HNGU PATAN. m.sc sem 3 botany cbo - 503

1. Transcription in Eukaryotes

2. Contents
• Introduction
• Mechanism of transcription
• Similarities between prokaryotic and eukaryotic transcription
• Difference between prokaryotic and eukaryotic transcription

3. Introduction
• Transcription has been defined in various ways.
• Some definitions of transcription are given here.
• The synthesis of RNA from a single strand of a
DNA molecule in the presence of enzyme RNA
polymerase is called transcription.
• In other words, the process of formation of a
messenger RNA molecule using a DNA molecule
as a template is referred to as transcription.

4. 1. Synthesis :-
• RNA is synthesized from a DNA template.
• The RNA is processed in to messenger RNA ( m RNA)
which is then used for synthesis of a protein.
• The RNA thus synthesized is called m RNA , because it
carries a genetic message from the DNA to the protein
synthesizing machinery of the cell.
• The main difference between RNA and DNA sequence
is the presence of U, or uracil in RNA instead of the
T of thymine of DNA.

5. 2. Template used :-
• The RNA is synthesized from a single strand or
template of a DNA molecule.
• The stretch of DNA that is transcribed into an RNA
molecule is called a transcription unit.
• A transaction unit codes the sequences that is
translated into protein.
• It also directs and regulates protein synthesis.

6. • The DNA strand which is used in RNA synthesis is
called template strand ; because it provides the
template for ordering the sequence of nucleotides in
an RNA transcript.
• The DNA strand which does not take part in DNA
synthesis is called coding strand because it's
nucleotide sequence is the same as that of the newly
created RNA transcript.

7. 3.Enzyme involved :-
• The process of transcription is catalyzed by the
specific enzyme called RNA polymerase.
• DNA sequence is enzymatically copied by RNA
polymerase to produce a complement tarry
nucleotide RNA strand.
• In eukaryotes , there are three classes of RNA
polymerase I, II and III which are involved in the
transcription of all protein genes.

8. 4. Genetic information copied :
• In this process the genetic information coded in DNA
is copied into a molecule of RNA.
• The genetic information is transcribed or copied from
DNA to RNA.
• In other words it results in the transfer of genetic
information from DNA into RNA.

9. 5. first step :-
• The expression of a gene consists of two major
steps vise ; transcription and translation.
• Thus transaction is the first step in the process of
gene regulation or protein synthesis.

10. 6. Direction of synthesis :-
• As in DNA replication , RNA is synthesized in the 5' –3'
direction.
• The DNA template strand is read 3'–5' by RNA polymerase
and the new RNA strand is synthesized in the 5' –3'
direction.
• RNA polymerase bind to the 3' end of a gene on the DNA
template strand and travels to ward the 5' end.
• The regulatory sequence that is before or 5' of the coding
sequence is called 5' untranslated region and sequence
found following or 3' of the coding sequence is called 3'
untranslated region.

11. Mechanism of Transcription
• The mechanism of transcription consists of three
major steps , viz;
1. Initiation
2. Elongation
3. Termination

12. 1. Initiation
Pre - initiation :-
• In initiation of transcription does not require a
prime to start.
• RNA polymerase simply binds to the DNA and
along with other cofactors . the DNA to create an
initiation bubble so that the RNA polymerase has
access to the single stranded DNA template.
• However, RNA polymerase does require a
promoter like sequence.

13. Proximal (core) promoters :-
• TATA promoters are found around –30bp to the
start site of transcription.
• Not all genes have TATA box promoters and there
exists TATA -less promoters and there exists
promoter consensus sequence is TATA (A/T) A
(A/T).

14. Initiation :–
• In eukaryotes and archaea, transcription initiation is
Far more complex.
• The main difference is that eukaryotic polymerases do
not recognize directly their core promoter sequence.
• In eukaryotes, a collection of proteins called
transcription factors mediate the binding of RNA
polymerase and the initiation of transcription.
• Only after attachment of certain transcription factors
to the promoter , the RNA polymerase binds to it.

15. • The complete assembly of transcription factors
and RNA polymerase bind to the promoter,
called transcription initiation complex.
• Initiation starts as soon as the complex is
opened and the first phosphodiester bond is
formed.
• This is the end of Initiation.
• RNA pol II does not contain a subunit similar to
the prokaryotic factor, which can recognize the
promoter and unwind the DNA double helix.

16. Con...
• In eukaryotes, these two functions are carried out
by a set of proteins called general transacritpton
factors.
• The RNA pol II is associated with six general
transcription factors, designated as TFIIA , TFIIB,
TFIID, TFIIE, TFIIF and TFIIH where
' TF ' strands for transcription factors and ' II ' for the
RNA pol II.
• TFIID consists of TBP ( TATA - box binding protein )
and TAFS ( TBP associated factors ).
• The role of TBP is the core promoter.

17. Con..
• TAFs may assist TBP in this process.
• In human cells TAFs are formed by 12 subunits.
• One of them , TAFs 250 ( with molecular weight 250KD
) , has the histone acetyl transfers activity which can
relieve the binding between DNA and histone in the
nucleosome.
• The transcription factor which catalyzes DNA melting
is TFIIH.
• However, before TFIIH can unwind DNA the RNA pol III
at least five general transcription factors have to form
a pre - initation complex.

19. Promoter clearance :-
• After the first bond is synthesized the RNA
polymerase must clear the promoter.
• During this time there is a tendency to release the
RNA transcript and produce truncated transcripts.
• This is called abortive imitation and is common for
both eukaryotes and prokaryotes.
• Once the transcript reaches approximately 23
nucleotides it no longer slips and elongation can
occur.
• This is an ATP dependent process.

20. 2.Elongation
• For RNA synthesis one strand of DNA known as the
template strand or non coding strand is used as a
template.
• 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 strand is usually used as
the reference point 50 transcription is said to go from
5'–3'.

21. • This produces an RNA molecule from 5'–3' an exact
copy of the coding strand ( except that thymines are
replaced uracil's and the nucleotides are composed of
a ribose sugar where DNA has deoxyribose in its sugar
phosphate back bone.
• After pre - initiation complex is assembled at the
promoter TFIIH can use its helicase activity to unwind
DNA.
• This requires energy released from ATP hydrolysis.
• The DNA melting starts from about 10bp.

22. • Then RNA pol II uses nucleoside triphosphate to
synthesize a RNA transcript.
• During RNA elongation TFIIF remains attached to the
RNA polymerase but all of the other transcription
factors have dissociated from PIC.
• The carboxyl terminal domain (CTD ) of the largest
subunit of RNA pol II is critical for elongation.
• In the imitation phase CTD is un -phosphorylated but
during elongation it has to be phosphorylated.
• This domain contains many proline, serine residues .

24. 3. Termination
• In eukaryotic transcription the mechanism of
termination is not very clear.
• In other words, it is not well understood.
• It involves cleavage of the new transcript followed by
template independent addition of as at its new 3' and in
a process called polyadenylation.
• Eukaryotic protein genes contain a poly -A signal located
down stream of the last exon.
• This signal is used to add a series of adenylate residues
RNA processing.
• Transcription often terminates at 0.5 - 2 kb downstream
of the poly A signal.

26. Similarities between prokaryotic and
eukaryotic transcription
1. In both groups DNA acts as the template for RNA
synthesis.
2. In both groups transcription produces RNA
molecule.
3. Chemical composition of transcript is similar in
both groups.
4. Transcription is facilitated by the enzyme RNA
polymerase in both groups.
5. In both groups one strand of the DNA duplex acts
as the template.

27. Difference between prokaryotic and eukaryotic
transcription
No
.
Prokaryotic Eukaryotic
1 . Coupled transcription translation
is the rule
Coupled transcription translation is not
possible
2 . Occurs in the cytoplasm Occurs in the nucleus
3 . There is not definite phase for its
occurrence
Take place in the G₁ and G₂ phase of
cell cycle
4 . A single RNA polymerase synthesis
all the three types of RNA (mRNA,
tRNA , rRNA )
The RNA polymerase I , II and III
synthesis rRNA , mRNA and tRNA
respectively
5 . RNAs are related and processed in
the cytoplasm
RNA are released and processes in the
nucleus
6 . RNA polymerase are complexes of
five polypeptides
RNA polymerases are complex of 10-15
polypeptides
7. Transcriptional units has one or
more genes
Transcriptional unit has only one gene

28. References
• Molecular biology by :-
P . S . Verma and V . K . Agraval
 https:// en . m . Wikipedia . Org

29. THANK YOU