very easy to understand transcription topic within a very less time.

1. TOPIC : TRANSCRIPTION
Dr. Shree Lakshmi.BN
1yr MvSc
Dept.VPT
LUVAS,
Hisar,Haryana

2. Transcription
• Cells are governed by a cellular chain of command
– DNA RNA protein
• Transcription
– Is the synthesis of RNA under the direction of
DNA
– Produces messenger RNA (mRNA)
– For this purpose, one of the 2 strands of DNA
serves as a template(anti sense or non coding
strand) and produces working copies of RNA
molecules.
– Other DNA strand referred as coding strand used
since with the exception of T for U

3. • Transcription is selective
• Primary transcript : product formed in
transcription ,are inactive. which undergo
alterations like splicing,terminal addition,base
modifications…etc known as post
transcriptional modifications to produce
functionally active RNA molecules.

5. Transcription in prokaryotes
• There is a single prokaryotic RNA polymerase that
synthesizes all types of RNA in the cell.
• The core polymerase responsible for making the RNA
molecule has the subunit structure (α2ββ`).
• A protein factor called sigma (σ) is required for the
initiation of transcription at a promoter. Sigma factor is
released immediately after-initiation of transcription.
• Termination of transcription sometimes requires a protein
called rho (ρ) factor.

6. Synthesis of an RNA Transcript
• The stages of
transcription are
– Initiation
– Elongation
– Termination
Promoter
Transcription unit
RNA polymerase
Start point
53 35
35
53
53 35
53 35
5
5
Rewound
RNA
RNA
transcript
3
3
Completed RNA
transcript
Unwound
DNA
RNA
transcript
Template strand of
DNA
DNA
1
Initiation. After RNA polymerase binds to
the promoter, the DNA strands unwind, and
the polymerase initiates RNA synthesis at the
start point on the template strand.
2
Elongation. The polymerase moves downstream, unwinding the
DNA and elongating the RNA transcript 5  3 . In the wake of
transcription, the DNA strands re-form a double helix.
3 Termination. Eventually, the RNA
transcript is released, and the
polymerase detaches from the DNA.

7. Initiation
The following events occur during the expression of a prokaryotic
gene:
1. With the help of sigma factor, RNA polymerase recognizes and
binds to the promoter, region.
• The bacterial promoter contains two "consensus" sequences,
called the Pribnow box (or TATA box) and the -35 sequence.
The promoter identifies the start site for transcription and
orients the enzyme on the template strand.

8. Elongation
• RNA polymerase locates genes in DNA by searching for promoter
regions.
– The promoter is the binding site for transcription factors and RNA
polymerase.
• RNA polymerase moves along the template strand in the 3' to 5'
direction as it synthesizes the RNA product in the 5' to 3' direction
using NTPs (ATP, GTP, CTP, UTP) as substrates. RNA polymerase
does not proofread its work. The RNA product is complementary and
antiparallel to the template strand.
• The coding (non-template) strand is not used during transcription. It is
identical in sequence to the RNA molecule, except that RNA contains
uracil instead of the thymine found in DNA.

9. Termination
There are two kinds of transcription terminators commonly found
in prokaryotic genes:
– Rho-independent termination occurs when the newly formed
RNA folds back on itself to form a GC-rich hairpin loop
closely followed by 6-8 U residues due to the presence of
Palindromes. These two structural features of the newly
synthesized RNA promote dissociation of the RNA from the
DNA template.
– Rho-dependent termination requires participation of rho factor.
This protein binds to the newly formed RNA and moves toward
the RNA polymerase that has paused at a termination site. Rho
then displaces RNA polymerase from the 3' end of the RNA.

10. Transcription in eukaryotes
RNA polymerases
There are three eukaryotic RNA polymerases
• RNA polymerase I is located in the nucleolus and synthesizes 28S,
18S, and 5.8S rRNAs.
• RNA polymerase II is located in the nucleoplasm and synthesizes
hnRNA/mRNA and some snRNA.
• RNA polymerase III is located in the nucleoplasm and synthesizes
tRNA, some snRNA, and 5S rRNA.
• Transcription factors (such as TFIIB for RNA polymerase II) help
to initiate transcription.

11. Promoter sites
Transcription of a typical eukaryotic gene occurs as follows:
1. With the help of proteins called transcription factors, RNA
polymerase II recognizes and binds to the promoter region. The
basal promoter region of eukaryotic genes usually has two
consensus sequences called the TATA box (also called Hogness
box) and the CAAT box.
2. RNA polymerase II separates the strands of the DNA over a short
region to initiate transcription and read the DNA sequence. The
template strand is read in the 3' to 5' direction as the RNA product
(the primary transcript) is synthesized in the 5' to 3' direction. Both
exons and introns are transcribed.

12. Initiation
• A large number of transcription factors interact
with eukaryotic promoter regions.
• TFIID,TFIIA,TEIIB,TFIIF,TFIIE,TFIIH

13. Transcription Enhancers and Silencers
• Both are Binding Sites for Transcription Factors (TF’s)
• Enhancers: Increase the amount of Transcription from a nearby
promoter (core + upstream elements)
• Silencers: Decrease amount of Transcription from nearby
promoters
• Initially Defined as being “Position and orientation independent”
– Found upstream, within, or downstream of genes
– Function in either orientation (not always true)

14. What Are Enhancers?
Enhancer = “non-promoter DNA elements that stimulate transcription”
• First found in eukaryotes and then found in bacteria
• Binding sites for transcription factors (= activators or enhancer binding
proteins).
• NOT the binding sites for RNA polymerase
• Can function over long distances (100 - 1000 bp) away from +1 sites
(upstream or downstream)
• They are also tissue-specific (rely on tissue-specific DNA-binding
proteins for activity).
• Sometimes a DNA element can act as an enhancer or a silencer
depending on what is bound to it.

15. Heterogenous nuclear RNA(hnRNA)
• The primary mRNA transcript produced by RNA Polymerase
II in eukaryotes is often referred to as heterogeneous nuclear
RNA. This is then processed to produce mRNA needed for
protein synthesis.

16. Post transcriptional modifications
• Transcription of RNA processing occur in the nucleus. After this, the
messenger RNA moves to the cytoplasm for translation.
• The cell adds a protective cap to one end, and a tail of A’s to the other end.
These both function to protect the RNA from enzymes that would degrade
• Most of the genome consists of non-coding regions called introns
– Non-coding regions may have specific chromosomal functions or have
regulatory purposes
– Introns also allow for alternative RNA splicing
• Thus, an RNA copy of a gene is converted into messenger RNA by doing 2
things
– Add protective bases to the ends
– Cut out the introns

17. Transcription Overview

18. Difference Between Prokaryotic and Eukaryotic Transcription
Prokaryotes
Location
Transcription
Transcription of mRNA
Type of mRNA
Occurs in cell cytoplasm
Transcription happen
simultaneously.
mRNA is transcribed
directly from template DNA
molecule.
The type of RNA
polymerase does not vary
with the bacterial type.
•Occurs in cell nucleus
•Transcription differ in
space and time
(transcription – nucleus,
translation – cytoplasm)
•Initially a pre-mRNA
molecule (primary
transcript) is formed and
then processed to yield a
mature mRNA
• The type of RNA varies
with the organisms.
eukaryotes

19. Prokaryotes eukaryotesRNA Polymerase
Subunits
Promoter Recognition
Type of Transcription
A single type of RNA
polymerase, which has a
core enzyme and other
subunits, is involved.
Prokaryotic RNA
polymerase consists of five
subunits.
In prokaryotes,
holoenzyme (RNA
polymerase + sigma factor)
recognizes and binds
directly to the promoter.
In prokaryotic, DNA is not
bound to the histone
proteins. Therefore,
transcription occurs
directly.
Type of RNA polymerase
varies according to the
type of RNA that is
transcribed.
Eukaryotic RNA
polymerase consists of 10
– 17 subunits.
In eukaryotes, promoter
recognition cannot be
carried out by RNA
polymerase alone.
a complex of histone
proteins and DNA should
be accessible, before the
transcription.

20. Promoter
Transcription Terminators
Binding to the RNA
Molecule
no such differentiation
can be seen.
Prokaryotic cells possess
two types of transcription
terminators; Rho-
dependent terminators
and Rho-independent
terminators.
Rho factor binds to the
growing RNA molecule in
the prokaryotic
transcription.
Eukaryotic DNA that is
identified by the RNA
polymerase II has two
parts of the promoter
known as core promoter
and regulatory promoter.
In eukaryotes
transcription, the three
RNA polymerases use
different mechanisms for
the termination.
Termination factor in
eukaryotes binds to the
template DNA molecule.