Transcription is the process of creating an RNA copy of a DNA sequence. It involves RNA polymerase binding to the promoter region of DNA and copying the sequence into RNA. There are three main stages: initiation, elongation, and termination. In initiation, RNA polymerase binds to the promoter and unwinds the DNA helix. In elongation, RNA is extended using the DNA as a template. Termination occurs when the RNA reaches a termination sequence and detaches from the DNA. The resulting pre-mRNA in eukaryotes undergoes processing including capping, polyadenylation, and splicing before being exported and used for protein synthesis.

1. TRANSCRIPTION

2. Transcription:
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 Transcription, or RNA synthesis, is the process
of creating an equivalent RNA copy of a
sequence of DNA
 Both RNA and DNA are nucleic acids, which use
base pairs of nucleotides as a complementary
language that can be converted back and forth
from DNA to RNA in the presence of the correct
enzymes
 During transcription, a DNA sequence is read by
RNA polymerase, which produces a
complementary, antiparallel RNA strand

3. Conti…
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 In eukaryotes several different enzymes are
used to synthesize different types of RNA
 They are called as:
 RNA Polymerase I
Synthesizes 28S, 18S & 5.8S ribosomal RNA in
nucleolus
 RNA Polymerase II
Synthesizes precursors of mRNA
 RNA Polymerase III
Synthesizes tRNA, 5S rRNA & snRNA

4. Stages of transcription:
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 Transcription is divided into 4 stages
 Formation of transcription complex
 Initiation
 Elongation
 Termination

5. Formation of transcription
complex:
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 RNA polymerase binds to specific sequence on
DNA which are called promoters
 The promoter size is variable
 In prokaryotes ranges from 20-200 bases
 Sigma factor recognize the promoter region

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 Following four steps occur
 Sigma factor recognize the promoter sequence
 RNA polymerase attaches to promoter region
 RNA polymerase melts the helical structure and
separates two strands of DNA locally
 RNA polymerase initiates RNA synthesis
 The site at which the first nucleotide is incorporated is
called the start site or start point

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 RNA polymerase is a core enzyme consisting of
five subunits: 2 α subunits, 1 β subunit, 1 β
subunit, and 1 omega subunit
 The core enzyme is associated with a sigma
factor that aids in finding the appropriate -35
and -10 base pairs downstream of promoter
sequences

8. Promoter region:
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 DNA molecules contain regions of nucleotide
called promoters that directs RNA polymerase to
begin transcription
 In bacterial cells, there are two sequences that
commonly act as promoter
 One of these found is 35 nucleotide from the
start of transcription and other one called
Pribnow box is found 10 nucleotide from the
start

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Up stream

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 In eukaryotic cells, there are several promoter
regions:
o TATA Box (Hogness Box) found 25 nucleotide
upstream from the start of transcription
o CAAT Box found about 75 nucleotide upstream

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Up
stream

12. Initiation:
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 Core enzyme starts initiation of transcription of
the complex
 As the enzyme moves along, the unwound
region moves with it
 The first base copied is always within six to nine
bases of the conserved T of the Pribnow box on
the unwound portion of 3’-5’ strand of DNA
 RNA polymerase have two sites for the binding
of nucleotide triphosphate

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 The first incoming NTP binds to RNA
polymerase at the start point of initiation site
 The second incoming NTP binds to the
elongation site on the polymerase
 After this phosphodiester bond formation the σ
factor is released

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15. Elongation:
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 One strand of DNA, the template strand (or
noncoding strand), is used as a template for RNA
synthesis
 As transcription proceeds, RNA polymerase
traverses the template strand and uses base
pairing complementarity with the DNA template to
create an RNA copy
 Although RNA polymerase traverses the template
strand from 3 → 5, the coding (non-template)
strand and newly-formed RNA can also be used
as reference points, so transcription can be
described as occurring 5 → 3

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 This produces an RNA molecule from 5 → 3, an
exact copy of the coding strand (except that
thymines are replaced with uracils, and the
nucleotides are composed of a ribose (5-carbon)
sugar where DNA has deoxyribose (one less
oxygen atom) in its sugar- phosphate backbone)
 Unlike DNA replication, mRNA transcription can
involve multiple RNA polymerases on a single DNA
template and multiple rounds of transcription
(amplification of particular mRNA), so many mRNA
molecules can be rapidly produced from a single
copy of a gene

17. Termination:
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 Bacteria use two different strategies for
transcription termination:
o Rho-independent termination
o Rho-dependent termination
 ρ-Independent termination:
 In Rho-independent transcription termination,
RNA transcription stops when the newly
synthesized RNA molecule forms a G-C rich
hairpin loop followed by a run of Us, which
makes it detach from the DNA

18. ρ-Independent termination:
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

19. ρ-dependent termination:
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 In the "Rho-dependent“ type of termination, a
protein factor called "Rho" destabilizes the
interaction between the template and the RNA,
thus releasing the newly synthesized mRNA
from the elongation complex
 rho (ρ) protein , which is a hexameric adenosine
tri phosphatase (ATPase) with helicase activity
 The ATP-dependent helicase activity of ρ
separates the RNA-DNA hybrid helix, causing
the release of the RNA

20. Post-Transcriptional processing of
mRNA:
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 Transcription occurs in the nucleus, where the
DNA is used to produce RNA molecule
 One type of RNA molecule that is formed is
called messenger RNA
 Messenger RNA is used as a template for
synthesizing protein in the cytoplasm of the cell
 Before the messenger makes the way out of the
nucleus it must undergo several modifications

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 The mRNA that is formed directly after
transcription is known as precursor messenger
RNA (pre-mRNA)
 This is the mRNA that has not yet undergone
the necessary modification processes
 In the nucleus, before the RNA molecule can
enter the cytoplasm, the pre-mRNA must
undergo three post-transcriptional modification

22. 5-Guanosine Triphosphate Cap:
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 In the nucleus of the cell, the 5’ end of the pre-
mRNA is altered by the attachment of a
Guanosine nucleotide via a 5’-5’ triphosphate
linkage
 The Guanosine nucleotide is quickly methylated
at the 7th position to form the 7-methylguanosine

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 One important function of this cap is to protect
the mRNA from degradation during protein
synthesis
 It also stabilize the mRNA and aids in transport
across the nuclear membrane

25. Polyadenylation of 3’-end:
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 In this type of post-transcriptional modification
the 3’end of the pre RNA is removed and a
series of adenosine molecules are added
which is called the polyadenine tail
 Just as 5’-cap aids stability and provides
resistance to degradation, the poly-A tail also
provides the mRNA with stability and keep the
tail from degrading

26. Splicing of the Exons:
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 Not all regions of the pre-mRNA molecule
code for protein
 Those regions that do code for proteins are
called exons, while those who do not are
called introns
 Before the mRNA exist in the nucleus, special
protein will cut out the introns and splice
together the axons

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