Transcript Chapter 11: Regulation of Gene Expression

Chapter 11
How Genes Are Controlled
PowerPoint Lectures for
Biology: Concepts & Connections, Sixth Edition
Campbell, Reece, Taylor, Simon, and Dickey
Lecture by Mary C. Colavito
Copyright © 2009 Pearson Education, Inc.
Introduction: Cloning to the Rescue?
 Cloning has been attempted to save endangered
species
– A clone is produced by asexual reproduction and is
genetically identical to its parent
– Dolly the sheep was the first cloned mammal
– Endangered animals that were cloned include cows,
oxen, sheep, wildcats, and wolves
 Disadvantages of cloning
– Does not increase genetic diversity
– Cloned animals may have health problems related to
abnormal gene regulation
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CONTROL OF GENE
EXPRESSION
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11.1 Proteins interacting with DNA turn
prokaryotic genes on or off in response to
environmental changes
 Gene expression is the overall process of
information flow from genes to proteins
– Mainly controlled at the level of transcription
– A gene that is “turned on” is being transcribed to
produce mRNA that is translated to make its
corresponding protein
– Organisms respond to environmental changes by
controlling gene expression
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11.1 Proteins interacting with DNA turn
prokaryotic genes on or off in response to
environmental changes
 An operon is a group of genes under coordinated
control in bacteria
 The lactose (lac) operon includes
– Three adjacent genes for lactose-utilization enzymes
– Promoter sequence where RNA polymerase binds
– Operator sequence is where a repressor can bind
and block RNA polymerase action
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11.1 Proteins interacting with DNA turn
prokaryotic genes on or off in response to
environmental changes
 Regulation of the lac operon
– Regulatory gene codes for a repressor protein
– In the absence of lactose, the repressor binds to the
operator and prevents RNA polymerase action
– Lactose inactivates the repressor, so the operator is
unblocked
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11.1 Proteins interacting with DNA turn
prokaryotic genes on or off in response to
environmental changes
 Types of operon control
– Inducible operon (lac operon)
– Active repressor binds to the operator
– Inducer (lactose) binds to and inactivates the repressor
– Repressible operon (trp operon)
– Repressor is initially inactive
– Corepressor (tryptophan) binds to the repressor and
makes it active
– For many operons, activators enhance RNA
polymerase binding to the promoter
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OPERON
Regulatory Promoter Operator
gene
Lactose-utilization genes
DNA
mRNA
Protein
Active
repressor
Operon turned off (lactose absent)
RNA polymerase
cannot attach to
promoter
DNA
mRNA
RNA polymerase
bound to promoter
Protein
Lactose
Inactive
repressor
Enzymes for lactose utilization
Operon turned on (lactose inactivates repressor)
Promoter Operator
Gene
DNA
Active
repressor
Active
repressor
Tryptophan
Inactive
repressor
Inactive
repressor
Lactose
lac operon
trp operon
11.2 Differentiation results from the expression
of different combinations of genes
 Differentiation involves cell specialization, in
both structure and function
 Differentiation is controlled by turning specific
sets of genes on or off
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Muscle cell
Pancreas cells
Blood cells
11.3 DNA packing in eukaryotic chromosomes
helps regulate gene expression
 Eukaryotic chromosomes undergo multiple levels
of folding and coiling, called DNA packing
– Nucleosomes are formed when DNA is wrapped
around histone proteins
– “Beads on a string” appearance
– Each bead includes DNA plus 8 histone molecules
– String is the linker DNA that connects nucleosomes
– Tight helical fiber is a coiling of the nucleosome
string
– Supercoil is a coiling of the tight helical fiber
– Metaphase chromosome represents the highest level
of packing
 DNA packing can prevent transcription
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11.3 DNA packing in eukaryotic chromosomes
helps regulate gene expression
Animation: DNA Packing
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Metaphase
chromosome
Tight helical fiber
(30-nm diameter)
DNA double helix
(2-nm diameter)
Linker
“Beads on
a string”
Nucleosome
(10-nm
diameter)
Histones
Supercoil
(300-nm diameter)
700 nm
11.4 In female mammals, one X chromosome is
inactive in each somatic cell
 X-chromosome inactivation
– In female mammals, one of the two X chromosomes
is highly compacted and transcriptionally inactive
– Random inactivation of either the maternal or
paternal chromosome
– Occurs early in embryonic development and all
cellular descendants have the same inactivated
chromosome
– Inactivated X chromosome is called a Barr body
– Tortoiseshell fur coloration is due to inactivation of X
chromosomes in heterozygous female cats
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Early embryo
Two cell populations
in adult
Cell division
and random
X chromosome
inactivation
X chromosomes
Allele for
orange fur
Allele for
black fur
Active X
Inactive X
Orange
fur
Inactive X
Active X
Black fur
11.5 Complex assemblies of proteins control
eukaryotic transcription
 Eukaryotic genes
– Each gene has its own promoter and terminator
– Are usually switched off and require activators to be
turned on
– Are controlled by interactions between numerous
regulatory proteins and control sequences
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11.5 Complex assemblies of proteins control
eukaryotic transcription
 Regulatory proteins that bind to control
sequences
– Transcription factors promote RNA polymerase
binding to the promoter
– Activator proteins bind to DNA enhancers and
interact with other transcription factors
– Silencers are repressors that inhibit transcription
 Control sequences
– Promoter
– Enhancer
– Related genes located on different chromosomes can be
controlled by similar enhancer sequences
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11.5 Complex assemblies of proteins control
eukaryotic transcription
Animation: Initiation of Transcription
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Enhancers
Promoter
Gene
DNA
Activator
proteins
Transcription
factors
Other
proteins
RNA polymerase
Bending
of DNA
Transcription
11.6 Eukaryotic RNA may be spliced in more
than one way
 Alternative RNA splicing
– Production of different mRNAs from the same
transcript
– Results in production of more than one polypeptide
from the same gene
– Can involve removal of an exon with the introns on
either side
Animation: RNA Processing
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Exons
1
DNA
RNA
transcript
1
1
2 3
5
4
3
2
RNA splicing
mRNA
4
3
2
5
or
5
1
2 4
5
11.7 Small RNAs play multiple roles in
controlling gene expression
 RNA interference (RNAi)
– Prevents expression of a gene by interfering with
translation of its RNA product
– Involves binding of small, complementary RNAs to
mRNA molecules
– Leads to degradation of mRNA or inhibition of
translation
 MicroRNA
– Single-stranded chain about 20 nucleotides long
– Binds to protein complex
– MicroRNA + protein complex binds to complementary
mRNA to interfere with protein production
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11.7 Small RNAs play multiple roles in
controlling gene expression
Such as RNAi or RNAsi molecules
Animation: Blocking Translation
Animation: mRNA Degradation
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Protein
miRNA
1
miRNAprotein
complex
2
Target mRNA
3
mRNA degraded
4
OR Translation blocked
11.8 Translation and later stages of gene
expression are also subject to regulation
 Control of gene expression also occurs with
– Breakdown of mRNA
– Initiation of translation
– Protein activation
– Protein breakdown
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Folding of
polypeptide and
formation of
S—S linkages
Initial polypeptide
(inactive)
Cleavage
Folded polypeptide
(inactive)
Active form
of insulin
11.9 Review: Multiple mechanisms regulate gene
expression in eukaryotes
 Many possible control points exist; a given gene
may be subject to only a few of these
– Chromosome changes (1)
– DNA unpacking
– Control of transcription (2)
– Regulatory proteins and control sequences
– Control of RNA processing
– Addition of 5’ cap and 3’ poly-A tail (3)
– Splicing (4)
– Flow through nuclear envelope (5)
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11.9 Review: Multiple mechanisms regulate gene
expression in eukaryotes
 Many possible control points exist; a given gene
may be subject to only a few of these
– Breakdown of mRNA (6)
– Control of translation (7)
– Control after translation
– Cleavage/modification/activation of proteins (8)
– Breakdown of protein (9)
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11.9 Review: Multiple mechanisms regulate gene
expression in eukaryotes
 Applying Your Knowledge
For each of the following, determine whether an
increase or decrease in the amount of gene
product is expected
– The mRNA fails to receive a poly-A tail during
processing in the nucleus
– The mRNA becomes more stable and lasts twice as
long in the cell cytoplasm
– The region of the chromatin containing the gene
becomes tightly compacted
– An enzyme required to cleave and activate the
protein product is missing
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11.9 Review: Multiple mechanisms regulate gene
expression in eukaryotes
These mechanisms may occur
During transcription
After transcription
Before translation
During translation or
After translation
Animation: Protein Degradation
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NUCLEUS
Chromosome
DNA unpacking
Other changes to DNA
Gene
Gene
Transcription
Exon
RNA transcript
Intron
Addition of cap and tail
Splicing
Tail
mRNA in nucleus
Cap
Flow through
nuclear envelope
mRNA in cytoplasm
CYTOPLASM
Breakdown of mRNA
Translation
Brokendown
mRNA
Polypeptide
Cleavage / modification /
activation
Active protein
Breakdown
of protein
Brokendown
protein
11.10 Cascades of gene expression direct the
development of an animal
 Role of gene expression in fruit fly development
– Orientation from head to tail
– Maternal mRNAs present in the egg are translated and
influence formation of head to tail axis
– Segmentation of the body
– Protein products from one set of genes activate other sets
of genes to divide the body into segments
– Production of adult features
– Homeotic genes are master control genes that
determine the anatomy of the body, specifying structures
that will develop in each segment
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11.10 Cascades of gene expression direct the
development of an animal
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Eye
Antenna
Leg
Head of a normal fruit fly
Head of a developmental mutant
Egg cell
Egg cell
within ovarian
follicle
Protein
signal
Follicle cells
1
Gene expression
“Head”
mRNA
2
Embryo
3
Cascades of
gene expression
Body
segments
Gene expression
Adult fly
4
11.12 Signal transduction pathways convert
messages received at the cell surface to
responses within the cell
 Signal transduction pathway is a series of
molecular changes that converts a signal at the
cell’s surface to a response within the cell
– Signal molecule is released by a signaling cell
– Signal molecule binds to a receptor on the surface of
a target cell
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11.12 Signal transduction pathways convert
messages received at the cell surface to
responses within the cell
– Relay proteins are activated in a series of reactions
– A transcription factor is activated and enters the
nucleus
– Specific genes are transcribed to initiate a cellular
response
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Signaling cell
Signaling
molecule
Plasma
Receptor membrane
protein
1
2
3
Target cell
Relay
proteins
Transcription
factor
(activated)
4
Nucleus
DNA
5
mRNA Transcription
New
protein
6
Translation
CLONING OF PLANTS AND
ANIMALS
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11.14 Plant cloning shows that differentiated cells
may retain all of their genetic potential
 Most differentiated cells retain a full set of genes,
even though only a subset may be expressed
– Evidence is available from
– Plant cloning
– A root cell can divide to form an adult plant
– Animal limb regeneration
– Remaining cells divide to form replacement
structures
– Involved dedifferentiation followed by redifferentiation
into specialized cells
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Root of
carrot plant
Single
cell
Root cells cultured Cell division
in culture
in nutrient medium
Plantlet
Adult plant
11.15 Nuclear transplantation can be used to
clone animals
 Nuclear transplantation
– Replacing the nucleus of an egg cell or zygote with a
nucleus from an adult somatic cell
– Early embryo (blastocyst) can be used in
– Reproductive cloning
– Implant embryo in surrogate mother for
development
– New animal is genetically identical to nuclear donor
– Therapeutic cloning
– Remove embryonic stem cells and grow in
culture for medical treatments
– Induce stem cells to differentiate
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Donor
cell
Nucleus from
donor cell
Reproductive
cloning
Implant blastocyst in
surrogate mother
Remove
nucleus
from egg
cell
Add somatic cell
from adult donor
Grow in culture
to produce an Therapeutic
early embryo cloning
(blastocyst)
Remove embryonic
stem cells from
blastocyst and
grow in culture
Clone of
donor is born
Induce stem
cells to form
specialized cells
11.16 CONNECTION: Reproductive cloning has
valuable applications, but human
reproductive cloning raises ethical issues
 Cloned animals can show differences from their
parent due to a variety of influences during
development
 Reproductive cloning is used to produce animals
with desirable traits
– Agricultural products
– Therapeutic agents
– Restoring endangered animals
 Human reproductive cloning raises ethical
concerns
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11.17 CONNECTION: Therapeutic cloning can
produce stem cells with great medical
potential
 Stem cells can be induced to give rise to
differentiated cells
– Embryonic stem cells can differentiate into a variety
of types
– Adult stem cells can give rise to many but not all
types of cells
 Therapeutic cloning can supply cells to treat
human diseases
 Research continues into ways to use and produce
stem cells
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Blood cells
Adult stem
cells in bone
marrow
Nerve cells
Cultured
embryonic
stem cells
Heart muscle cells
Different culture
conditions
Different types of
differentiated cells
THE GENETIC BASIS
OF CANCER
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11.18 Cancer results from mutations in genes
that control cell division
 Mutations in two types of genes can cause cancer
– Oncogenes
– Proto-oncogenes normally promote cell division
– Mutations to oncogenes enhance activity
– Tumor-suppressor genes
– Normally inhibit cell division
– Mutations inactivate the genes and allow uncontrolled
division to occur
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11.18 Cancer results from mutations in genes
that control cell division
 Oncogenes
– Promote cancer when present in a single copy
– Can be viral genes inserted into host chromosomes
– Can be mutated versions of proto-oncogenes, normal
genes that promote cell division and differentiation
– Converting a proto-oncogene to an oncogene can
occur by
– Mutation causing increased protein activity
– Increased number of gene copies causing more protein to
be produced
– Change in location putting the gene under control of new
promoter for increased transcription
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11.18 Cancer results from mutations in genes
that control cell division
 Tumor-suppressor genes
– Promote cancer when both copies are mutated
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Proto-oncogene DNA
Mutation within
the gene
Multiple copies
of the gene
New promoter
Oncogene
Hyperactive
growthstimulating
protein in
normal
amount
Gene moved to
new DNA locus,
under new controls
Normal growthstimulating
protein
in excess
Normal growthstimulating
protein
in excess
Tumor-suppressor gene
Mutated tumor-suppressor gene
Normal
growthinhibiting
protein
Defective,
nonfunctioning
protein
Cell division
under control
Cell division not
under control
11.19 Multiple genetic changes underlie the
development of cancer
 Four or more somatic mutations are usually
required to produce a cancer cell
 One possible scenario for colorectal cancer
includes
– Activation of an oncogene increases cell division
– Inactivation of tumor suppressor gene causes
formation of a benign tumor
– Additional mutations lead to a malignant tumor
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Colon wall
1
2
Cellular Increased
changes: cell division
Growth of polyp
DNA
Oncogene
changes: activated
Tumor-suppressor
gene inactivated
3
Growth of malignant
tumor (carcinoma)
Second tumorsuppressor gene
inactivated
Chromosomes
Normal
cell
1
mutation
2
mutations
3
mutations
4
mutations
Malignant
cell
11.20 Faulty proteins can interfere with normal
signal transduction pathways
 Path producing a product that stimulates cell
division
 Product of ras proto-oncogene relays a signal when
growth hormone binds to receptor
 Product of ras oncogene relays the signal in the
absence of hormone binding, leading to uncontrolled
growth
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11.20 Faulty proteins can interfere with normal
signal transduction pathways
 Path producing a product that inhibits cell division
– Product of p53 tumor-suppressor gene is a
transcription factor
– p53 transcription factor normally activates genes for
factors that stop cell division
– In the absence of functional p53, cell division
continues because the inhibitory protein is not
produced
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Growth factor
Receptor
Target cell
Hyperactive
relay protein
(product of
ras oncogene)
issues signals
on its own
Normal product
of ras gene
Relay
proteins
Transcription
factor
(activated)
DNA
Nucleus
Protein that
Stimulates
cell division
Transcription
Translation
Growth-inhibiting
factor
Receptor
Relay
proteins
Transcription
factor
(activated)
Nonfunctional transcription
factor (product of faulty p53
tumor-suppressor gene)
cannot trigger
transcription
Normal product
of p53 gene
Transcription
Protein that
inhibits
cell division
Translation
Protein absent
(cell division
not inhibited)
11.21 CONNECTION: Lifestyle choices can
reduce the risk of cancer
 Carcinogens are cancer-causing agents that
damage DNA and promote cell division
– X-rays and ultraviolet radiation
– Tobacco
 Healthy lifestyle choices
– Avoiding carcinogens
– Avoiding fat and including foods with fiber and
antioxidants
– Regular medical checkups
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A typical operon
Regulatory
Promoter Operator
gene
Gene 1
DNA
Encodes repressor
that in active form
attaches to operator
Gene 2
Gene 3
Switches operon
RNA
polymerase on or off
binding site
Nucleus from
donor cell
Early embryo
resulting from
nuclear transplantation
Surrogate
mother
Clone
of donor
Early embryo
Nucleus from resulting from
donor cell
nuclear transplantation
Embryonic
stem cells
in culture
Specialized
cells
prokaryotic
genes often
grouped into
Gene
regulation
is a normal gene that
can be mutated to an
in eukaryotes
may involve
operons
(a)
when
abnormal
may lead to
oncogene
controlled by
protein called
can cause
are
switched
on/off by
(b)
(c)
in active
form binds to
(d)
(e)
(f)
(g)
occurs in
are proteins
that promote
can produce
female
mammals
transcription
multiple mRNAs
per gene
You should now be able to
1. Explain how prokaryotic gene control occurs in
the operon
2. Describe the control points in expression of a
eukaryotic gene
3. Describe DNA packing and explain how it is
related to gene expression
4. Explain how alternative RNA splicing and
microRNAs affect gene expression
5. Compare and contrast the control mechanisms
for prokaryotic and eukaryotic genes
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You should now be able to
6. Distinguish between terms in the following
groups: promoter—operator; oncogene—tumor
suppressor gene; reproductive cloning—
therapeutic cloning
7. Define the following terms: Barr body,
carcinogen, DNA microarray, homeotic gene;
stem cell; X-chromosome inactivation
8. Describe the process of signal transduction,
explain how it relates to yeast mating, and
explain how it is disrupted in cancer development
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You should now be able to
9. Explain how cascades of gene expression affect
development
10. Compare and contrast techniques of plant and
animal cloning
11. Describe the types of mutations that can lead to
cancer
12. Identify lifestyle choices that can reduce cancer
risk
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