COMPARISON altering the local conformation of DNA. l

COMPARISON
BETWEEN REPLICATION AND TRANSCRIPTION

 

Replication

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a.         
Synthesis of DNA
on DNA template.

a.         
Synthesis of RNA
on DNA template.

b.         
This process
involves DNA polymerase.

b.         
This process
involves RNA polymerase.

c.         
Requirement of
primer is essential.

c.         
No primer is
required.

d.         
DNA chain grows
in the 5′ to 3′ direction.

d.         
RNA chain grows
in the 5′ to 3′ direction.

e.         
Replication is
more prominent during the cell division.

e.         
During the
development of the stem cell to cells with specialized function that is
during the cell differentiation, the transcription takes place.

f.          
Replication rate
is much higher than that of transcription.

f.          
Transcription
rate is slower.

g.         
Replication
requires dATP, dGTP, dCTP and dTTP.

g.         
Transcription
requires ATP, GTP, CTP and TTP.

h.         
The starting
point of replication is the ori C on the DNA template.

h.         
The starting
point of transcription is the promoter sites on the DNA template.

i.          
The replication
results in the formation of two copies of double stranded DNA.

i.          
A single stranded
RNA which is identical in sequence with one of the duplex DNA is generated
through the transcription.

j.          
Error rate
during the replication is less.

j.          
Error rate
during the transcription is more.

 

IN EUKARYOTIC SYSTEM

 

-10 region of the prokaryotes’
promoter and TATA box region of the eukaryotes’ promoter resemble to each
other. This TATA box involves the proper positioning of the enzyme to initiate
the transcription in eukaryotes correctly.

Enhancers

l  
These
transcriptional activators possess variable positions and orientations.

l  
For the full
promoter activity, requirement of enhancers is essential.

l  
They favors the
RNA polymerase II binding by altering the local conformation of DNA.

l  
Transcription
factors or TFs are able to recognize the enhancers. These TFs are specific
proteins. These specific proteins lead to the stimulation of RNA polymerase II
binding to the nearby the promoter.

 

Three distinct types of RNA
polymerases can be found in eukaryotic nuclei.

Types

Localization

Cellular transcript formed

RNA polymerase I

Nucleus

18S; 55.8S; 28S rRNA

RNA polymerase II

Nucleoplasm

mRNA and hnRNA

RNA polymerase III

Nucleoplasm

tRNA

 

Transcription factors

There are three classes of
transcription factors are present which regulate the initiation of the
transcription by the help of RNA polymerase II. These TFs are:

1.         
General transcription factors or GTF: mRNA synthesis requires GTF, the basal factors.
Transcriptional initiation sites are selected by these basal factors and RNA
polymerase II is delivered to that site. This event leads to the formation of a
complex which initiates the transcription. This GTF-RNA polymerase II-promoter
containing DNA complex is known as the pre-initiation complex. It is situated
near the transcriptional start site. Basal level transcriptions are supported
by this pre-initiation complex. Several GTFs are observed. Examples are: TF
IIA; TF IIB; TF IID; TF IIF; TF IIE; TF IIH and TF IIJ. Here II stands for the
class II genes.

Steps
for formation of initiation complex and process of initiation

l   TBP protein that is TF IID or TATA box binding proteins
tend to bind to the TATA box.

l   TF IIA and TF IIB bind to that growing complex and
stabilizes it to initiate the transcription process.

l   By binding to the RNA polymerase II enzyme, TF IIF
exorts it to the complex.

l   At last, the remaining TF IIE; TF IIH and TF IIJ are
recruited sequentially to the complex. This leads to complete the formation of
pre-initiation complex or PIC.

l   After the formation of PIC, TF IIH mediates the
ATP-dependent activation step. This step is essential for the initiation of the
transcription process.

l   The associated PIC is stimulated by the co-operative
binding of several transcriptional factors to their target promoter and
enhancer sites. This leads to increase the rate of the transcription initiation
by that stimulated PIC.

l   Several types of zinc fingers and DNA binding motifs
are present in the TFs.

2.         
Upstream transcription factors or UTF: Cell
specific UTFs which are bound to the promoters and enhancer elements mediates
the transcriptional initiation of mRNA. There is a co-operative interaction
between the UTFs (each other) and UTFs with the pre-initiation complex.

3.         
Inducible transcription factors or ITF: ITFs
are steroid hormone receptors. These non-polar steroid hormones are able to
pass through the plasma membrane of their target cells to their cytosol. In the
cytosol, they tend to bind to their receptors. After binding to their
receptors, these steroid-receptor complexes enter into the nucleus and bind to
specific segments. These specific segments are of chromosomal enhancers. These
are known as response elements.

 

HOUSE KEEPING GENES

House keeping genes are RNA
polymerase II promoters which lack TATA boxes. Their expression in cells is
observed at relatively low rates.

 

DIFFERENCE BETWEEN TRANSCRIPTION IN
PROKARYOTES AND EUKARYOTES

PROKARYOTIC SYSTEM

EUKARYOTIC SYSTEM

1.         
Much simpler

1.         
More complex

2.         
Only one RNA
polymerase, holoenzyme synthesizes tRNA; mRNA and rRNA.

2.         
The rRNA; mRNA
and tRNA are synthesized by three different types of RNA polymerases I, II
and III respectively.

3.         
It does not
involve splicing.

3.         
Cleavage and
joining of the sequences from a large transcript (heterogenous nuclear RNA)
that is splicing is required for the derivation of mRNA in eukaryotes.

4.         
A specific
termination site is present in prokaryotes.

4.         
No such specific
termination site is present in them.

5.         
Transcription
and translation are coupled in this case.

5.         
Termination and
translation are not coupled.

6.         
Polycistronic
mRNA is observed.

6.         
Monocistronic
mRNA is observed.

7.         
The formed mRNA
is not modified further.

7.         
These mRNAs
contains 7-methyl guanosine modified caps at their 5′ ends and a long poly A
tail is observed at their 3′ ends.

8.         
Pribnow box that
is -10 sequence and -35 hexamer are conserved sequences in promoter region.

8.         
Hogness box that
is -25 sequence and CCAAT box that is -80 sequence are the conserved sequence
of promoter region.

9.         
5S; 16S and 23S
rRNA are formed.

9.         
5.8S; 18S and
28S rRNA are formed.

10.      
Inhibitor of
prokaryotic transcription are rifamycin.

10.      
Inhibitor of eukaryotic
transcription is alpha-amanitin.