transcription in prokaryotes and eukaryotes

1. MS. NEHA
Asst. Prof. (Zoology)

2. • Central dogma of molecular biology proposes a unidirectional or one
way flow of information from DNA to RNA (transcription) and from
RNA to protein (translation).
• The concept was given by Watson and Crick.

3. • First step is transcription (synthesis of RNA from DNA), but in case of
reverse transcription DNA is synthesizes from RNA in retrovirus. That
concept is given by Temin and Baltimore in Rous sarcoma virus, also
known as teminism and enzyme catalyze this reaction is reverse
transcriptase or RNA dependent DNA polymerase.
• In second step, messenger RNA (mRNA) moves the information from
the DNA to the ribosomes to direct the production of
protein. Translation represents a change in the language from the
nucleotide letters in RNA to the amino acid letters in protein

4. • RNA synthesis (transcription) is a transfer of the information from
the DNA where it is stored into RNA which can be transported and
interpreted
• Three Major Classes of RNA
These are synthesized by transcription of the appropriate genes and
are involved in protein synthesis.
1) mRNA- carries the message from the DNA to the ribosome.
2) rRNA- are major structural components of the protein-synthesizing
ribosome.
3) tRNA- act as adaptor molecules in aligning the amino acids
according to the sequence present in the mRNA.

5. • Transcription is catalyzed by RNA polymerase which makes RNA using DNA
as a template.
• The strand of DNA which is transcribed is the template/sense/non-coding
strand in 3’-5’ direction.
• The other strand is called non sense/non template/coding strand which is in
5’-3’ direction.
• Prokaryotes have only one RNA polymerase which synthesis all the three
types of RNA’s.

6.  RNA polymerase moves along
the template strand of the DNA
in the 3’ to 5’ direction, and the
RNA molecule grows in the 5’ to
3’ direction.
 RNA polymerase consists of five
polypeptide chains-β,β’,2 α & σ.
 The σ subunit recognizes
promoter on DNA template.
 β subunit helps in formation of
phosphodiester bond
 β’ subunit helps in binding with
template DNA
Fig. RNA
polymerase

7. Binding - DNA promoter region is a stretch of about 40 bp
adjacent to and including the transcription start point. Promoter have
a start point (designated +1 and usually an A), the six-nucleotide -10
sequence, and the six-nucleotide -35 sequence, located approx. 10
nucleotides and 35 nucleotides upstream from the start point.
• RNA polymerase binds at promoter by σ subunit.

8. • RNA synthesis starts from start point
• Ribonucleotide triphosphates(rNTPs) acts as substrate and hydrogen
bonded to complementary bases of template strand at start point
• RNA polymerase form phosphodiester bond between 3’-OH end of 1st
rNTP and 5’-P end of next rNTP & PPi releases
• After adding upto 9 rNTPs, σ subunit detaches from RNA polymerase
enzyme
Fig. Initiation of transcription

9. Elongation
• Successive rNTPs are added to 3’-OH end of growing RNA chain
about 18 base pairs of DNA are unwound
• The most recently synthesized RNA is still hydrogen-bonded to the
DNA, forming a short RNA-DNA hybrid(about 12 bp long)
• This unwinding and rewinding causes by RNA polymerase
but supercoiling is reduced by topoisomerase
Fig. Elongation of
transcription

10. Fig. Transcription process in
prokaryotes

11. Rho independent
• The newly formed RNA has GC rich
sequence followed by poly U
sequence
• GC rich sequence form hairpin loop
which pulls RNA chain and weak
bond between A=U broken & RNA
released
Rho dependent
• Rho factors/proteins causes
disruption of RNA-DNA hybrid
• Rho protein is a specialized
helicase which recognizes 50-90 bp
upstream of terminator in mRNA
sequence
• Rho protein releases newly formed
RNA

12. Name Location Product
α-Amanitin
sensitivity
RNA
polymerase I
(Pol I, Pol A)
Nucleolus
larger ribosomal RNA (rRNA)
(28s,18s and 5.8s)
Resistant
RNA
Polymerase
II (Pol II, Pol
B)
nucleus
messenger RNA ,most small
nuclear RNAs ,small interfering
RNA and micro RNA
Very sensitive
RNA
Polymerase
III (Pol III,
Pol C)
nucleus (and
possibly the
nucleolus-
nucleoplasm
interface)
transfer RNA ,other small RNAs
(including the small 5S ribosomal
RNA (5s rRNA) ,snRNA U6, signal
recognition particle RNA (SRP
RNA) and other stable short RNAs
Moderately
sensitive
Mitochondri
a
Mitochondria Mitochondrial RNA’s
Resistant
Chloroplast Chloroplast Chloroplast RNA’s Resistant

13. -180 -107 -45 +1 +20
Transcriptio
n
5’ 3’
Fig. Promoter of RNA polymerase I
Fig. Promoter of RNA polymerase II
Fig. Promoter of RNA
polymerase III

14. • Transcription factors and the
polymerase undergo binding to
initiate transcription
1) TFIID binds to the TATA box
followed by
2) The binding of TFIIA and
TFIIB.
3) The resulting complex is now
bound by the polymerase, to
which TFIIF has already
attached.
4) The initiation complex is
completed by the addition of
TFIIE, TFIIJ, and TFIIH.
5) Activation step requiring ATP-
dependent phosphorylation of the
RNA polymerase molecule which
initiates transcription

15. • Elongation in eukaryotes is same as in prokaryotes
Termination
RNA polymerase I – requires protein factors which recognize 18
nucleotides terminal sequence in newly formed RNA and then
termination occurs
RNA polymerase II – recognizes poly U sequence on template DNA but
hairpin loop is not formed
RNA polymerase III – newly formed mRNA cleaved at 10-35
nucleotides downstream from AAUAAA sequence in growing mRNA. At
cleaved site in mRNA poly A tail is formed

16. Fig. Termination in eukaryotes