➢ It is the synthesis of an RNA molecule from
➢ The template strand is the strand from which
the RNA is actually transcribed. It is also
termed as Antisense Strand.
➢ The coding strand is also called as Sense
✓ An enzyme that catalyses RNA synthesis.
✓ It does not need a Primer for initiation of
✓ Eukaryotes have 3 RNA polymerases but bacteria
have only a single RNA polymerase.
✓ Shape: RNA polymerase resembles a crab claw.
• Transcription in eukaryotes is undertaken by different
• Several Initiation factors are required for efficient &
promoter- specific initiation in eukaryotes, and are
called as General Transcription Factors(GTFs).
• Sometimes additional factors are required such as
Mediator Complex, DNA binding regulatory proteins
and chromatin modifying enzymes.
Pol II core Promoter
• The Eukaryotic core promoter refers to the minimal set
of sequence elements required for accurate
transcription initiation by the Pol II machinery.
PRE INITIATION COMPLEX
• RNA Polymerase II Forms a Preinitiation Complex with General
Transcription Factors at the Promoter.
• Once assembled at the Promoter, Pol II leaves the preinitiation
complex upon addition of the nucleotide precursors required for RNA
synthesis and after phosphorylation of serine residues within the
➢ Elongation requires another set of factors, such as
TFIIS & hSPT5, Known as Elongation factors.
➢ Various Proteins are thought to stimulate elongation
by Pol II.
➢ The protein P- TEFB stimulates elongation in 3
separate steps- 1). Phosphorylation at ser-2 position of CTD
2). It also activates another protein ,called hSPT5 which is an
3). It activates another elongation factor called TAT-SF1.
POLYADENYLATION & TERMINATION
➢ Once the elongation is completed, it proceeds through the
RNA processing events i.e., polyadenylation &
➢ The CTD tail of polymerase carries 2 protein complexes-
CPSf and CstF.
➢ The sequences which one transcribed into RNA, trigger
the transfer of these factors to the RNA, are called as poly-
➢ Once CPSF & CstF bound to the RNA, it results in RNA
clevage and then polyadenylation.
RNA POLYMERASE I
• This enzyme is related to Pol. II, but they initiate transcription
from distinct promoters and transcribed different genes.
• Pol. I is required for the expression of only one gene that
encoding the rRNA precursor.
• The promoter of rRNA genes comprises 2 parts: core elements
• In addition to Pol. I, initiation requires 2 other factors, called
SL1 & UBF.
• SL1 comprises TBP & three TAF’s specific for transcription.
• These complex bound to the UCE in the presence of UBF and
stimulates transcription for core promoter by recruiting Pol I.
RNA POLYMERASE III
• This enzyme is also related to Pol.II and they initiate transcription
from distinct promoters and transcribed distinct genes.
• Pol. III promoters come in various forms and being located
downstream of the transcription start site.
• Some Pol III promoters consist of 2 regions called Box A & Box B.
• Same as Pol I & II, transcription by Pol III also requires
transcription factors alongwith polymerase.
• The factors are TFIIIB &TFIIIC.
• Firstly TFIIIC binds to the promoter region.
• After that the factor TFIIIB containing TBP is also binds to the
• This complex is forced which stimulates the initiation of
Differences Between Prokaryotic vs
Prokaryotic Transcription Eukaryotic Transcription
• Coupled Transcription-
Translation is the rule.
• Occurs in the cytoplasm.
• A single RNA Polymerase
synthesises all three types
of RNA .
• RNAs are released &
processed in the cytoplasm.
• σ factor present for
• mRNA is devoid of
5’guanosine cap and Poly A
• Coupled Transcription-
Translation is not possible.
• Occurs in the Nucleus.
• The RNA polymerases I, II,III
synthesises rRNA,mRNA, tRNA
• RNAs are released & processed
in the nucleus.
• σ absent and initiation of
transcription require proteins
called Transcription factors.
• Capping occurs at 5’ end and
Poly A tail at 3’ position of
1). Watson et.al (2009), Molecular Biology of gene, 5th Edition,
Pearson Education, New Delhi.
2). Lewin B. (2004), Gene VIII, Pearson Prentice and Hall, New Delhi.
3). Lodish et al (2013), Molecular Cell Biology, 7th Edition, W.H. Freeman