
protein) is required for all three RNA polymerases .
• Transcription factors (TFs) can be defined as proteins, which are required
for the initiation of transcription, but not for the RNA polymerase
component. T
• hey assist in the synthesis of DNA polymerase RNA to form the so-called
pre-activation complex or transcript. After the construction of this complex
synthesis of text occurs. All known transcription factors may recognize DNA
sequences, other elements or RNA polymerases.
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26. Initiation of Eukaryotic Transcription
• Transcriptosome formation by RNA pol II. The promoter sequence
responsible for the positive expression of a common gene (also
called a housekeeping gene) in all cells, is said to be a promoter
of some people. Generic promotions cannot deliver controlled
calls (e.g., tissue or a specific genetic attribute, called a primitive
type).
• Activation of normal promoter transcription by RNA polymerase II
requires the action of different transcription factors (TFs) in the
following order: (i) TF II D binds to the TATA box; (ii) step (i) to
allow for the merging of TF IIA and TF IIB; (iii) TF II B forms the so-
called DB complex and RNA polymerase II that link to the
developmental site; (iv) RNA pol II interacts with the TF II F
promoter to form a transcription factor; (v) the systematic
addition of TF II E, TF II H and TF II J facilitates the initiation
process.
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27. Elongation of RNA Chain in Eukaryotes
• There are certain transcription factor proteins, called
elongation factor, that enhance the overall activity of RNA
polymerase II and lead to an increase in elongation rate.
• At least two proteins such as these are known: (1) TF II F
accelerates RNA chain growth evenly in combination with
RNA polymerase II. 2. The TF II S (also called S II) aids in the
elevation of the RNA chain by removing the barrier along
the entry pathway.
• TF II S is known to act first by inducing hydrolytic cleavage
at the end of the 3NA RNA, thus, assisting in the
progression of RNA polymerase from any block to
elongation.
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28. Termination of Eukaryotic Transcription
• In eukaryotes, the actual elimination of RNA
polymerase II activity during transcription is possible
through elimination sites similar to those found in
prokaryotes.
• However, the status of individual sites is unknown.
Sites of such termination are believed to be located at
a distance (sometimes up to one address away from
the 3'end mRNA site).
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29. Eukaryotic Transcription Mechanism
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30. Thank you
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