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Chapter 18
Regulation of Gene
Expression
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: Conducting the Genetic Orchestra
• Prokaryotes and eukaryotes alter gene
expression in response to their changing
environment
• In multicellular eukaryotes, gene expression
regulates development and is responsible for
differences in cell types
• RNA molecules play many roles in regulating
gene expression in eukaryotes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-1
Fruit fly wing Color coded mRNA
from 3 different Genes; blue, red,
green (yellow is when red/green
Appear together)
Concept 18.1: Bacteria often respond to
environmental change by regulating transcription
• Natural selection has favored bacteria that
produce only the products needed by that cell
• A cell can regulate the production of enzymes
by feedback inhibition or by gene regulation
• Gene expression in bacteria is controlled by
the operon model
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
E. coli and tryptophan
• No tryptophan (an amino acid) in the environment
(colon)
• E. coli activates another pathway that makes
tryptophan from another compound
• LATER:
– If human eats tryptophan then the E. coli will
stop synthesizing from the alternative pathway
and use the resources that are readily
available
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Two Ways this can be accomplished
• First: cells can adjust the activity levels of the
enzymes they have present
(quick response)
– The end product can inhibit the productivity of
the first enzyme in the pathway (feed back
inhibition)
Second: cells adjust the production levels that
create the enzymes (regulate the expression of
genes)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-2
Precursor
Feedback
inhibition
trpE gene
Enzyme 1
trpD gene
Regulation
of gene
expression
Enzyme 2
trpC gene
trpB gene
Enzyme 3
trpA gene
Tryptophan
(a) Regulation of enzyme
activity
(b) Regulation of enzyme
production
In bacteria: the OPERON MODEL
Bozeman: Gene Regulation
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Operons: The Basic Concept
• A cluster of functionally related genes can be
under coordinated control by a single on-off
“switch”
• The regulatory “switch” is a segment of DNA
called an operator usually positioned within
the promoter or between the promoter and the
enzyme-coding genes
• An operon is the entire stretch of DNA that
includes the operator, the promoter, and the
genes that they control
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• The operon can be switched off by a protein
repressor
• The repressor prevents gene transcription by
binding to the operator and blocking RNA
polymerase
• The repressor is the product of a separate
regulatory gene
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• The repressor can be in
an active or inactive
form, depending on the
presence of other
molecules
• A corepressor is a
molecule that
cooperates with a
repressor protein to
switch an operon off
• For example, E. coli can
synthesize the amino
acid tryptophan
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
E. Coli trp operon how it really works
• Tryptophan synthesis is a
multistep process
– There are 5 genes that
code for the subunits of the
enzymes that are specific
to each step in the
pathway
One promoter serves all 5
genes (transcription unit)
So, transcription creates the
mRNA that codes for all 5
proteins in this pathway
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-2
Precursor
Feedback
inhibition
trpE gene
Enzyme 1
trpD gene
Regulation
of gene
expression
Enzyme 2
trpC gene
trpB gene
Enzyme 3
trpA gene
Tryptophan
(a) Regulation of enzyme
activity
One promoter
Controls the
Transcription of
5 genes that
make
The enzymes
for this process
(b) Regulation of enzyme
production
continued
• The mRNA can translate into 5 different proteins
because there are various start/stop codons
• If the E coli is lacking tryptophan, the genes
synthesize all the enzymes for the pathway
• The OPERATOR controls access of the RNA
polymerase to the genes- IN THIS CASE “ON”
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-3a
The operon is made of the promoter
The operator and the DNA sequence
Required for enzyme production
trp operon
Promoter
Promoter
Genes of operon
DNA
trpR
Regulatory
gene
mRNA
5
Protein
trpE
3
Operator
Start codon
mRNA 5
RNA
polymerase
Inactive
repressor
E
trpD
trpC
trpB
trpA
B
A
Stop codon
D
C
Polypeptide subunits that make up
enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on
The transcription of the mRNA is done in order to
Create the subunits for the enzymes
• If there is a supply of tryptophan supplied, the
operon can be switched off by a REPRESSOR (it
binds to the operator and blocks RNA polymerase
from binding to the promoter)
• The repressor is active only in the presence of its
corepressor tryptophan; thus the trp operon is
turned off (repressed) if tryptophan levels are high
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-3b-1
DNA
No RNA made
mRNA
Active
repressor
Protein
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off
The cell doesn’t “waste” its efforts in creating the
Enymes to convert the precursor to tryptophan
Fig. 18-3b-2
DNA
No RNA made
mRNA
Active
repressor
Protein
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off
• Trp repressor is the result of a
REGULATORY GENE trpR (located further
down from the operon and has its own
promoter)
• These are continuously made, but the levels
determine whether the signal is permanently
turned off
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Repressible and Inducible Operons: Two Types of
Negative Gene Regulation
• A repressible operon is one that is usually on;
binding of a repressor to the operator shuts off
transcription
• The trp operon is a repressible operon
(tryptophan binds allosterically to a regulatory
protein)
• An inducible operon is one that is usually off; a
molecule called an inducer inactivates the
repressor and turns on transcription
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• The lac operon is an inducible operon and
contains genes that code for enzymes used in
the hydrolysis and metabolism of lactose
• By itself, the lac repressor is active and
switches the lac operon off
• A molecule called an inducer inactivates the
repressor to turn the lac operon on
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-4a
Regulatory
gene
Promoter
Operator
lacI
DNA
lacZ
No
RNA
made
3
mRNA
5
Protein
RNA
polymerase
Active
repressor
(a) Lactose absent, repressor active, operon off
a. No need to metabolize lactose
Fig. 18-4b
lac operon
DNA
lacZ
lacY
-Galactosidase
Permease
lacI
3
mRNA
5
RNA
polymerase
mRNA 5
Protein
Allolactose
(inducer)
lacA
Inactive
repressor
(b) Lactose present, repressor inactive, operon on
b. Lactose present, must
metabolize
Transacetylase
• Inducible enzymes usually function in catabolic
pathways; their synthesis is induced by a
chemical signal
• Repressible enzymes usually function in
anabolic pathways; their synthesis is repressed
by high levels of the end product
• Regulation of the trp and lac operons involves
negative control of genes because operons are
switched off by the active form of the repressor
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Positive Gene Regulation
• Some operons are also subject to positive
control through a stimulatory protein, such as
catabolite activator protein (CAP), an activator
of transcription
• When glucose (a preferred food source of E.
coli) is scarce, CAP is activated by binding with
cyclic AMP (so the E. coli will make more of
the enzymes for the lactose pathway)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Activated CAP attaches to the promoter of the
lac operon and increases the affinity of RNA
polymerase, thus accelerating transcription
• When glucose levels increase, CAP detaches
from the lac operon, and transcription returns
to a normal rate
• CAP helps regulate other operons that encode
enzymes used in catabolic pathways
– It attaches to the promoter and stimulates
gene expression
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-5
Promoter
(a) Lactose present,
glucose scarce
(cAMP level
DNA
lacI
high): abundant
CAP-binding site
lac mRNA
synthesized
Operator
lacZ
RNA
polymerase
binds and
transcribes
Active
CAP
cAMP
Inactive lac
repressor
Inactive
CAP
Allolactose
(b) Lactose
present, glucose
present (cAMP
level low): little
lac mRNA
synthesized
BECAUSE
Glucose usage is
preferred
Promoter
DNA
lacI
CAP-binding site
Inactive
CAP
Operator
lacZ
RNA
polymerase less
likely to bind
Inactive lac
repressor
Concept 18.2: Eukaryotic gene expression can be
regulated at any stage
• All organisms must regulate which genes are
expressed at any given time
• In multicellular organisms gene expression is
essential for cell specialization
• http://www.youtube.com/watch?v=4PKjF7Oum
Yo
• (DNA packaging)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Differential Gene Expression
• Almost all the cells in an organism are
genetically identical (except cells of the
immune system)
• Differences between cell types result from
differential gene expression, the expression
of different genes by cells with the same
genome
• Errors in gene expression can lead to diseases
including cancer
• Gene expression is regulated at many stages
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-6a
Signal
NUCLEUS
Chromatin
Chromatin modification
DNA
Gene available
for transcription
Gene
Transcription
RNA
Exon
Primary transcript
Intron
RNA processing
Tail
Cap
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
Fig. 18-6b
CYTOPLASM
mRNA in cytoplasm
Degradation
of mRNA
Translation
Polypeptide
Protein processing
Active protein
Degradation
of protein
Transport to cellular
destination
Cellular function
Regulation of Chromatin Structure
• Genes within highly packed heterochromatin
are usually not expressed (too difficult to get to)
• The packing of the chromatin into a nucleus
also allows for “convenient” storage but acts as
a regulator for what actually gets expressed
• The location of the gene’s promoter relative to
the nucleosome (and where the DNA attaches
to the cs scaffolding) also affects expression
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Chemical modifications to histones and DNA of
chromatin influence both chromatin structure and
gene expression
• http://www.youtube.com/watch?v=eYrQ0EhVCYA
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Histones play a role
in the regulation of
gene transcription.
• The histone (protein)
tails stick out from
the nucleosome (the
N terminus)
• The amino acids
here are accessible
for chemical
modification
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Histone Modifications
• In histone acetylation, acetyl
groups are attached to positively
charged lysines in histone tails
• This process loosens chromatin
structure, thereby promoting the
initiation of transcription
• It is thought that the histone
acetylation enzymes may promote
the initiation of transcription by
remodeling the chromatin and/or
binding to transcription machinery
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-7
Histone
tails
DNA
double helix
Amino
acids
available
for chemical
modification
(a) Histone tails protrude outward from a
nucleosome
Unacetylated histones
Acetylated histones
(b) Acetylation of histone tails promotes loose
chromatin structure that permits transcription
• The addition of methyl groups
(methylation) can condense
chromatin; the addition of
phosphate groups
(phosphorylation) next to a
methylated amino acid can
loosen chromatin
• The histone code hypothesis
proposes that specific
combinations of modifications
help determine chromatin
configuration and influence
transcription
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
DNA Methylation
• DNA methylation, the addition of methyl
groups to certain bases in DNA, is associated
with reduced transcription in some species
• DNA methylation can cause long-term
inactivation of genes in cellular differentiation
• In genomic imprinting, methylation
regulates expression of either the maternal or
paternal alleles of certain genes at the start
of development (not from sex chromosomes)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Epigenetic Inheritance
• Although the chromatin modifications just
discussed do not alter DNA sequence (adding
a methyl, acetyl or phophate group etc
DOESN’T CHANGE THE DNA but can change
the expression), they may be passed to future
generations of cells
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• The inheritance of traits
transmitted by mechanisms
not directly involving the
nucleotide sequence is called
epigenetic inheritance
• DNA changes can’t be
reversed, but changes that
occur on the chromatin CAN
BE reversed
• This allows for an organism to
continually adjust its gene
expression to fit the
environment
• But researchers don’t know
how, this is an up and coming
topic
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Regulation of Transcription Initiation
• Chromatin-modifying enzymes provide initial
control of gene expression by making a region
of DNA either more or less able to bind the
transcription machinery
• Once the chromatin is in the optimal position
transcription factors can bind and transcription
can occur
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Organization of a Typical Eukaryotic Gene
• Associated with most eukaryotic genes are
control elements, segments of noncoding
DNA that help regulate transcription by binding
certain proteins
• Control elements and the proteins they bind
are critical to the precise regulation of gene
expression in different cell types
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-8-1
Enhancer
(distal control elements)
Poly-A signal
sequence
Termination
region
Proximal
control elements
Exon
Intron
Exon
Intron Exon
DNA
Upstream
Promoter
Downstream
Remember: the promoter is the DNA
Sequence where RNA polymerase binds
To start transcription
Fig. 18-8-2
Control elements: DNA sequence located
Near or far from the promoter
Enhancer
(distal control elements)
Poly-A signal
sequence
Termination
region
Proximal
control elements
Exon
Intron
Exon
Intron Exon
DNA
Upstream
Downstream
Promoter
Primary RNA
5
transcript
Transcription
Exon
Intron
Exon
Intron Exon
Cleaved 3 end
of primary
transcript
Poly-A
signal
The primary RNA transcript is a lot shorter
Than the original DNA strand because some
Of the DNA is for regulation and doesn’t get
Transcribed.
Fig. 18-8-3
Enhancer
(distal control elements)
Poly-A signal
sequence
Termination
region
Proximal
control elements
Exon
Intron
Exon
Intron Exon
DNA
Upstream
Downstream
Promoter
Primary RNA
5
transcript
Transcription
Exon
Intron
Exon
Intron Exon
RNA processing
Cleaved 3 end
of primary
transcript
Poly-A
signal
Intron RNA
Coding segment
mRNA
3
5 Cap
5 UTR
Start
codon
Stop
codon
3 UTR Poly-A
tail
The Roles of Transcription Factors
• To initiate transcription, eukaryotic RNA
polymerase requires the assistance of proteins
called transcription factors
• General transcription factors are essential for
the transcription of all protein-coding genes
• In eukaryotes, high levels of transcription of
particular genes depend on control elements
interacting with specific transcription factors
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• Only a few transcription factors bind directly to the
DNA
• Others will bind proteins
• Others bind RNA polymerase II
• Only under proper binding/interactions can
transcription start
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Enhancers and Specific Transcription Factors
• Proximal control elements are located close to the
promoter
• Distal control elements, groups of which are called
enhancers, may be far away from a gene or even
located in an intron
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• An activator is a protein that binds to an
enhancer and stimulates transcription of a
gene
• Bound activators cause mediator proteins to
interact with proteins at the promoter
• A promoter binds to a DNA sequence and
allows RNA polymerase to bind
Animation: Initiation of Transcription
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-9-1
Activators
Promoter
DNA
Enhancer
Distal control
element
Gene
TATA
box
1. Activator proteins bind to distal control element
enhancer
Fig. 18-9-2
Promoter
Activators
DNA
Enhancer
Distal control
element
Gene
TATA
box
General
transcription
factors
DNA-bending
protein
Group of
mediator proteins
DNA bending protein allows the strand to bend
So that it is in closer contact with the mediator proteins
The transcription factors and the promoter region
Fig. 18-9-3
Promoter
Activators
DNA
Enhancer
Distal control
element
Gene
TATA
box
General
transcription
factors
DNA-bending
protein
Group of
mediator proteins
Activators bind
To the mediator
And the
transcription
Factors forming
The initiation
complex
On the promoter
RNA
polymerase II
RNA
polymerase II
Transcription
initiation complex
RNA synthesis
• Some transcription factors function as
repressors, inhibiting expression of a particular
gene
– Bind directly to the control element DNA
– Block activator binding
– Turning off transcription
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Some activators and repressors act indirectly by
influencing chromatin structure to promote or
silence transcription
– Some activators recruit proteins that help to
acetylate histones, causing the chromatin to
loosen
– Some repressors recruit proteins that cause
deacetylation and cause the chromatin to
tighten
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Combinatorial Control of Gene Activation
• A particular combination of control elements
can activate transcription only when the
appropriate activator proteins are present
• There are about a dozen control element
sequences available but a lot of different
combinations that result from these
combinations
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Example: liver and lens cell
• Both have the DNA to make crystallin and albumin
proteins
–
Only liver cells make albumin (blood protein)
–
Only lens cells make crystallin (the main protein of lenses)
The specific transcription factors in the cell
determine which genes are expressed
There might be common elements to this, but in order for the
correct protein to be made for the correct cell there are SPECIFIC
transcription factors needed for the correct type of cell
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-10
Enhancer Promoter
Control
elements
Activators specific
To the cell type are
Present and
Determine which
Genes will be
Transcribed and
expressed
Albumin gene
Crystallin gene
LIVER CELL
NUCLEUS
Available
activators
LENS CELL
NUCLEUS
Available
activators
Albumin gene
not expressed
Albumin gene
expressed
Crystallin gene
not expressed
(a) Liver cell
Crystallin gene
expressed
(b) Lens cell
Coordinately Controlled Genes in Eukaryotes
• Unlike the genes of a prokaryotic operon, each
of the coordinately controlled eukaryotic genes
has a promoter and control elements
–
Remember: prokaryotes have an operon region that can be
repressed/induced but THE ENTIRE gene sequence for a
particular pathway are clustered together into an operon
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
In eukaryotes
• These genes can be scattered over different
chromosomes, but each has the same
combination of control elements
• Copies of the activators recognize specific control
elements and promote simultaneous transcription
of the genes
• http://www.youtube.com/watch?v=vi-zWoobt_Q
• Regulated transcription
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Coordinated control of dispersed genes
• Response to a chemical signal from outside the
cell like a steroid
– Binds to receptor protein forming transcription
activator
– This hormone/receptor complex will be
recognized by a control element
– Causing activation of genes
Other things bind on the membrane, but signal a
transduction pathway to that lead to activation of
transcription
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Mechanisms of Post-Transcriptional Regulation
• Transcription alone does not account for gene
expression (just because you make a copy
doesn’t mean it is going to be expressed or
expressed correctly)
• Regulatory mechanisms can operate at various
stages after transcription
• Such mechanisms allow a cell to fine-tune
gene expression rapidly in response to
environmental changes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
RNA Processing
• In alternative RNA splicing, different mRNA
molecules are produced from the same primary
transcript, depending on which RNA segments
are treated as exons and which as introns
– Resulting in different but possibly related
proteins as the end product
Animation: RNA Processing
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-11
Exons
DNA
Troponin T gene
Primary
RNA
transcript
RNA splicing
mRNA
or
mRNA Degradation
• The life span of mRNA molecules in the
cytoplasm is a key to determining protein
synthesis
• Eukaryotic mRNA is more long lived (a few
hours, days, or weeks) than prokaryotic mRNA
(few minutes)
• The mRNA life span is determined in part by
sequences in the leader and trailer regions
Animation: mRNA Degradation
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• What happens (they think, in yeast)
– Enzymes start to shorten the Poly-A tail,
– This activates the removal of the 5’ cap
– Nuclease enzymes break up the mRNA
Thought: there are nucleotide sequences that are
in the UTR (untranslated region) that “tell” the
mRNA how long to be stable
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Initiation of Translation
• The initiation of translation of selected
mRNAs can be blocked by regulatory proteins
that bind to sequences or structures of the
mRNA (so some regions can be translated
while other regions can’t)
• Alternatively, translation of all mRNAs
in a cell may be regulated simultaneously
Animation: Blocking Translation
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• For example, translation initiation factors are
simultaneously activated in an egg following
fertilization
– How? The poly- A tail isn’t long enough to
allow translation initiation
– During development, a cytoplasmic enzyme
adds more Adenine to the 3’ end, allowing
translation to begin
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Protein Processing and Degradation
• After translation, various types of protein
processing
–
–
–
–
including cleavage (chopping a long polypeptide into smaller lengths)
the addition of chemical groups, are subject to control
Regulatory proteins undergo Addition/removal of a phosphate group to
activate/deactivate
Selective degradation so the length of time each protein functions is
regulated
Animation: Protein Processing
Animation: Protein Degradation
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Proteasomes are giant protein complexes that
bind protein molecules and degrade them
– A protein is marked for degradation usually by
ubiquitin then the proteasomes degrade them
– If this pathway doesn’t work, this can
sometimes lead to cancer
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-12
Ubiquitin
Proteasome
Protein to
be degraded
Ubiquitinated
protein
Proteasome
and ubiquitin
to be recycled
Protein entering a
proteasome
Protein
fragments
(peptides)
Concept 18.3: Noncoding RNAs play multiple roles
in controlling gene expression
• Only a small fraction of DNA codes for
proteins, rRNA, and tRNA (1.5%)
• A significant amount of the genome may be
transcribed into noncoding RNAs
– Introns, UTR, etc
– Noncoding RNAs regulate gene expression
at two points: mRNA translation and
chromatin configuration
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Effects on mRNAs by MicroRNAs and Small
Interfering RNAs
• MicroRNAs (miRNAs) are small singlestranded RNA molecules that can bind to
mRNA forming hairpin structures
– Hairpin (double stranded) gets cut away,
the dicer cuts it up; one strand is degraded,
the other joins with one/more proteins; finds
the complement and then degrades the
mRNA or blocks translation
• These can degrade mRNA or block its
translation
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-13
Hairpin
miRNA
Hydrogen
bond
Dicer
miRNA
5 3
(a) Primary miRNA transcript
miRNAprotein
complex
If the miRNA is a complete match for
The complementary mRNA, it will be
Degraded; if it only closely matches
The mRNA will be blocked from
translation
mRNA degraded
Translation blocked
(b) Generation and function of miRNAs
• The phenomenon of inhibition of gene
expression by RNA molecules is called RNA
interference (RNAi)- scientists injected small
sections of double stranded RNA which ended
up interfering with translation
• RNAi is caused by small interfering RNAs
(siRNAs)
• siRNAs and miRNAs are similar but form from
different RNA precursors (siRNAs are made
from a longer double strand than the miRNAs)
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Chromatin Remodeling and Silencing of
Transcription by Small RNAs
• siRNAs play a role in heterochromatin
formation and can block large regions of the
chromosome
• Small RNAs may also block transcription of
specific genes
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Concept 18.4: A program of differential gene
expression leads to the different cell types in a
multicellular organism
• During embryonic development, a fertilized egg
gives rise to many different cell types
• Cell types are organized successively into
tissues, organs, organ systems, and the whole
organism
• Gene expression orchestrates the
developmental programs of animals
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A Genetic Program for Embryonic Development
• The transformation from zygote to adult results
from cell division, cell differentiation, and
morphogenesis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Introduction to Animals
Animal Characteristics
Early Development
 The zygote undergoes mitosis and a
series of cell divisions to form new cells.
 The cells continue to divide, forming a fluidfilled ball of cells called the blastula.
 The blastula continues to undergo cell
division as some cells move inward to
form a gastrula.
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Introduction to Animals
Animal Characteristics
Invagination
Occurs
And tissue
Layers start to form
(Gastrulation)
Hollow ball
Of cells
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Fig. 18-14
(a) Fertilized eggs of a frog
(b) Newly hatched tadpole
• Cell differentiation is the process by which
cells become specialized in structure and
function
• The physical processes that give an organism
its shape constitute morphogenesis
• Differential gene expression results from genes
being regulated differently in each cell type
• Materials in the egg can set up gene regulation
that is carried out as cells divide
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Cytoplasmic Determinants and Inductive Signals
• An egg’s cytoplasm contains RNA, proteins,
and other substances that are distributed
unevenly in the unfertilized egg
• Cytoplasmic determinants are maternal
substances in the egg that influence early
development (RNA and proteins that are
encoded by the mother’s DNA)
• As the zygote divides by mitosis, cells contain
different cytoplasmic determinants, which lead
to different gene expression
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-15a
Unfertilized egg cell
Sperm
Fertilization
Nucleus
Two different
cytoplasmic
determinants
Zygote
Mitotic
cell division
Two-celled
embryo
(a) Cytoplasmic determinants in the egg
Because of the
Different cytoplasmic
Determinants these two
Cells will have different
outcomes
Fig. 18-15b
The bottom cells induce the cells above
It to change their gene expression
Early embryo
(32 cells)
Signal
transduction
pathway
Signal
receptor
Signal
molecule
(inducer)
(b) Induction by nearby cells
NUCLEUS
• The other important source of developmental
information is the environment around the cell,
especially signals from nearby embryonic cells
• In the process called induction, signal
molecules from embryonic cells cause
transcriptional changes in nearby target cells
• Thus, interactions between cells induce
differentiation of specialized cell types
Animation: Cell Signaling
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Introduction to Animals
Tissue Development
 Endoderm
 inner layer of cells in the gastrula
 Ectoderm
 outer layer of cells in the gastrula
 Mesoderm
 layer of cells between the endoderm
and ectoderm
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Introduction to Animals
Animal Characteristics
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Sequential Regulation of Gene Expression During
Cellular Differentiation
• Determination commits a cell to its final fate
• Determination precedes differentiation
• Cell differentiation is marked by the production
of tissue-specific proteins
– mRNAs made first
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• Myoblasts produce muscle-specific proteins
and form skeletal muscle cells (myoblasts fuse
to become multinucleate, mature, skeletal
cells)
• MyoD is one of several “master regulatory
genes” that produce proteins that commit the
cell to becoming skeletal muscle
• The MyoD protein is a transcription factor that
binds to enhancers of various target genes
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• MyoD also stimulates expression of the myoD
gene itself, and also encodes other musclespecific transcription factors
• Thought to be coordinately controlled with MyoD
activating the enhancer control elements
recognized by myoD
• Secondary transcription factors activate the genes
for myosin and actin (proteins for skeletal muscle)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-16-1
Nucleus
Master regulatory gene myoD
Embryonic
precursor cell
Other muscle-specific genes
DNA
OFF
OFF
Fig. 18-16-2
Nucleus
Master regulatory gene myoD
Embryonic
precursor cell
Myoblast
(determined)
Other muscle-specific genes
DNA
OFF
OFF
mRNA
OFF
MyoD protein
(transcription
factor)
Signals from other cells lead to activation of the
Master regulatory gene; the cell makes MyoD
The cell is a myoblast and is now “determined”
Fig. 18-16-3
Nucleus
Master regulatory gene myoD
Embryonic
precursor cell
Other muscle-specific genes
DNA
Myoblast
(determined)
OFF
OFF
mRNA
OFF
MyoD protein
(transcription
factor)
mRNA
MyoD
Part of a muscle fiber
(fully differentiated cell)
MyoD stimulates the gene to make
More muscle-specific transcription factors;
The MyoD also turns on genes that block
The cell cycle, the myoblasts then fuse to
Make muscle fiber
mRNA
Another
transcription
factor
mRNA
mRNA
Myosin, other
muscle proteins,
and cell cycle–
blocking proteins
Pattern Formation: Setting Up the Body Plan
• Pattern formation is the development of a
spatial organization of tissues and organs
• In animals, pattern formation begins with the
establishment of the major axes
– Relative positions of head, tail, right and left
are determined
– http://www.bing.com/videos/search?q=gene
tic+tool+kit+video&view=detail&mid=DDC59
C3047B05C15D1EFDDC59C3047B05C15
D1EF&first=0&FORM=NVPFVR
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• Positional information, the molecular cues that
control pattern formation, tells a cell its location
relative to the body axes and to neighboring cells
– Cytoplasmic determinants and inductive
signals
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• Pattern formation has been extensively studied
in the fruit fly Drosophila melanogaster
• Combining anatomical, genetic, and
biochemical approaches, researchers have
discovered developmental principles common
to many other species, including humans
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-17a
Head
Thorax
Abdomen
0.5 mm
Dorsal
BODY
AXES
(a) Adult
Anterior
Left
Ventral
Right
Posterior
Fig. 18-17b
Follicle cell
1 Egg cell
Nucleus
developing within
ovarian follicle
Egg
cell
Nurse cell
In Drosophila, cytoplasmic
determinants in the
unfertilized egg determine
the axes before fertilization
Egg
shell
2 Unfertilized egg
Depleted
nurse cells
Fertilization
Laying of egg
3 Fertilized egg
After fertilization, the
embryo develops into a
segmented larva with
three larval stages
Embryonic
development
4 Segmented
embryo
0.1 mm
Body
segments
Hatching
5 Larval stage
(b) Development from egg to larva
Genetic Analysis of Early Development: Scientific
Inquiry
• Edward B. Lewis, Christiane Nüsslein-Volhard,
and Eric Wieschaus won a Nobel 1995 Prize
for decoding pattern formation in Drosophila
• Lewis demonstrated that genes direct the
developmental process
– Discovered where the genes were located
on the chromosomes; discovered the
homeotic genes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-18
Eye
Leg
Antenna
Wild type
Mutant
Legs aren’t supposed
To grow from the antenna spot
• Nüsslein-Volhard and Wieschaus studied
segment formation
• They created mutants, conducted breeding
experiments, and looked for corresponding
genes
• Breeding experiments were complicated by
embryonic lethals, embryos with lethal
mutations (they weren’t able to reproduce)
• They found 120 genes essential for normal
segmentation
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Axis Establishment
• Maternal effect genes encode for cytoplasmic
determinants that initially establish the axes of the
body of Drosophila (mutations are generally
embryonic lethals)
• These maternal effect genes are also called eggpolarity genes because they control orientation of
the egg and consequently the fly
• If the female has mutations, the offspring will have
a mutation, regardless of the offspring’s own
genotype
Animation: Development of Head-Tail Axis in Fruit Flies
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Bicoid: A Morphogen Determining Head
Structures
• One maternal effect gene, the bicoid (“twotailed”) gene, affects the front half of the body
• An embryo whose mother has a mutant bicoid
gene lacks the front half of its body and has
duplicate posterior structures at both ends
• So a wild type bicoid would have normal head
development
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-19a
EXPERIMENT
Tail
Head
T1
T2
T3
A1 A2
A6
A3 A4 A5
A7
A8
Wild-type larva
Tail
Tail
A8
A8
A7
Mutant larva (bicoid)
A6
A7
Bicoid gene
Is mutated
Giving two
tails
Fig. 18-19b
RESULTS
100 µm
Bicoid mRNA in mature
unfertilized egg
Fertilization,
translation
Anterior end
of bicoid
Bicoid protein in early
mRNA
embryo
Bicoid mRNA was located at the anterior portion of
The egg; after fertiliation bicoid protein is located at
The anterior portion of the embryo
Fig. 18-19c
CONCLUSION
Nurse cells
Egg
bicoid mRNA
Developing egg
Bicoid mRNA in mature
unfertilized egg
Bicoid protein
in early embryo
• This phenotype suggests that the product of
the mother’s bicoid gene is concentrated at the
future anterior end
• This hypothesis is an example of the gradient
hypothesis, in which gradients of substances
called morphogens establish an embryo’s
axes and other features
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• The bicoid research is important for three
reasons:
– It identified a specific protein required for some
early steps in pattern formation
– It increased understanding of the mother’s role
in embryo development –we rock
– It demonstrated a key developmental principle
that a gradient of molecules can determine
polarity and position in the embryo
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Concept 18.5: Cancer results from genetic changes
that affect cell cycle control
• The gene regulation systems that go wrong
during cancer are the very same systems
involved in embryonic development
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Types of Genes Associated with Cancer
• Cancer can be caused by mutations to genes
that regulate cell growth and division
• Tumor viruses can cause cancer in animals
including humans
– Like Epstein Barr has been associated with
Burkitt’s lumphoma, some types of
leukemia
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Oncogenes and Proto-Oncogenes
• Oncogenes are cancer-causing genes
• Proto-oncogenes are the corresponding
normal cellular genes that are responsible for
normal cell growth and division
• Conversion of a proto-oncogene to an
oncogene can lead to abnormal stimulation
of the cell cycle
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-20
Proto-oncogene
DNA
Translocation or
transposition:
Point mutation:
Gene amplification:
within a control element
New
promoter
Normal growthstimulating
protein in excess
Oncogene
Normal growth-stimulating
protein in excess
Normal growthstimulating
protein in excess
within the gene
Oncogene
Hyperactive or
degradationresistant protein
• Proto-oncogenes can be converted to
oncogenes by
– Movement of DNA within the genome: if it ends
up near an active promoter, transcription may
increase
– Amplification of a proto-oncogene: increases
the number of copies of the gene
– Point mutations in the proto-oncogene or its
control elements: causes an increase in gene
expression
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Tumor-Suppressor Genes
• Tumor-suppressor genes help prevent
uncontrolled cell growth (inhibit cell division)
• Mutations that decrease protein products of
tumor-suppressor genes may contribute to
cancer onset
• Tumor-suppressor proteins
– Repair damaged DNA
– Control cell adhesion
– Inhibit the cell cycle in the cell-signaling
pathway
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Interference with Normal Cell-Signaling Pathways
• Mutations in the ras proto-oncogene and p53
tumor-suppressor gene are common in human
cancers
• Mutations in the ras gene can lead to
production of a hyperactive Ras protein and
increased cell division
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-21a
1 Growth
factor
1
MUTATION
Hyperactive
Ras protein
(product of
oncogene)
issues
signals
on its own
Ras
3 G protein
GTP
Ras
GTP
2 Receptor
4 Protein kinases
(phosphorylation
cascade)
NUCLEUS
5 Transcription
factor (activator)
DNA
Gene expression
Protein that
stimulates
the cell cycle
(a) Cell cycle–stimulating pathway
If Ras is mutated cell cycle
Continually runs
Fig. 18-21b
2 Protein kinases
MUTATION
3 Active
form
of p53
UV
light
1 DNA damage
in genome
Defective or
missing
transcription
factor, such
as p53, cannot
activate
transcription
DNA
Protein that
inhibits
the cell cycle
(b) Cell cycle–inhibiting pathway
Activated p53 promotes transcription
Of a protein that inhibits the cell cycle
Fig. 18-21c
EFFECTS OF MUTATIONS
Protein
overexpressed
Cell cycle
overstimulated
(c) Effects of mutations
Protein absent
Increased cell
division
Cell cycle not
inhibited
• Suppression of the cell cycle can be important
in the case of damage to a cell’s DNA; p53
prevents a cell from passing on mutations due
to DNA damage
• Mutations in the p53 gene prevent suppression
of the cell cycle
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The Multistep Model of Cancer Development
• Multiple mutations are generally needed for
full-fledged cancer; thus the incidence
increases with age
• At the DNA level, a cancerous cell is usually
characterized by at least one active oncogene
(cancer causing) and the mutation of several
tumor-suppressor genes (so they don’t stop the
cell cycle)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 18-22a
Colon
Colon wall
Normal colon
epithelial cells
Fig. 18-22b
1 Loss of tumorsuppressor gene
APC (or other)
Small benign
growth (polyp)
Fig. 18-22c
2 Activation of
ras oncogene
3 Loss of
tumor-suppressor
gene DCC
Larger benign
growth (adenoma)
Fig. 18-22d
4 Loss of
tumor-suppressor
gene p53
5 Additional
mutations
Malignant tumor
(carcinoma)
Inherited Predisposition and Other Factors
Contributing to Cancer
• Individuals can inherit oncogenes or mutant
alleles of tumor-suppressor genes
• Inherited mutations in the tumor-suppressor
gene adenomatous polyposis coli are common
in individuals with colorectal cancer
• Mutations in the BRCA1 or BRCA2 gene are
found in at least half of inherited breast cancers
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