Gene
Expression
AP Biology
Questions to Ponder…..
 How
do your cells “know” what kind of
cell they are?
 How do your cells “know” when to make
a particular protein? When to stop making
it?
 How does the environment affect your
cells?
 ANSWER: Gene Expression
What makes cells from the same individual look different?
Stem Cells
Liver Cells
Red Blood Cells
Cartilage Cells
DNA sequence in each cell is the same, but different cell types have
different “GENE EXPRESSION PATTERNS”
• When a gene is “on” and its protein or
RNA product is being made, scientists
say that the gene is being EXPRESSED.
• The on and off states of all of a cell’s
genes is known as a GENE EXPRESSION
PROFILE.
• Each cell type has a unique gene
expression profile.
Insulin
Muscle Cell
DNA?
Protein?
X
Pancreatic Cell
Slide adapted from Genetic Science Learning Center, University of Utah  20
Gene Expression in Bacteria


Bacteria are singlecelled organisms
who are surrounded
on all sides by their
environment.
They must be able to
regulate expression
of their genes in
response to
environmental
changes.
Bacteria Respond by
Regulating Transcription


Bacteria cells that
can conserve
resources and
energy have a
selective advantage
over cells that
cannot do so.
Natural selection has
favored bacteria
that express only the
genes they need.
E. Coli Regulation of
Tryptophan



An individual E. coli cell living in the erratic
environment of the human colon, is
dependent for its nutrients on the whimsical
eating habits of its host—you!
If the environment is lacking in the amino acid
tryptophan, which the bacterium needs to
survive, it responds by activating a metabolic
pathway that makes tryptophan from another
compound.
If tryptophan becomes available, it shuts
down this pathway.
Regulation of a Metabolic
Pathway
 In
the pathway for tryptophan synthesis,
an abundance of tryptophan can both
inhibit the activity of the first enzyme (a
rapid response) OR repress expression of
the genes encoding the enzymes in the
pathway (a longer response).
 This is an example of feedback inhibition.
It allows for a cell to adapt to short-term
fluctuation in the supply of a substance it
needs.
Fig. 18-2
Precursor
Feedback
inhibition
trpE gene
Enzyme 1
trpD
gene
Enzyme 2
Regulation
of gene
expression
trpC
gene
trpB gene
Enzyme 3
trpA
gene
Tryptophan
(a) Regulation of
enzyme
activity
(b) Regulation of enzyme
production
Gene Expression Controls Which
Enzymes are Made and When
 In
many cases, this occurs in the process
of transcription.
 Many genes may be switched on or off by
changes in the metabolic status of the
cell.
 One example was discovered in 1961 by
Francois Jacob and Jacques Monod at
the Pasteur Institute in Paris. This method is
called the Operon Model.
Operons: The Basic Concept
A
cluster of functionally related genes
that can be under coordinated control by
a single on-off “switch”.
 The regulatory “switch” is a segment of
DNA called an operator usually positioned
within the promoter.
 An operon is the entire stretch of DNA that
includes the operator, the promoter, and
the genes that they control.
 The
operon can be switched off by a
protein repressor
 The repressor prevents gene transcription
by binding to the operator and blocking
RNA polymerase
 The repressor is the product of a separate
regulatory gene
•
•
The repressor can be in an active or
inactive form, depending on the presence
of other molecules.
A corepressor is a molecule that
cooperates with a repressor protein to
switch an operon off.
•
•
•
•
Bacteria can synthesize tryptophan by utilizing
the trp operon.
By default, the trp operon is on and the genes
for tryptophan synthesis are transcribed
When tryptophan is present, it binds to the trp
repressor protein, which turns the operon off
The repressor is active only in the presence of
its co-repressor tryptophan; thus the trp
operon is turned off (repressed) if tryptophan
levels are high
Fig. 18-3a
trp operon
Promoter
Promoter
DNA
trpR
Regulatory
gene
mRNA
Protein
5
Genes of operon
trpE
Operator
Start codon
3
mRNA
RNA
5
polymerase
trpD
trpB
trpA
B
A
Stop codon
E
Inactive
repressor
trpC
D
C
Polypeptide subunits that make up
enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on
Fig. 18-3b-1
DNA
No RNA made
mRNA
Active
repressor
Protein
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off
Fig. 18-3b-2
DNA
No RNA made
mRNA
Active
repressor
Protein
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off
Video for Gene Expression
 http://education-
portal.com/academy/lesson/regulationof-gene-expression-transcriptionalrepression-and-induction.html
Different Types of Operons
A
repressible operon is one that is usually
ON—binding a repressor to the operator
turns off transcription. (The trp operon is
like this)
 An inducible operon is one that is usually
OFF—a molecule called an inducer
inactivates the repressor and starts
transcription. (The lac operon is this type)
The lac Operon



The lac operon is an inducible operon (usually
off) and contains genes that code for
enzymes that break down the sugar lactose
(found in dairy products)
By itself, the lac repressor is active and
therefore shuts the lac operon off most of the
time.
A molecule called an inducer inactivates this
repressor which turns the lac operon on.
Fig. 18-4
Regulatory
gene
Promoter
Operato
r
lacZ
lacI
DNA
No
RNA
made
3
mRNA
RNA
polymerase
5
Active
repressor
Protein
(a) Lactose absent, repressor active, operon off
lac operon
lacZ
lacI
DNA
3
mRNA
5
lacA
RNA
polymerase
mRNA
5
-Galactosidase
Protein
Allolactose
(inducer)
lacY
Inactive
repressor
(b) Lactose present, repressor inactive, operon on
Permease
Transacetylase
Fig. 18-4a
Regulatory
gene
Promoter
Operato
r
lacZ
lac
I
DNA
No
RNA
made
3
mRNA
Protein
5
RNA
polymerase
Active
repressor
(a) Lactose absent, repressor active, operon
off
Fig. 18-4b
lac
operon
lacI
DNA
lacZ
3
mRNA
5
lac
A
RNA
polymerase
mRNA
5
Galactosidase
Protein
Allolactose
(inducer)
lacY
Inactive
repressor
(b) Lactose present, repressor inactive, operon
on
Permeas
e
Transacetylase
Inducible Enzymes
 Inducible
enzymes (such as those found in
the lac operon) are usually catabolic
enzymes, which means they break things
apart.
 Their synthesis is usually induced by some
kind of signal.
 In the lac operon, the signal is the
presence of the lactose sugar molecule.
Repressible Enzymes
 Repressible
enzymes (such as those in the
trp operon) usually function in anabolic
pathways which build things or put things
together.
 Since these are almost always ON, they
are repressed (shut down) when there are
high levels of the end-product present.