This document provides an overview of gene regulation in prokaryotes. It discusses operons, which are units of DNA that control the transcription of a set of genes. It specifically examines the lac and tryptophan operons in E. coli. The lac operon codes for enzymes involved in lactose digestion and is induced by the presence of lactose. The tryptophan operon codes for enzymes in the tryptophan synthesis pathway and is repressed by the presence of tryptophan. Both operons use a repressor protein that binds to the operator region to control transcription in response to environmental conditions.

1. GENERAL BIOLOGY
HDL 121
GENE REGULATION
PREPARED BY:MANEGA
SCHOOL OF MLT
FACULTY OF HEALTH SCIENCE

2. GENE REGULATION
Learning Outcomes
After completing this lecture, students will be able to:
(a) Define gene regulation & operon
(b) Describe lac operon – operator, promoter &
terminator
(c) Explain tryptophan operon
Topic
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3. GENE REGULATION
Topic Outlines
1.1. Operon
1.2. Lac Operon
1.3 Tryptophan Operon
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© 2010 Cosmopoint

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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10. GENE REGULATION
1.2 Lac Operon
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11. GENE REGULATION
1.2 Lac Operon
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12. GENE REGULATION
1.2 Lac Operon
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13. GENE REGULATION
1.2 Lac Operon
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14. GENE REGULATION
1.2 Lac Operon
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15. GENE REGULATION
1.2 Lac Operon
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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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17. GENE REGULATION
1.2 Lac Operon
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18. GENE REGULATION
1.2 Lac Operon
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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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20. GENE REGULATION
1.3 Trypthon Operon
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21. GENE REGULATION
1.3 Trypthon Operon
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22. GENE REGULATION
1.3 Trypthon Operon
Enzyme: Tryptophan synthethase
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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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