The document summarizes key differences between transcription in prokaryotes and eukaryotes. In prokaryotes, transcription and translation occur simultaneously in the cytoplasm using a single type of RNA polymerase. In eukaryotes, transcription occurs separately from translation in the nucleus using three distinct RNA polymerases and requires numerous transcription factors to initiate transcription. Eukaryotic transcription also involves more complex post-transcriptional processing including splicing of pre-mRNA and alternative splicing to generate multiple mRNA isoforms from a single gene.

1. TRANSCRIPTION
Dr.Sohil Takodara

2. Expression of Genetic Information
The genetic master plan
of an organism is
contained in the form of
DNA.
Ribonucleic acid (RNA)—
the “working copies” of
the DNA— that the
master plan is expressed

3. Definition
“The copying process, during which a DNA strand
serves as a template for the synthesis of RNA, is
called transcription. “
DNA to RNA to Protein
Transcription is highly selective process, only few
region of whole DNA is transcribed into RNA

4. TRANSCRIPTION
IN
PROKARYOTES

5. Requirements
 Ribonucleotides – ATP, GTP,CTP, UTP
 RNA Polymerase
 No RNA primer needed

6. Prokaryotic RNA polymerase
Enzyme Assembly
5’-3’ RNA
Polymerase
Activity
Template Binding
Helps RNA
polymerase in
Recognize the
promoter region
Stability

7. • RNA is synthesized from its 5'-end to its 3'-end, antiparallel
to its DNA template strand.
• The template is copied as it is in DNA synthesis, in which a
G on the DNA specifies a C in the RNA, a C specifies a G, a
T specifies an A, but an A specifies a U instead of a T
• The RNA, then, is complementary to the DNA template
(antisense) strand and identical to the coding (sense) strand,
with U replacing T.
• Within the DNA molecule, regions of both strands can serve
as templates for transcription.
• For a given gene, however, only one of the two DNA strands
can be the template. Which strand is used is determined by
the location of the promoter for that gene.

9. 5’ A T G C A T G 3’ Coding Strand (DNA)
3’ T A C G T A C 5’ Template strand (DNA)
5’ AU G C A U G 3’ mRNA

10. Transcription – 3 Process
Initiation
Elongation
Termination
 A transcription unit extends from the
promoter to the termination region, and the
initial product of transcription by RNA
polymerase is termed the primary
transcript.

11. INITIATION
BINDING OF RNA POLYMERASE
TO THE PROMOTER REGIONS

12. Prokaryotic Promoter region

13. Elongation

14. Once the promoter region has been recognized and
bound by the holoenzyme, local unwinding of the DNA
helix continues mediated by the polymerase
Unwinding generates supercoils in the DNA that can be
relieved by DNA topoisomerases
RNA polymerase begins to synthesize a transcript of the
DNA sequence, and several short pieces of RNA are
made and discarded. The elongation phase is said to
begin when the transcript (typically starting with a purine)
exceeds ten nucleotides in length
Sigma is then released, and the core enzyme is able to
leave (“clear”) the promoter and move along the template
strand in a progressive manner.
During transcription, a short DNA-RNA hybrid helix is
formed Like DNA polymerase, RNA polymerase uses
nucleoside triphosphates as substrates and releases
pyrophosphate each time a nucleoside monophosphate is
added to the growing chain.

15. As with replication, transcription is
always in the 5'→3' direction.
In contrast to DNA polymerase,
RNA polymerase does not require
a primer and does not appear to
have proofreading activity.

16. Rho (ρ)
independent
termination

17. This requires the participation of an
additional protein, rho (ρ), which is a
hexameric adenosine triphosphatase
(ATPase) with helicase activity.
ρ binds a C-rich “rho recognition site”
near the 3'-end of the nascent RNA and,
using its ATPase activity, moves along the
RNA until it reaches the RNA polymerase
paused at the termination site.
The ATP- dependent helicase activity of ρ
separates the RNA-DNA hybrid helix,
causing the release of the RNA.
ρ dependent termination

18. Rifampicin & Actinomyocin D
RNA polymerase blocker
Rifampicin – Anti tuberculosis drug
Actinomycin and Mitomycin –
Anticancer drug
Alpha Amanitin- mushroom toxin
inhibits RNA polymerase II of
eukaryotes

19. TRANSCRIPTION
IN
EUKARYOTES

20.  Its more complicated process than transcription in prokaryotes.
Eukaryotic transcription
 Involves separate polymerases for the synthesis of rRNA, tRNA, and
mRNA.
 In addition, a large number of proteins called transcription factors
(TFs) are involved.
 TFs bind to distinct sites on the DNA—either within the core
promoter region, close (proximal) to it, or some distance away
(distal).
 They are required both for the assembly of a transcription complex
at the promoter and the determination of which genes are to be
transcribed.
 Each eukaryotic RNA polymerase has its own promoters and TFs.
 For TFs to recognize and bind to their specific DNA sequences, the
chromatin structure in that region must be altered (remodeled) to
allow access to the DNA.

21. Chromatin changes-Histone Modification
• DNA with histones ( nucleosomes) has impact on Transcription process
• Most actively transcribed genes are found in a relatively relaxed form of
chromatin called euchromatin
• whereas most inactive segments of DNA are found in highly condensed
heterochromatin.
• The interconversion of these forms is called chromatin remodeling
• A major mechanism by which chromatin is remodeled is through acetylation of
lysine residues at the amino terminus of histone protein.
• Acetylation, mediated by histone acetyltransferases (HATs), eliminates the
positive charge on the lysine and thereby decreases the interaction of the
histone with the negatively charged DNA.
• Removal of the acetyl group by histone deacetylases (HDACs) restores the
positive charge, and fosters stronger interactions between histones and DNA.

22. RNA polymerase
 Type I- ribosomal RNA
 Type II- messenger RNA, sn RNA, mi RNA
 Type III- transfer RNA
 Mitochondrial RNA polymerase ( resembling prokaryotic RNA
polymerase)

23. Eukaryotic promoter sequences
RNA polymerase II

24. Transcription Factors
(TFs)
• Must for recognition of the
promoter, recruitment of RNA
polymerase II to the promoter,
and initiation of transcription
• TFIID recognizes and binds the
TATA box
• TFIIF brings the polymerase to
the promoter.
• The helicase activity of TFIIH
melts the DNA and its kinase
activity phosphorylates
polymerase, allowing it to clear
the promoter

25. Locations of enhancer sequences
• Enhancers contain DNA sequences called
“response elements” that bind specific TFs that
function as transcriptional activators.
• Cause bending or looping the DNA
• Closing the gap of TFs, promoter region thereby
stimulating transcription

26. Post Transcriptional changes
 PRIMARY TRANSCRIPT
Edited By RIBONUCLEASE
 SECONDARY TRANSCRIPT

27. Post transcriptional changes in rRNA
• Mainly by Ribonucleases
• Then by Exonuclease
• Modification of few Bases and Sugar

28. Post transcriptional changes in tRNA

29. Post transcriptional changes in mRNA

30. Removal of Introns
Maturation of eukaryotic mRNA usually involves
the removal of RNA sequences (introns, or
intervening sequences), which do not code for
protein from the primary transcript.
The remaining coding sequences, the exons,
are joined together to form the mature mRNA.
The process of removing introns and joining
exons is called splicing.
The molecular complex that accomplishes
these tasks is known as the spliceosome.

31. SnRNP
 Protein + Uracil rich small RNA ( SnRNAs)
 Also known as SNURPS ( U1, U2, U3)
, an often fatal
inflammatory disease, results from an autoimmune response in
which individuals produce antibodies against their own nuclear
proteins such as snRNPs.

32. Splicing of RNA

33. Alternative splicing

34.  This produces multiple variations of the mRNA and,
therefore, of its protein product
 This appears to be a mechanism for producing a diverse set
of proteins from a limited set of genes.
 For example, in eukaryotic cells the mRNA for tropomyosin,
an actin filament-binding protein of the cytoskeleton (and of
the contractile apparatus in muscle cells), undergoes
extensive tissue-specific alternative splicing with production
of multiple isoforms of the tropomyosin protein.

35. TRANCRIPTION PROKARYOTES EUKARYOTES
Location Cytoplasm Nucleus
Process Less complicated More complicated
Transcription &
Translation
Coupled Separate
RNA polymerase Single type Three Types
Transcription Factors Not needed Must
RHO Factor Needed for
termination
Not needed
mRNA Polycistronic Monocistronic
Post Transcriptional
Changes
Do not occurs Many changes

36. Summery

37. Reference
 Textbook of Biochemistry, Lippincott