Understanding the lac Operon and Gene Induction in E. coli

 
Metabolism & Survival
 
Key Area 1c
Lac
 operon
 
Learning 
I
ntentions
 
Understand how the 
lac
 operon
works as an examp
le of gene
induction
 
Control of pathways
 
Genes that are not required can be
induced
 (switched on) only when
needed.
 
The Jacob-Monod hypothesis gives a
possible model for this action.
The Jacob-Monod hypothesis
 
Hypothesised by two famous scientists,
this model illustrates how certain genes
can be switched on (active) or switched off
(inactive) and thus save resources being
wasted by making unrequired proteins.
 
A good example of the mechanism
by which genes can be switched on
and off is found in the bacterium
Escherichia coli
.
 
The Lac Operon
 
Escherichia coli
 produces the enzyme
-galactosidase 
to digest lactose, to
release the glucose that it needs for
respiration.
The process of switching on a gene
only when the enzyme is needed is
called ENZYME INDUCTION
 
The enzyme is only made if 
lactose is
present
. When no lactose is present the
gene that codes for the synthesis of
-galactosidase is switched off.
 
Lactose
 
+
 
- a sugar found
 
in milk
The Jacob-Monod hypothesis
The lac Operon 
(the 1
st
 stage)
 
The gene which codes for 
-galactosidase
is called the 
structural gene 
and is part of
a DNA chain which includes a 
regulator
gene 
and a sequence of bases called the
operator
.
 
 
Regulator gene 
-is a section of 
DNA
responsible for producing the 
repressor
molecule
 
Repressor molecule 
- is responsible for
combining to the operator gene (lactose
in this case).
 
Structural gene
 
- 
 is responsible for the
production of enzymes used in biochemical
pathways
 
Operator gene  
- determines whether the
structural gene is switched on
The Jacob-Monod hypothesis
(the 2
nd
 stage)
 
The 
regulator gene 
is continually
transcribed into messenger RNA. This is
used to synthesise a 
repressor protein
.
The Jacob-Monod
hypothesis (the 2
nd
 stage)
 
When there is 
no lactose present 
the
 
repressor
protein becomes attached to the operator
.
This prevents the structural gene being
transcribed, and 
no enzyme is made
.
The Jacob-Monod
hypothesis (the 3
rd
 stage)
 
If molecules of 
lactose
 
are introduced,
they
 
bind to the repressor protein
molecules.
The Jacob-Monod
hypothesis (the 3
rd
 stage)
 
This prevents the repressor protein binding to the
operator. Because the 
operator is free 
the
structural gene is transcribed and the 
enzyme is
made
 
Lactose
 
is called the 
inducer
 
because
its presence 
induces synthesis of the
enzyme
.
The
 
-galactosidase 
breaks down any
lactose which is present and when
there is none left the repressor
protein once again binds to the
operator and switches off the
structural gene.
View the experiment
:  Scholar Unit 3, Fig 3.2: The effect of β-galactosidase
enzyme on lactose in milk
http://courses.interactiveuniversity.net/vle/scholar/session.controller?action=vi
ewContent&contentGUID=e40b97ac-3f87-f6c4-1ddf-df59d0cfc27c
 
 Q’s 1-3 p239
 Q’s 1-2 p245-246
 Try the BBC Bitesize Test
Do the Biology-online.org tutorial
 Revise with Scholar Unit 3, Ch3
 
How can I learn this theory?
View the animation
:  Scholar Unit 3, Fig 3.4: Absence of lactose
http://courses.interactiveuniversity.net/vle/scholar/session.controller?action=viewContent&co
ntentGUID=27961b23-db58-80f8-1d2a-20cbe3d5fad8
View the animation
:  Scholar Unit 3, Fig 3.5: Presence of lactose
http://courses.interactiveuniversity.net/vle/scholar/session.controller?action=viewContent&co
ntentGUID=c9e09eea-6d89-b12c-aa09-2e02312a0c96
 
The structure and function of all cells are
determined by their genes.  Each gene on a strand
of DNA codes for each protein that the cell
requires.
 
To prevent resources being
wasted the genes for the
proteins required are switched
on only when needed.
 
Enzyme induction 
is the name for the process of
switching on a gene, only when the 
enzyme
 
that it
codes for is needed.
Role of Genes
Role of Genes
Image source: 
library.thinkquest.org
  
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Explore the mechanism of gene induction through the lac operon in Escherichia coli, where genes are switched on or off based on the presence of lactose. Learn how enzymes like β-galactosidase are produced only when needed, following the Jacob-Monod hypothesis. This system allows for efficient resource utilization and the regulation of metabolic pathways in response to environmental conditions.


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  1. Metabolism & Survival Key Area 1c Lac operon

  2. Learning Intentions Understand how the lac operon works as an example of gene induction

  3. Control of pathways Genes that are not required can be induced (switched on) only when needed. The Jacob-Monod hypothesis gives a possible model for this action.

  4. The Jacob-Monod hypothesis Hypothesised by two famous scientists, this model illustrates how certain genes can be switched on (active) or switched off (inactive) and thus save resources being wasted by making unrequired proteins.

  5. The Lac Operon A good example of the mechanism by which genes can be switched on and off is found in the bacterium Escherichia coli.

  6. Escherichia coli produces the enzyme -galactosidase to digest lactose, to release the glucose that it needs for respiration. The enzyme is only made if lactose is present. When no lactose is present the gene that codes for the synthesis of -galactosidase is switched off. The process of switching on a gene only when the enzyme is needed is called ENZYME INDUCTION

  7. Glucose Galactose Lactose - a sugar found in milk - Galactosidase Enzyme that catalyses the breakdown of lactose + Glucose Galactose

  8. The Jacob-Monod hypothesis The lac Operon (the 1st stage) The gene which codes for -galactosidase is called the structural gene and is part of a DNA chain which includes a regulator gene and a sequence of bases called the operator.

  9. Regulator gene -is a section of DNA responsible for producing the repressor molecule Operator gene - determines whether the structural gene is switched on Structural gene - is responsible for the production of enzymes used in biochemical pathways Repressor molecule - is responsible for combining to the operator gene (lactose in this case).

  10. The Jacob The Jacob- -Monod hypothesis Monod hypothesis (the 2 (the 2nd nd stage) stage) The regulator gene is continually transcribed into messenger RNA. This is used to synthesise a repressor protein.

  11. The Jacob-Monod hypothesis (the 2nd stage) When there is no lactose present the repressor protein becomes attached to the operator. This prevents the structural gene being transcribed, and no enzyme is made.

  12. The Jacob-Monod hypothesis (the 3rd stage) If molecules of lactose are introduced, they bind to the repressor protein molecules.

  13. The Jacob-Monod hypothesis (the 3rd stage) This prevents the repressor protein binding to the operator. Because the operator is free the structural gene is transcribed and the enzyme is made

  14. Lactose is called the inducer because its presence induces synthesis of the enzyme. The -galactosidase breaks down any lactose which is present and when there is none left the repressor protein once again binds to the operator and switches off the structural gene. View the experiment: Scholar Unit 3, Fig 3.2: The effect of -galactosidase enzyme on lactose in milk http://courses.interactiveuniversity.net/vle/scholar/session.controller?action=vi ewContent&contentGUID=e40b97ac-3f87-f6c4-1ddf-df59d0cfc27c

  15. How can I learn this theory? Q s 1-3 p239 Q s 1-2 p245-246 Try the BBC Bitesize Test Do the Biology-online.org tutorial Revise with Scholar Unit 3, Ch3 View the animation: Scholar Unit 3, Fig 3.4: Absence of lactose http://courses.interactiveuniversity.net/vle/scholar/session.controller?action=viewContent&co ntentGUID=27961b23-db58-80f8-1d2a-20cbe3d5fad8 View the animation: Scholar Unit 3, Fig 3.5: Presence of lactose http://courses.interactiveuniversity.net/vle/scholar/session.controller?action=viewContent&co ntentGUID=c9e09eea-6d89-b12c-aa09-2e02312a0c96

  16. Role of Genes The structure and function of all cells are determined by their genes. Each gene on a strand of DNA codes for each protein that the cell requires. To prevent resources being wasted the genes for the proteins required are switched on only when needed. Enzyme induction is the name for the process of switching on a gene, only when the enzyme that it codes for is needed. Image source: library.thinkquest.org

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