Chapter 15
Gene Regulation
Prokaryotic Regulation:
Gene Regulation
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 Bacteria do not need the same enzymes
and other proteins all of the time.
- They need only:
1. The enzymes required to break
down the nutrients available to
them or
2. The enzymes required to
synthesize whatever metabolites
are absent under the present
circumstances.
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Gene Regulation
Prokaryotic Regulation:
The Operon Model (Jacob & Monod 1961)
An operon consists of three components:
1. Promoter
-DNA sequence where RNA polymerase
first attaches
-Short segment of DNA
2. Operator
-DNA sequence where active repressor
binds
-Short segment of DNA
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Gene Regulation
Prokaryotic Regulation:
The Operon Model (Jacob & Monod 1961)
3. Structural Genes
-One to several genes coding for
enzymes of a metabolic pathway
-Translated simultaneously as a block
-Long segment of DNA
A regulator gene is located outside of the
operon. It codes for a repressor that
controls whether the operon is active or
not.
Gene Regulation
Repressible Operons:
The trp Operon - Normally turned ON
If tryptophan (an amino acid) is ABSENT:
Repressor is unable to attach to the
operator (expression is normally “on”)
RNA polymerase binds to the promoter
Transcription & translation occur
Enzymes for synthesis of tryptophan are
produced
Tryptophan will be produced by E. coli
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The trp Operon
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Gene Regulation
Repressible Operons:
The trp Operon - Genes repressed
If tryptophan IS present enzymes are not
needed and following occurs:
Tryptophan combines with repressor,
causing it to change shape, thus acting
as a corepressor
Repressor becomes functional
Blocks transcription & synthesis of
enzymes and tryptophan is NOT
produced
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The trp Operon
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Summary of repressible trp operon
Operon usually ON, must be turned OFF
Repressor
Transcription
bound ?
occurs?
NO
------->
YES
YES ------->
NO
*** Corepressors are frequently the
products in the pathway. In this
case, tryptophan is the corepressor.
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Gene Regulation 10
Inducible Operons:
The lac Operon - Normally turned OFF
When E. coli is denied glucose & is given
lactose instead, it immediately begins to
make three enzymes needed for the
metabolism of lactose.
These enzymes are encoded by three
structural genes which are adjacent to one
another on the chromosome. They are
controlled by one regulator gene that
codes for a one repressor.
Gene Regulation 11
Inducible Operons:
The lac Operon - Normal OFF state
If lactose (a sugar that can be used for
food) is absent:
Repressor attaches to the operator
RNA polymerase cannot bind to promoter
Transcription of structural genes is
blocked
Enzymes needed to digest lactose NOT
made
The lac Operon
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Gene Regulation 13
Inducible Operons:
The lac Operon - Induced state
If lactose IS present:
It combines with repressor and renders it
unable to bind to operator by causing
shape of repressor to change
RNA polymerase binds to the promoter
Transcription of genes occurs
The three enzymes necessary for lactose
catabolism are produced
Lactose will be digested by enzymes
The lac Operon
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Summary of inducible lac operon
Operon usually OFF, must be turned ON
Repressor
Transcription
bound ?
occurs?
YES
-------> NO
NO
------->
YES
*** Inducers are frequently the
reactants in the pathway. In this
case, the lactose is the inducer.
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Gene Regulation
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The lac Operon - Further control
E. coli preferentially break down glucose.
Thus, they have a way to ensure that the
lac operon is only turned on maximally
when glucose is absent.
This involves use of cyclic AMP which is
abundant when glucose is absent.
- Cyclic AMP binds to a molecule called
catabolite activator protein (CAP).
Gene Regulation
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The lac Operon - Further control (2)
The cAMP-CAP complex then binds to a
CAP binding site next to the lac operon
promoter.
• When CAP binds to DNA, the DNA
bends.
- This exposes the promoter to RNA
polymerase which is now better able
to bind to the promoter.
Gene Regulation
The lac Operon - Further control (2)
When glucose IS present:
 There is little cAMP in the cell
- CAP is not activated by cAMP
- lac operon does NOT function
maximally and cell will preferentially
use glucose as its food source.
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Action of CAP
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Gene Regulation
Animations for the Operons
Trp Operon
http://highered.mcgraw-hill.com/olc/dl/120080/bio26.swf
lac Operon
http://highered.mcgraw-hill.com/olc/dl/120080/bio27.swf
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Gene Regulation
Eukaryotic Regulation
A variety of mechanisms to control gene
expression:
Five primary levels of control:
Nuclear levels
- Chromatin Packing
- Transcriptional Control
- Posttranscriptional Control
Cytoplasmic levels
- Translational Control
- Posttranslational Control
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Regulation of Gene Expression:
Levels of Control in Eukaryotes
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Gene Regulation
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Chromatin Structure
Eukaryotic DNA associated with histone proteins
Together make up chromatin
As seen in the interphase nucleus
Nucleosomes:
DNA wound around balls of eight molecules of
histone proteins
Looks like beads on a string
Each bead a nucleosome
The levels of chromatin packing determined by
degree of nucleosome coiling
Levels of Chromatin Structure
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Gene Regulation
Chromatin Packing
Euchromatin
Loosely coiled DNA
Appears lightly stained in micrographs
Transcriptionally active - capable of
being transcribed
Heterochromatin
Tightly packed DNA
Appears darkly stained in micrographs
Transcriptionally inactive
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Gene Regulation
Chromatin Packing
Barr Bodies
Females have two X chromosomes, but
only one is active
Other is tightly packed along its entire
length
Inactive X chromosome is called a Barr
body
Inactive X chromosome does not
produce gene products
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X-Inactivation in Mammalian Females
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Gene Regulation
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Transcriptional Control
Transcription controlled by DNA-binding proteins
called transcription factors
Bind to a promoter adjacent to a gene
Transcription activators bind to regions of DNA
called enhancers. Might be brought near
region of promoter by hairpin loops in DNA.
Always present in cell, but most likely have to
be activated before they will bind to DNA
Lampbrush Chromosomes
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Initiation of Transcription
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Gene Regulation
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Transcriptional Control (2)
Transposons are specific DNA sequences
that have the ability to move within and
between chromosomes.
Their movement to a new location
sometimes alters neighboring genes by
decreasing their expression
- Thus, they can act like regulator genes
- They also can be a source of mutations.
Gene Regulation
Posttranscriptional Control
Posttranscriptional control operates within
the nucleus on the primary mRNA
transcript
Given a specific primary transcript:
Excision of introns can vary
Splicing of exons can vary
Thus, differing versions of the mRNA
transcript might leave the nucleus
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Gene Regulation
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Posttranscriptional Control
Posttranscriptional control may also control
speed of mRNA transport from nucleus to
cytoplasm
Will affect the number of transcripts
arriving at rough ER
And therefore the amount of gene
product realized per unit time
Processing of mRNA Transcripts
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Gene Regulation
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Translational Control
Translational control determines degree to
which mRNA is translated into a protein
product
Presence of 5′ cap and the length of
poly-A tail on 3′ end can determine
whether translation takes place and how
long the mRNA is active
- Example: Long life of mRNA in RBCs
that code for hemoglobin attributed to
presence of 5’ cap and 3’ poly-A tail
Gene Regulation
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Posttranslational Control
Some proteins are not immediately active
after synthesis.
Some need to be activated
- Folding and breaking into chains must
occur in bovine insulin before it is
active
Some are degraded quickly
- Cyclin proteins that control cell cycle
Gene Regulation
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Animations for Eukaryotic Control
Control of gene expression in eukaryotes
http://highered.mcgraw-hill.com/olc/dl/120080/bio31.swf
Transcription Complex and Enhancers
http://highered.mcgraw-hill.com/olc/dl/120080/bio28.swf
Effect of Mutations on
Protein Activity
Gene Regulation
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A mutation is a permanent change in the
sequence of bases in DNA.
Effects on proteins can range from no
effect to complete inactivity
Germ-line mutations
-Occur in sex cells; can be passed on to
future generations
Somatic mutations
-Occur in body cells; can’t be passed on
to future generations
-Can lead to development of cancer
Effect of Mutations on
Protein Activity
Gene Regulation
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Point Mutations
Involve change in a single DNA
nucleotide
Changes one codon to a different codon
Could change one amino acid for another
Effects on protein vary:
-Drastic - completely nonfunctional
-Reduced functionality
-Unaffected
Effect of Mutations on
Protein Activity
Gene Regulation
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Frameshift Mutations
One or two nucleotides are either
inserted or deleted from DNA
Can lead to completely new codon order
Protein can rendered nonfunctional
-Normal :
THE CAT ATE THE RAT
-After deletion:THE ATA TET HER AT
-After insertion:
THE CCA TAT ETH ERA T
Point Mutation
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Gene Regulation
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Nonfunctional Proteins
Examples of nonfunctional proteins:
Hemophilia due to the transposon Alu
Phenylketonuria (PKU) due to faulty code
for one enzyme
Cystic fibrosis due to inheritance of faulty
code for a chloride ion channel
Androgen insensitivity due to a faulty
receptor for androgens (male sex
hormones)
Gene Regulation
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Carcinogenesis
Development of cancer involves a series of
mutations:
•Proto-oncogenes – Stimulate cell cycle but are
usually turned off. Can mutate and become
oncogenes which are turned on all the time.
•Tumor suppressor genes – inhibit cell cycle
Mutation in oncogene and tumor suppressor
gene:
-Stimulates cell cycle uncontrollably
-Leads to tumor formation
Carcinogenesis
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Gene Regulation
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Causes of Mutations
Spontaneous Errors:
Happen for no apparent reason
Example of spontaneous germ-line mutation is
achondroplasia, a type of dwarfism
Replication Errors:
-DNA polymerase proofreads new strands
-Generally corrects errors
-1 in 1,000,000,000 replications error occurs
Gene Regulation
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Causes of Mutations
Environmental Mutagens
A mutagen is an environmental agent
that increases the chances of a mutation
Carcinogens - Mutagens that increase
the chances of cancer
-Many agricultural & industry chemicals
-Many drugs
-Tobacco smoke chemicals
-Radiation (X-rays, gamma rays, UV)
Achondroplasia and
Xeroderma Pigmentosum
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