Definition, Principle, gene expression in eukaryotes, levels of gene control, genes and regulatory elements, Operon system

1. Control of gene expression
Presented by :
Gurleen Kaur
Roll no. = 60235
M.Sc. Biotechnology

2. Gene expression is the process by which information from
gene is used in the synthesis of a functional gene products .

3. General Principle of gene regulation
One of the major themes of molecular genetics is the central
dogma
Which stated that genetic information flows from DNA to RNA
to proteins and provided a molecular basis for the connection
between genotype and phenotype. Although the central dogma
brought coherence to early research in molecular genetics, it
failed to address a critical issue:
How is the flow of information along the molecular
pathway regulated?
Consider Escherichia coli a bacterium that resides in your
intestine your eating habits determine the nutrients available to
this bacteria it can’t seek out nourishment when nutrients are
scare; nor can it move away when confronted with unpleasant
changes.

4. E. coli makes, up for its inability to alter the external
environmental by a being internally flexible.
For example, if Glucose is present, E. coli uses it to
generate ATP;
If there is no Glucose it utilizes Lactose, Arabinose,
Maltose, Xylose or many other no. of sugars.
If amino acids are available E. coli uses to synthesize
proteins.
If particular amino acid is absent, E. coli produces the
enzymes needed to synthesize that amino acid.

5. Thus, E. coli responds to the environmental changes by rapidly
altering its biochemistry.
Producing all enzymes necessary for every environmental
conditions is highly energetically expensive.
E. coli maintain this biochemical flexibility by gene regulation.
CONSEPTS
Gene regulation is key to both unicellular flexibility and
multicellular
specialization and it is critical to success of all living
organisms.

6. In bacteria
Gene regulation maintains internal flexibility , turning genes ON
and OFF in response to environmental changes.
In multicellular eukaryotes
Gene regulation brings about cellular differentiation .
LEVELS OF GENE CONTROL
A gene may be regulated at no. of points along the pathway of
information flow from genotype to phenotype.
First Regulation ;
First regulation arises through the alteration of structure of gene.
Modification to DNA or its packaging may influence which
sequence are available for transcription.

7. DNA methylation and changes in chromatin are two processes
that play a pivotal role in gene regulation.
Second point ;
At level of transcription gene can be regulated.
For sake of cellular economy it makes sence to limit protein
production early in the transfer of information from DNA to
protein.
Transcription is an important point of gene regulation in both
bacterial and eukaryotic cell .
Third point;
mRNA processing is third point of gene regulation. Eukaryotic
messenger RNA is extensively modified before its translation.
A 5’ cap is added.
3’ end is cleaved and polyadenylation .

8. Introns are removed.
These modifications determine the stability of mRNA;
Whether mRNA can be translated.
The rate of translation
The amino acid sequence of the protein produced.
Fourth point;
Regulation of RNA stability for the control of gene expression.
The amount of protein produced depends not only the amount
of the m RNA but also on the rate at which m RNA is degraded,
so RNA plays an important role in gene expression.

9. Fifth point;
At level of translation.
Gene regulation at this level requires a large number of enzymes
, protein factors and RNA molecules.
All of these factors, Availability of amino acids , Sequences in m
RNA .
Influences the rate at which proteins are produced and therefore
provides points at which gene expression may be controlled.

10. Concepts;
Gene expression may be controlled at any of a number of points
along the molecular pathway from DNA to protein , including
gene structure, transcription, m RNA processing, RNA stability,
translation and post translation modifications.

11. Gene expression may be controlled at multiple levels

12. Genes and regulatory elements;
Genes and DNA sequences that are transcribed into RNA.
Regulatory elements are DNA sequences that are not
transcribed but effect the expression of genes.
Regulatory gene’s product either RNA or proteins, interact
with other sequences and effect their transcription or
translation.
Genes include DNA sequences that encode proteins, as well as
sequences that encodes r RNA , t RNA, sn RNA AND Other
types of RNA.
Structural genes encodes proteins that are used in cellular
metabolism .Much of gene regulation take place through the
action of proteins ,produced by regulatory genes that recognize
and bind to regulatory elements.

13. DNA binding proteins
Much of gene regulation is accomplished by proteins that binds
to DNA sequences and influences their expression.
Regulatory proteins have function parts- domains, consisting of
60-90 amino acids- that are responsible for binding with DNA.
Within a domain only a few amino acids actually make contact
with DNA . ( asparagine, arginine, glutamine, glycine, lycine)
often form hydrogen bonds with the bases or interect with the
sugar phosphate backbone of DNA.

14. DNA binding proteins can be grouped into several distinct types
on the basis of characteristic structure called a motif, found
within the domain.
Common DNA – binding motifs
Location : Bacterial regulatory proteins;
related motifs in eukaryotic proteins.
Characteristics: Two alpha helices
Binding site: Major groove of DNA

15. Location; Eukaryotic regulatory and
other
proteins.
Characteristic: Loop of amino acids
with
zinc at base.
Binding site in DNA : Major groove

16. Location: Eukaryotic transcription factors
Characteristics : helix of lucine residues
and basic arm; two lucine rasidue
interdigitate.
Binding site in DNA two adjacent major
grooves

17. Location: Eukaryotic proteins
Characteristics: Two alpha helices
separated by a loop of amino acids.
Binding site in DNA: Major groove

18. Gene regulation in bacterial cells
The mechanism of gene regulation were first investigated in
bacterial cells. When the study of these mechanism in eukaryotic
cell begin , it seemed clear that bacterial and eukaryotic gene
regulation were quite different .
* One significant difference in bacterial and eukaryotic gene
control lies in the organization of functionally related genes .
*In bacterial cells the genes that have related function are
clustered
and under control of single promoter . These genes are often
transcribed in single mRNA.
*Eukaryotic genes in contrast are dispersed , and typically each is
transcribed into a separate mRNA.

19. OPERON SYSTEM
An operon is part of genetic material which act as single
regulated unit having one or more structural genes , an operator,
a promoter, a regulator gene, a repressor , an inducer or
corepressor.
Operons are of two types ;
*Inducible operon system - Lac operon
*Represible operon system

21. Inducible operon system – Lac operon
It is unit of genetic material which is switched ON in response
to the presence of chemicals. It comprised of following parts:
Structural genes - synthesizes m RNA
controls metabolic activity of cytoplasm
An operon has one or more structural genes (Z,Y,A). They
transcribe polycistronic m RNA molecules that helps in the
synthesis of three molecules:
•Beta galactosidase
•galactosidase permease
•thiogalactosidase acetylase
The three enzymes are however,produced in different molar
concentrations. Initial entry of Lactose into bacterium would
only occur due to this activity .

22. Operator gene
•It comprises of only 27 base pairs .
•It directly control the synthesis of the m RNA over the structural
genes.
•It is switched OFF by the presence of a repressor. An inducer can
take away the repressor and switch ON the gene.
Promoter gene
•Act as initiation signal for RNA polymerase.
•It is functional only when operator gene allows passage of RNA
polymerase to structural gene.

23. Regulator gene
•It produces inhibitor or repressor for blocking of operator gene.
•It controls the function of operator gene.
•It function through the formation of an m RNA of repressor.
•Repressor is proteinaceous substance synthesized by the regulator
gene i gene.
•Repressor as two allosteric sites one for attaching to operator and
second for binding to inducer.
•After coming in contact with inducer the repressor undergoes
conformational changes in such a way that it is unable to combine
with operator.

24. Inducer
*It is chemical which after coming contact with the repressor,
changes into the non DNA binding state so as to free the operator
gene.
CAP (catabolic activator pathway)
*It exerts negative control in lac operon .
*In its absence RNA polymerase is unable to recognise promoter
gene.
•CAP site is present near the lac promoter.
•CAP activates Lac genes only when glucose is absent.

25. The trp operon of E. coli
The trp operon controls the biosynthesis of the amino acid
tryptophan in E. coli and it is repressible operon .
In repressible operon transcription is normally turned on and
must be repressed

26. When trp is low:
The trp repressor is normally inactive, it cannot bind to the
operator and transcription takes place.
When trp is high :
The trp binds to the repressor and makes it active.
The trp repressor then binds to the operator and shuts
transcription OFF.

32. When glucose level is high, cAMP levels are low, c AMP is likely
to bind to CAP.
RNA polymerase can’t bind to DNA as efficiently so
transcription is at a low rate.
When glucose level is low c AMP levels are high.
CAP readily binds c AMP, and the CAP-c AMP complex bind
DNA, increasing the efficiency of polymerase binding.
The result is high rate of transcription and translation of the
structural genes and the production of glucose from lactose.