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Gene Regulation
Chapter 14
Learning Objective 1
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Why do bacterial and eukaryotic cells have
different mechanisms of gene regulation?
Prokaryotes
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Bacterial cells
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grow rapidly
have a short life span
Transcriptional-level control
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usually regulates gene expression
Eukaryotic Cells
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Have long life span
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One gene
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respond to many different stimuli
may be regulated in different ways
Transcriptional-level control
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and control at other levels of gene expression
KEY CONCEPTS
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Cells can synthesize thousands of proteins
•
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but not all proteins are required in all cells
Cells regulate which parts of the genome
will be expressed, and when
Learning Objective 2
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What is an operon?
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What are the functions of the operator and
promoter regions?
Operon
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A gene complex
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structural genes with related functions
controlled by closely linked DNA sequences
Regulated genes in bacteria
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are organized into operons
Promoter Region
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Each operon has a promoter region
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upstream from protein-coding regions
where RNA polymerase binds to DNA before
transcription
Operator (1)
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Regulatory switch for transcriptional-level
control of operon
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Repressor protein
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binds to operator sequence
prevents transcription
Operator (2)
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RNA polymerase
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bound to promoter
is blocked from transcribing structural genes
If repressor is not bound to operator
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transcription proceeds
Learning Objective 3
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What is the difference between inducible,
repressible, and constitutive genes?
Inducible Genes (1)
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An inducible operon
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such as lac operon
is normally turned off
Repressor protein
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is synthesized in active form
binds to operator
Inducible Genes (2)
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If lactose is present
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is converted to allolactose (inducer)
binds to repressor protein
changes repressor’s shape
Altered repressor
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cannot bind to operator
operon is transcribed
The lac Operon
lac operon
Repressor
gene
Promoter Operator
lac Z
lac Y lac A
DNA
Repressor
protein
Transcription
mRNA
Ribosome
Translation
Fig. 14-2a, p. 307
Repressor
gene
mRNA
lac operon
Promoter Operator
lac Z lac Y
RNA
polymerase
lac A
Transcription
mRNA
Translation
Inducer
(allolactose)
Repressor
protein
(inactive)
Transacetylase
Lactose
permease
β-galactosidase
Enzymes for lactose metabolism
Fig. 14-2b, p. 307
Repressible Genes (1)
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A repressible operon (trp operon)
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Repressor protein
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is normally turned on
is synthesized in inactive form
cannot bind to operator
A metabolite (metabolic end product)
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acts as corepressor
Repressible Genes (2)
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With high intracellular corepressor levels
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corepressor molecule binds to repressor
changes repressor’s shape
Altered repressor
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binds to operator
turns off transcription of operon
The trp
Operon
Repressor
gene
trp operon
Promoter Operator trp E trp D trp C trp B trp A
DNA
mRNA
RNA
polymerase
Transcription
mRNA
Translation
Repressor
protein
(inactive)
Enzymes of the tryptophan
biosynthetic pathway
Tryptophan
(a) Intracellular tryptophan levels low.
Fig. 14-4a, p. 310
Repressor
gene
trp operon
Promoter Operator trp E
DNA
trp D trp C trp B trp A
Active repressor –
corepressor complex
mRNA
Inactive repressor
protein
Tryptophan
(corepressor)
(b) Intracellular tryptophan levels high.
Fig. 14-4b, p. 310
Constitutive Genes (1)
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Are neither inducible nor repressible
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active at all times
Regulatory proteins
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produced constitutively
• catabolite activator protein (CAP)
• repressor proteins
Constitutive Genes (2)
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Regulatory proteins
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recognize and bind to specific base
sequences in DNA
Activity of constitutive genes
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controlled by binding RNA polymerase to
promoter regions
Learning Objective 4
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What is the difference between positive
and negative control?
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How do both types of control operate in
regulating the lac operon?
Negative Control
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Repressible and inducible operons are
under negative control
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When repressor protein binds to operator
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transcription of operon is turned off
Positive Control (1)
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Some inducible operons are under positive
control
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Activator protein binds to DNA
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stimulates transcription of gene
Positive Control (2)
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CAP activates lac operon
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To bind to lac operon
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binds to promoter region
stimulates transcription by tightly binding RNA
polymerase
CAP requires cyclic AMP (cAMP)
cAMP levels increase
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as glucose levels decrease
Positive Control
Promoter
Repressor
gene
CAPRNA
binding polymerase –
site
binding site Operator lac Z lac Y lac A
DNA
mRNA
RNA polymerase
binds poorly
CAP
(inactive)
Allolactose
Repressor
protein (inactive)
(a) Lactose high, glucose high, cAMP low.
Fig. 14-5a, p. 311
Promoter
Repressor
gene
CAPRNA
binding polymerase –
site
binding site Operator lac Z lac Y lac A
DNA
RNA
Transcription
polymerase
binds efficiently
mRNA
CAP
mRNA
Translation
cAMP
Allolactose
Repressor
protein (inactive)
(b) Lactose high, glucose low, cAMP high.
Galactoside
transacetylase
Lactose permease
β -galactosidase
Enzymes for lactose metabolism
Fig. 14-5b, p. 311
Binding CAP
DNA
cAMP
CAP dimer
Fig. 14-6, p. 312
Learning Objective 5
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What are the types of posttranscriptional
control in bacteria?
Posttranscriptional Controls
in Bacteria
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Translational control
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regulates translation rate of particular mRNA
Posttranslational controls
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include feedback inhibition of key enzymes in
metabolic pathways
KEY CONCEPTS
•
Prokaryotes regulate gene expression in
response to environmental stimuli
KEY CONCEPTS
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Gene regulation in prokaryotes occurs
primarily at the transcription level
Learning Objective 6
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Discuss the structure of a typical
eukaryotic gene and the DNA sequences
involved in regulating that gene
Eukaryotic Genes
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Are not normally organized into operons
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Regulation occurs at levels of
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Transcription
mRNA processing
Translation
Modifications of protein product
Transcription
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Requires
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Transcription initiation site
• where transcription begins
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Promoter
• to which RNA polymerase binds
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In multicellular eukaryotes
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RNA polymerase binds to promoter (TATA
box)
Transcription
TATA box
UPE
T T
TATA A
A A
Transcription
initiation
site
pre-mRNA
(a) Eukaryotic promoter elements.
Fig. 14-9a, p. 316
TATA box
UPE
T T
TATA A
A A
Transcription
initiation
site
pre-mRNA
(b) A weak eukaryotic promoter.
Fig. 14-9b, p. 316
UPE
UPE
UPE
UPE
TATA box
T T
TATA A
A A
Transcription
initiation
site
pre-mRNA
(c) A strong eukaryotic promoter.
Fig. 14-9c, p. 316
Enhancer
UPE
UPE
TATA box
T T
TATA A
A A
Transcription
initiation
site
pre-mRNA
(d) A strong eukaryotic promoter plus an enhancer.
Fig. 14-9d, p. 316
Regulated Eukaryotic Gene
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Promoter
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RNA polymerase-binding site
short DNA sequences (upstream promoter
elements (UPEs) or proximal control elements)
UPEs
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number and types within promoter region
determine efficiency of promoter
Enhancers (1)
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Located far away from promoter
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control some eukaryotic genes
Help form active transcription initiation
complex
Enhancers (2)
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Specific regulatory proteins
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bind to enhancer elements
activate transcription by interacting with
proteins bound to promoters
Enhancers
Enhancer
Target
proteins
RNA
polymerase
TATA box
DNA
(a) Little or no transcription.
Fig. 14-11a, p. 317
Enhancer
DNA
TATA box
Activator
(transcription factor)
(b) High rate of transcription.
Fig. 14-11b, p. 317
Learning Objective 7
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In what ways may eukaryotic DNA-binding
proteins bind to DNA?
Transcription Factors
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DNA-binding protein regulators control
eukaryotic genes
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some transcriptional activators
some transcriptional repressors
Transcription Factors
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Each has DNA-binding domain
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3 types of regulatory proteins
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Helix-turn-helix
Zinc fingers
Leucine zippers
Helix-Turn-Helix
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Inserts one helix into DNA
Turn
α -helix
DNA
(a) Helix-turn-helix.
Fig. 14-10a, p. 317
Zinc Fingers
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Loops of amino acids
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held together by zinc ions
each loop has α-helix that fits into DNA
COO–
Finger 2
Finger 3
Zinc
ion
Finger 1
NH3+
DNA
(b) Zinc fingers.
Fig. 14-10b, p. 317
Leucine Zipper Proteins
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Associate as dimers that insert into DNA
Leucine
zipper
region
DNA
(c) Leucine zipper.
Fig. 14-10c, p. 317
Learning Objective 8
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How may a change in chromosome
structure affect the activity of a gene?
Gene Activity (1)
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Changes in chromosome structure
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inactivates genes
Heterochromatin
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densely packed regions of chromosomes
contain inactive genes
Gene Activity (2)
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Active genes
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associated with loosely packed chromatin
structure (euchromatin)
Cells change chromatin structure
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from heterochromatin to euchromatin
by chemically modifying histones (proteins
associated with DNA to form nucleosomes)
Chromatin Structure
Heterochromatin: genes
silent
Chromatin
decondensation
Nucleosome
Histones
DNA
Transcribed
region
Euchromatin: genes active
Fig. 14-7, p. 314
Gene Activity (3)
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Histone tail
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string of amino acids that extends from the
DNA-wrapped nucleosome
Methyl groups, acetyl groups, sugars, and
proteins
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may chemically attach to the histone tail
may expose or hide genes (turn on or off)
Gene Activity (4)
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Epigenetic inheritance
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changes how a gene is expressed
important mechanism of gene regulation
DNA methylation
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perpetuates gene inactivation
patterns repeat in successive cell generations
mechanism for epigenetic inheritance
Gene Amplification
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Some genes
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products are required in large amounts
have multiple copies in the chromosome
Gene amplification
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some cells selectively amplify genes by DNA
replication
Gene Amplification
Drosophila chorion gene
Gene amplification by
repeated DNA replication
of chorion gene region
Chorion gene in ovarian cell
Fig. 14-8, p. 315
Learning Objective 9
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How may a gene in a multicellular
organism produce different products in
different types of cells?
Differential mRNA Processing
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Single gene produces different forms of
protein in different tissues
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depending on how pre-mRNA is spliced
Gene contains a segment that can be
either intron or exon
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as intron, sequence is removed
as exon, sequence is retained
Differential mRNA Processing
Potential splice sites
Exon
Intron
Exon
or intron
Exon
pre-mRNA
Differential
mRNA processing
Exon
Exon
Exon
Functional mRNA in
tissue A
Exon
Exon
Functional mRNA in
tissue B
Fig. 14-12, p. 318
Learning Objective 10
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What types of regulatory controls operate
in eukaryotes after mature mRNA is
formed?
mRNA Stability
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Certain regulatory mechanisms increase
RNA stability
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allowing more protein synthesis before mRNA
degradation
Sometimes under hormonal control
Posttranslational Control (1)
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In eukaryotic gene expression
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feedback inhibition
modification of protein structure
Protein function change
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by kinases adding phosphate groups
by phosphatases removing phosphates
Protein Degradation (1)
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Proteins targeted for destruction
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covalently bonded to ubiquitin
Protein tagged by ubiquitin
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degraded in a proteasome
Protein Degradation (2)
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Proteasome
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large macromolecular structure
recognizes ubiquitin tags
Proteases
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protein-degrading enzymes
associated with proteasomes
degrade protein into peptide fragments
Protein
Degradation
Target protein
Ubiquitin
1
Ubiquitin molecules
attach to protein targeted for degradation.
2
Protein enters
proteasome.
Ubiquitinylated
protein
Proteasome
3 Ubiquitins are
released and
available for
reuse. Protein
is degraded
into peptide
fragments.
Peptide
fragments
Fig. 14-13, p. 318
Target protein
Ubiquitin
1 Ubiquitin molecules
attach to protein targeted for degradation.
Ubiquitinylated
protein
2
Protein enters
proteasome.
Proteasome
3
Ubiquitins are
released and
available for
reuse. Protein
is degraded
into peptide
fragments.
Peptide
fragments
Stepped Art
Fig. 14-13, p. 318
KEY CONCEPTS
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Gene regulation in eukaryotes occurs at
the levels of transcription,
posttranscription, translation, and
posttranslation
Animation: Controls of
Eukaryotic Gene Expression
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