CONCEPT ON GENE EXPRESSION, GENE THEORY, STAGES IN PROKARYOTES AND EUKARYOTES, TRANSCRIPTION AND TRANSLATION...

1. MANSI YADAV
SECTION-C
ROLL NO.-204
MEDICAL SC. FIRST YEAR..
REGULATION OF GENE
EXPRESSION…

2. MODERN CONCEPT OF GENES…
A gene may be defined as a segment of DNA
which is responsible for inheritance and
expression of a particular character. It provides
instructions for the synthesis of a specific
protein or RNA. They are the functional unit of
hereditary, variation, mutation and evolution…
Modern Concept of Gene
Seymour Benzer in 1955 introduced the 3 terms-:
1) CISTRON { Unit of FUNCTION }
 Segment of DNA having information of
synthesis of particular protein or RNA.
 Responsible for expression of a trait.
 Can be several bp long.
2) MUTON { Unit of MUTATION }
 Segment of DNA that undergoes mutation
 Consists of few nucleotides (1 or few bp)
3) RECON { Unit of RECOMBINATION }
 Segment of DNA that participates in
recombination through crossing over during
meiosis
 Consists of few to many bp

3. Objective :
 What is Gene expression?
 Necessity of regulation of
gene expression
 Different types of gene
regulation
 Gene regulation in
prokaryotes
Gene regulation in
Eukaryotes
A. Lac operon
B. Tryptophan operon
1. At transcription level
2. At translation
3. Modification of DNA

4. ∂ What is gene expression?
Formation of gene products
 RNA and Protein
 Gene expression is the process by which the
instructions in our DNA are converted into a
functional product, such as a protein.
 Normal growth , development and functioning
of an organism
 Production of gene product according to the
requirement of the body
∂ Necessity of Gene Expression and regulation

6. Because it is the first stage of gene
expression.
Gene regulation at the first stage will be
quick and will save a lot of energy in the
organism that can be utilized by the cell
for other useful purposes.
QUESTION ?
Why is transcription a particularly
important level of gene regulation in both
bacteria and eukaryotes?
ANSWER -

7. Differences in the Regulation of Gene
Expression of Prokaryotic and Eukaryotic
Organisms
Prokaryotic Organisms Eukaryotic Organisms
Lack a membrane-bound nucleus. Contain nucleus.
DNA is found in the cytoplasm. DNA is confined to the nuclear
compartment.
RNA transcription and protein
formation occur almost
simultaneously.
RNA transcription occurs prior to
protein formation, and it takes place
in the nucleus. Translation of RNA to
protein occurs in the cytoplasm.
Gene expression is regulated
at the transcriptional level.
Gene expression is regulated at many
levels (epigenetic, transcriptional,
nuclear shuttling, post-transcriptional,
translational, and post-translational).

8. Regulation of
Gene Expression
in Prokaryotes…

9. A. Constitutive gene expression - essential genes for living cells
& housekeeping genes
B. Inducible gene expression - enzymes in catabolic pathways
C. Repressible gene expression - enzymes in anabolic pathways
Type of genes expression
A. Positive regulation----
Enhancer/Activator
B. Negative regulation----
Repressor/ Silencer
Regulation of gene expression
In positive control, binding
of activator protein triggers
the transcription whereas in
negative control, binding of
repressor protein inhibits
the transcription.

10. Bacterial gene organization
Cluster of genes under the
control of the same promoter
and operator
Promoter, operator &
structural gene(s)
In E.coli , 25 operons
controlling 250 genes
Inducible & Repressible
operons
Lac operon and Trp operon
What is “Operon”?
Model
 Described by Jacob and Monod
in 1961 .
 Hypothesis: Based on the
regulation of lactose metabolism
by the intestinal bacterium E coli
 Led to the discovery of basic
principles of Gene transcription
activation and Repression

11. An operon is made up of 4 basic DNA
components:
1) Promoter – a nucleotide sequence that enables
a gene to be transcribed. The promoter is
recognized by RNA polymerase, which then
initiates transcription.
2) Regulator – These genes control the operator
gene in cooperation with certain compounds
called inducers and corepressors present in the
cytoplasm.
3) Operator – a segment of DNA that a repressor
binds to. It is classically defined in the lac
operon as a segment between the promoter
and the genes of the operon.
4) Structural genes – the genes that are co-
regulated by the operon.
TypeS-
I. Catabolic (inducible) such as Lac operon
II. Anabolic (repressible) such as Trp operon
Ligands which bind to the activators to “switch on” gene expression in
positive control are called inducers ; those binding to the repressors and
“switching off” gene expression are called co-repressors. Inducers and co-
repressors are known as effectors….

12. Induction
1. It turns the operon on.
2. It starts transcription and
translations.
3. It is caused by a new metabolite
which needs enzymes to get
metabolized.
4. It operates in a catabolic pathway.
5. Repressor is prevented by the
inducer from joining the operator
gene.
Repression
1. It turns the operon off.
2. It stops transcription and translation.
3. It is caused by an excess of existing
metabolite.
4. It operates in an anabolic pathway.
5. Apo repressor is enabled by a co-
repressor to join the operator gene.
TYPE OF OPERON PRESENCE OF
METABOLITE
EFFECT ON
OPERON
EXAMPLE
INDUCIBLE LACTOSE ON Lac Operon
REPRESSIBLE TRYPTOPHAN OFF Trp Operon

13. 1. Regulator gene (Lac I)
2. Promoter
3. Operator
4. Structural genes
a) Beta-galactosidase (lacZ) -- Breaks lactose into
glucose + galactose. Converts lactose to the
allolactose, regulates lac operon.
b) Beta Galactoside permease (lacY) -- Transports
lactose across cytoplasmic membrane.
c) Thiogalactoside transacetylase (lacA) -- Trans-
acetylation of lactose.
*Catabolite gene activator protein (CAP)
*Cyclic Adenosine Monophosphate (cAMP)
 Two regulators turn the operon "on" and "off" in
response to lactose and glucose levels:
the lac repressor and CAP.
 The lac repressor acts as a lactose sensor. It normally
blocks transcription of the operon, but stops acting as
a repressor when lactose is present. The lac repressor
senses lactose indirectly, through its
isomer allolactose.
 CAP acts as a glucose sensor. It activates transcription
of the operon, but only when glucose levels are low.
CAP senses glucose indirectly, through the "hunger
signal" molecule cAMP.
The lac operon is considered
an inducible operon because it is
usually turned off (repressed), but
can be turned on in the presence of
the inducer allolactose.

14. INDUCER IS PRESENT
INDUCER IS ABSENT
The repressor of the operon is synthesized
from the regulatory gene. The repressor
protein binds to the operator region of the
operon and prevents RNA polymerase from
transcribing the operon.
In the presence of an inducer, such as lactose
or allolactose, the repressor is inactivated by
interaction with the inducer. This allows RNA
polymerase access to the promoter and
transcription proceeds.
*Regulation of lac operon by
repressor is referred to as
negative regulation…

15. POSITIVE CONTROL AND CATABOLITE REPRESSION
1. cAMP―adenosine-3′,5′-cyclic monophosphate
2. The concentration of cAMP is inversely proportional to
the level of available glucose.
• CAP is only active when glucose levels are low (cAMP
levels are high). Thus, the lac operon can only be
transcribed at high levels when glucose is absent.
• This strategy ensures that bacteria only turn on
the lac operon and start using lactose after they have
used up all of the preferred energy source (glucose).
a. The positive effect is activated by CAP.
b. cAMP is bound to CAP, together CAP–cAMP complex
binds to a site slightly upstream from the lac gene
promoter.

16. Q-What is the meaning of this two-stages growing pattern ?
A-In stage 1, bacteria grow using glucose as carbon source. When
glucose is totally consumed, bacteria will stop growing (first
plateau). After this lag phase, bacteria grow again (stage 2) using
lactose until this second sugar is also finished (second plateau)….
Summary of Lac Operon Responses
GLUCOSE LACTOSE CAP BINDS REPRESSOR LEVEL OF
TRANSCRI
PTION
+ - - + NO
+ + - - LOW-
LEVEL
- - + + NO
- + + - HIGH-
LEVEL

17. 1. The trp operon, found in E. coli bacteria, is a group of genes that encode biosynthetic
enzymes for the amino acid tryptophan needed for their survival by building proteins.
2. The trp operon is regulated by the trp repressor. When bound to tryptophan,
the trp repressor blocks expression of the operon.
3. E. coli can also make their own tryptophan using enzymes that are encoded by 5 genes.
These 5 genes are located next to each other in what is called the trp operon i.e.
E,D,C,B,A.

18. The trp repressor does not always bind to DNA.
Instead, it binds and blocks transcription only
when tryptophan is present. When tryptophan is
around, it attaches to the repressor molecules and
changes their shape so they become active. A small
molecule like tryptophan, which switches a
repressor into its active state, is called
a corepressor.
When there is little tryptophan in the cell, on the other
hand, the trp repressor is inactive (because no
tryptophan is available to bind to and activate it). It does
not attach to the DNA or block transcription, and this
allows the trp operon to be transcribed by RNA
polymerase.
Trp ABSENT
Trp PRESENT

19. Regulation of
Gene Expression
in Eukaryotes….

20. In eukaryotes like humans, gene expression
involves many steps, and gene regulation can
occur at any of these steps. However, many genes
are regulated primarily at the level of
transcription.
 Chromatin accessibility. The structure of
chromatin (DNA and its organizing
proteins) can be regulated. More open or
“relaxed” chromatin makes a gene more
available for transcription.
 Transcription. Transcription is a key
regulatory point for many genes. Sets
of transcription factor proteins bind to
specific DNA sequences in or near a gene
and promote or repress its transcription
into an RNA.
 RNA processing. Splicing, capping, and
addition of a poly-A tail to an RNA molecule
can be regulated, and so can exit from the
nucleus. Different mRNAs may be made
from the same pre-mRNA by alternative
splicing.
 RNA stability. The lifetime of an mRNA
molecule in the cytosol affects how many
proteins can be made from it. Small regulatory
RNAs called miRNAs can bind to target
mRNAs and cause them to be chopped up.
 Translation. Translation of an mRNA may be
increased or inhibited by regulators. For
instance, miRNAs sometimes block translation
of their target mRNAs (rather than causing
them to be chopped up).
 Protein activity. Proteins can undergo a
variety of modifications, such as being chopped
up or tagged with chemical groups. These
modifications can be regulated and may affect
the activity or behavior of the protein.

22. • At the chromatin structure level, genes can be silenced
by changing the degree of compacting or by chemical
modifications of the DNA. Genes can also be
hyperactivated through amplification. Proteins
participating in the changeset of chromatin structure
are called nucleosome modifiers.
• Epigenetic inheritance is a gene expression control
relying on control of chromatin structure independent
of any DNA sequence changes. Epigenetic inheritance is
mainly based on histone modifications and DNA
methylation. Epigenetic regulations include X-
inactivation and parental imprinting which is crucial for
normal embryonic development. Abnormal epigenetic
inheritance can cause cancers and many genetic
disorders.
*Modification of Histone Proteins is an Example of
Epigenetic Control…

23. Transcription
Factors
 Groups of transcription
factor binding sites
called enhancers and s
ilencers can turn a
gene on/off in specific
parts of the body.
 Transcription factors
that
are activators boost a
gene's
transcription. Repress
ors decrease
transcription.
 It allow cells to perform
logic operations and
combine different
sources of information
to "decide" whether to
express a gene.
 They are proteins that
help turn specific
genes "on" or "off" by
binding to nearby
DNA.
Transcriptional regulation is control of
whether or not an mRNA is transcribed
from a gene in a particular cell.

24.  Transcription involves RNA Polymerase II and
transcription factors.
 RNA polymerase II - attaches to the promoter (TATA box).
 The purpose of the promoter is to bind transcription
factors that control the initiation of transcription. Within
the promoter region, just upstream of the transcriptional
start site, resides the TATA box (a repeat of thymine and
adenine dinucleotides).
 Control elements – non coding sequences of DNA where
the transcription factors attach.
A. Enhancer – control element far from a gene or
intron.
B. Activator – bind to enhancers to turn on
transcription of a gene
C. Repressors – inhibit gene expression
• Turn off transcription
• Block activators from binding to enhancers
Transcription factors + enhancer + activator + RNA
Polymerase II = transcription initiation complex
 Needed for transcription to begin

25. Distal control
element
Activators
Enhancer
Promoter
Gene
TATA
box
General
transcription
factors
DNA-bending
protein
Group of
Mediator proteins
RNA
Polymerase II
RNA
Polymerase II
RNA synthesis
Transcription
Initiation complex
Chromatin changes
Transcription
RNA processing
mRNA
degradation
Translation
Protein processing
and degradation
A DNA-bending protein
brings the bound activators
closer to the promoter.
Other transcription factors,
mediator proteins, and RNA
polymerase are nearby.
2
Activator proteins bind
to distal control elements
grouped as an enhancer in
the DNA. This enhancer has
three binding sites.
1
The activators bind to
certain general transcription
factors and mediator
proteins, helping them form
an active transcription
initiation complex on the promoter.
3

26. a) Alternative RNA splicing is a mechanism that allows
different combinations of introns, and sometimes exons,
to be removed from the primary transcript. This allows
different protein products to be produced from one
gene. It acts as a mechanism of gene regulation.
b) Control of RNA Stability : Proteins, called RNA-binding
proteins (RBPs) can bind to the regions of the RNA just
upstream or downstream of the protein-coding region.
These regions in the RNA that are not translated into
protein are called the untranslated regions, or UTRs. The
region just before the protein-coding region is called
the 5′ UTR, whereas the region after the coding region is
called the 3′ UTR.
c) microRNAs, or miRNAs, can also bind to the RNA
molecule. miRNAs are short (21–24 nucleotides) RNA
molecules that are made in the nucleus. miRNAs bind to
mRNA along with a ribonucleoprotein complex called
the RNA-induced silencing complex (RISC). The RISC-
miRNA complex rapidly degrades the target mRNA.
Post-transcriptional regulation occurs after the
mRNA is transcribed but before translation begins.
This regulation can occur at the level of mRNA
processing, transport from the nucleus to the
cytoplasm, or binding to ribosomes.
Alternative RNA splicing
Control of RNA Stability

27. a. After an mRNA has been transported to the
cytoplasm, it is translated into proteins. Control
of this process is largely dependent on the mRNA
molecule.
b. Translation can also be regulated at the level of
binding of the mRNA to the ribosome. Once the
mRNA bound to the ribosome, the speed and level
of translation can still be controlled.
c. An example of translational control occurs in
proteins that are destined to end up in an
organelle called the endoplasmic reticulum (ER).
d. The first few amino acids of these proteins are a
tag called a signal sequence. As soon as these
amino acids are translated, a signal recognition
particle (SRP) binds to the signal sequence and
stops translation while the mRNA-ribosome
complex is shuttled to the ER. Once they arrive,
the SRP is removed and translation resumes.
Translational Control

28.  The final level of control of gene expression
in eukaryotes is post-translational regulation.
This type of control involves modifying the
protein after it is made to affect its
activity.
 When an enzyme is no longer needed, it is
inhibited by a competitive or allosteric
inhibitor, which prevents it from binding to
its substrate. The inhibition is reversible, so
that the enzyme can be reactivated later.
This is more efficient than degrading the
enzyme.
 The activity or stability of proteins can also
be regulated by adding functional groups,
such as methyl, phosphate, or acetyl groups.
 A phosphate group is attached to
a protein. The effect of
phosphorylation varies from protein
to protein: some are activated by
phosphorylation, while others are
deactivated, and others yet simply
change their behavior.

29. 1. https://cnx.org/resources/0e5cca3cd82aa8ea1b9
6841daebcb1dae341e5a3/7-
REGULATION%20OF%20GENE%20EXPRESSI
ON%20IN%20EUKARYOTES.pdf
2. https://www.slideshare.net/MetheeSri/regulatio
n-of-gene-expression-41008675
3. https://www.ncbi.nlm.nih.gov/pmc/articles/PM
C3610329/
4. https://rwu.pressbooks.pub/bio103/chapter/reg
ulation-of-gene-expression/
5. https://www.nature.com/scitable/topicpage/reg
ulation-of-transcription-and-gene-expression-in-
1086/