4. GENE REGULATION
1.1. Operon
Definition
Gene regulation: process that cells use to turn the
information on genes into gene products
Operon: a functioning unit of key nucleotide sequences
of DNA including an operator, a common promoter, &
one or more structural genes, which is controlled as a
unit to produce messenger RNA (mRNA), in the process
of transcription by an RNA polymerase
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5. GENE REGULATION
1.1. Operon
Introduction
How are genes regulated?
They are turned off if there is no need for the
enzymes they code for or turned on when the
environment changes and the enzymes are once
again needed
Example
E. coli in an environment without lactose does not
produce the enzymes for lactose digestion. When
lactose is present the enzymes for digestion are
produced.
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6. GENE REGULATION
1.1. Operon
Why is this off/on switch important?
Energy is not wasted. It would be similar to having
all the electrical appliances in your house on at
once. Which of course would be very wasteful. Also
unnecessary materials would lead to sluggish
functioning.
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7. GENE REGULATION
1.1. Operon
How are the genes for a particular metabolic
pathway turned on or off?
On the prokaryote chromosome a combination of
genes and regulatory DNA sequences known as the
operon accomplishes this.
An example of an operon discovered by Jacob and
Monod in E. coli
The lac operon which is off if no lactose is present
but can be induced to turn on in the presence of
lactose.
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8. GENE REGULATION
1.2 Lac Operon
Lactose Operon
Francois Jacob & Jacques Monod demonstrated how
some genes in the E. coli are regulated at the
biochemical level.
The E. coli contains a set of genes that encodes for three
proteins that the bacteria use to obtain energy from the
sugar lactose.
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9. GENE REGULATION
1.2 Lac Operon
Repressor/Regulator Gene - Produces a repressor
protein that fits in the operator to turn the operon
off
Promoter - RNA polymerase attaches here to begin
transcription of the genes
Operator - The active repressor fits in notches to
block RNA polymerase and turn off transcription.
Structural Genes - Metabolic pathway genes with
code for enzymes to digest lactose
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16. GENE REGULATION
1.2 Lac Operon
Three enzymes produced
β-galactosidase: catalyst the hydrolysis
process of lactose into glucose & galactose
Lactose permease: absorption of lactose by
bacteria
Transacetylase: function not clear
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19. GENE REGULATION
1.3 Trypthon Operon
Trp Operon
Trp operon – gene activity is repressed when a
tryptophan is added unlike Lac operon which is
induced when lactose is added
5 structural genes (trp A, trp B, trp C, trp D & trp E)
The presence of tryptophan in the cell shut down the
operon
When Trp is present, it binds to a site on the Trp
repressor & enables the Trp repressor to bind to the
operator
When Trp is not present, the repressor leaves its
operator, & transcription of the 5 structural genes
begins
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23. GENE REGULATION
1.3 Trypthon Lac Operon
Repression
The repressor for the trp operon is produced upstream
by the trp R gene, which is continually expressed
When tryptophan is present, it binds to the tryptophan
repressor tetramers, & causes a change in
conformation, which allows the repressor to bind the
operator, which prevents RNA polymerase from binding
or transcribing the operon, so tryptophan is not
produced
When tryptophan is not present, the repressor cannot
bind the operator, so transcription can occur.
Therefore, this is called negative feedback
mechanism
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