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LECTURE PRESENTATIONS

For CAMPBELL BIOLOGY, NINTH EDITIONJane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson

© 2011 Pearson Education, Inc.

Lectures by

Erin Barley

Kathleen Fitzpatrick

Regulation of Gene Expression

Chapter 18



Overview: Conducting the Genetic Orchestra

• Prokaryotes and eukaryotes alter gene expressionin 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




Figure 18.1



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




Precursor

Feedbackinhibition

Enzyme 1

Enzyme 2

Enzyme 3

Tryptophan

(a) (b)Regulation of enzyme

activity

Regulation of enzyme

production

Regulationof geneexpression

−

−

trpE gene

trpD gene

trpC gene

trpB gene

trpA gene

Figure 18.2



Operons: The Basic Concept

• A cluster of functionally related genes can beunder coordinated control by a single “on-off

switch”

• The regulatory “switch” is a segment of DNA called

an operator usually positioned within the promoter

• An operon is the entire stretch of DNA that

includes the operator, the promoter, and the genes

that they control




• The operon can be switched off by a proteinrepressor

• The repressor prevents gene transcription by

binding to the operator and blocking RNA

polymerase

• The repressor is the product of a separate

regulatory gene




• 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




• By default the trp operon is on and the genes for tryptophan synthesis are transcribed

• When tryptophan is present, it binds to the trp

repressor protein, which turns the operon off

• 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




Promoter

DNA

Regulatorygene

mRNA

trpR

5′

3′

Protein Inactiverepressor

RNApolymerase

Promoter

trp operon

Genes of operon

Operator

mRNA 5′

Start codon Stop codon

trpE trpD trpC trpB trpA

E D C B A

Polypeptide subunits that make upenzymes for tryptophan synthesis

(a) Tryptophan absent, repressor inactive, operon on

(b) Tryptophan present, repressor active, operon off

DNA

mRNA

Protein

Tryptophan(corepressor)

Activerepressor

No RNAmade

Figure 18.3



Figure 18.3a

Promoter

DNA

Regulatorygene

mRNA

trpR

5′

3′

Protein Inactiverepressor

RNApolymerase

Promoter

trp operon

Genes of operon

Operator

mRNA 5′

Start codon Stop codon

trpE trpD trpC trpB trpA

E D C B A

Polypeptide subunits that make upenzymes for tryptophan synthesis

(a) Tryptophan absent, repressor inactive, operon on



Figure 18.3b-1


DNA

mRNA

Protein


Activerepressor



Figure 18.3b-2


DNA

mRNA

Protein


Activerepressor

No RNAmade



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

• An inducible operon is one that is usually off; a

molecule called an inducer inactivates the

repressor and turns on transcription




• The lac operon is an inducible operon andcontains 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




(a) Lactose absent, repressor active, operon off

(b) Lactose present, repressor inactive, operon on

Regulatorygene

Promoter

Operator

DNA lacZ lac I

lac I

DNA

mRNA5′

3′

NoRNAmade

RNApolymerase

Activerepressor Protein

lac operon

lacZ lacY lacADNA

mRNA

5′

3′

Protein

mRNA 5′

Inactiverepressor

RNA polymerase

Allolactose(inducer)

β-Galactosidase Permease Transacetylase

Figure 18.4



Figure 18.4a

(a) Lactose absent, repressor active, operon off

Regulatorygene

Promoter

Operator

DNA lacZ lac I DNA

mRNA5′

3′

NoRNA

madeRNApolymerase

Activerepressor Protein



Figure 18.4b

(b) Lactose present, repressor inactive, operon on

lac I

lac operon

lacZ lacY lacADNA

mRNA5′

3′

Protein

mRNA 5′

Inactiverepressor

RNA polymerase

Allolactose

(inducer)

β-Galactosidase Permease Transacetylase



• Inducible enzymes usually function in catabolicpathways; 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 areswitched off by the active form of the repressor




Positive Gene Regulation

• Some operons are also subject to positive controlthrough 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 (cAMP)

• 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 fromthe lac operon, and transcription returns to a

normal rate

• CAP helps regulate other operons that encode

enzymes used in catabolic pathways


Figure 18 5



Figure 18.5 Promoter

DNA

CAP-binding site

lacZ lac I

RNApolymerasebinds andtranscribes

Operator

cAMPActiveCAP

InactiveCAP

Allolactose

Inactive lac repressor

(a) Lactose present, glucose scarce (cAMP level high):abundant lac mRNA synthesized

Promoter

DNA

CAP-binding site

lacZ lac I

Operator

RNApolymerase lesslikely to bind


InactiveCAP

(b) Lactose present, glucose present (cAMP level low):

little lac mRNA synthesized

Figure 18 5a



Figure 18.5a

Promoter

DNA

CAP-binding site

lacZ lac I

RNApolymerase

binds andtranscribes

Operator

cAMP ActiveCAP

InactiveCAP

Allolactose

Inactive lac

repressor

(a) Lactose present, glucose scarce (cAMP level high):abundant lac mRNA synthesized

Figure 18 5b



Figure 18.5b

Promoter

DNA

CAP-binding site

lacZ lac I

Operator

RNA

polymerase lesslikely to bind


InactiveCAP

(b) Lactose present, glucose present (cAMP level low):little lac mRNA synthesized



Concept 18.2: Eukaryotic gene expression

is regulated at many stages

• All organisms must regulate which genes are

expressed at any given time

•

In multicellular organisms regulation of geneexpression is essential for cell specialization




Differential Gene Expression

• Almost all the cells in an organism are geneticallyidentical

• Differences between cell types result from

differential gene expression, the expression of

different genes by cells with the same genome

• Abnormalities in gene expression can lead to

diseases including cancer

• Gene expression is regulated at many stages


Figure 18 6 Si l



Figure 18.6 Signal

NUCLEUS

Chromatin

Chromatin modification:DNA unpacking involvinghistone acetylation and

DNA demethylationDNA

Gene

Gene available

for transcription

RNA Exon

Primary transcript

Transcription

Intron

RNA processing

Cap

Tail

mRNA in nucleus

Transport to cytoplasm

CYTOPLASM

mRNA in cytoplasm

TranslationDegradationof mRNA

Polypeptide

Protein processing, suchas cleavage andchemical modification

Active proteinDegradation

of proteinTransport to cellular

destination

Cellular function (suchas enzymatic activity,structural support)

Figure 18 6aSi l



Figure 18.6aSignal

NUCLEUS

Chromatin

Chromatin modification:DNA unpacking involvinghistone acetylation and

DNA demethylationDNA

Gene

Gene availablefor transcription

RNA Exon

Primary transcript

Transcription

Intron

RNA processing

Cap

Tail

mRNA in nucleus

Transport to cytoplasm

CYTOPLASM

Figure 18 6b



Figure 18.6b

CYTOPLASM

mRNA in cytoplasm

Translation

Degradationof mRNA

Polypeptide

Protein processing, such

as cleavage andchemical modification

Active proteinDegradation

of protein

Transport to cellular destination

Cellular function (suchas enzymatic activity,structural support)



Regulation of Chromatin Structure

• Genes within highly packed heterochromatin areusually not expressed

• Chemical modifications to histones and DNA of

chromatin influence both chromatin structure and

gene expression




Histone Modifications

• In histone acetylation, acetyl groups are attachedto positively charged lysines in histone tails

• This loosens chromatin structure, thereby

promoting the initiation of 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





Animation: DNA Packing

Right-click slide / select “Play”

Figure 18.7



g

Amino acidsavailablefor chemicalmodification

Histone

tails

DNAdoublehelix

Nucleosome(end view)

(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 histone code hypothesis proposes thatspecific combinations of modifications, as well as

the order in which they occur, help determine

chromatin configuration and influence transcription




DNA Methylation

• DNA methylation, the addition of methyl groupsto 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




Epigenetic Inheritance

• Although the chromatin modifications justdiscussed do not alter DNA sequence, they may

be passed to future generations of cells

• The inheritance of traits transmitted by

mechanisms not directly involving the nucleotide

sequence is called epigenetic inheritance




Regulation of Transcription Initiation

• Chromatin-modifying enzymes provide initialcontrol of gene expression by making a region of

DNA either more or less able to bind the

transcription machinery




Organization of a Typical Eukaryotic Gene

• Associated with most eukaryotic genes aremultiple control elements, segments of

noncoding DNA that serve as binding sites for

transcription factors that help regulate transcription

• Control elements and the transcription factors they

bind are critical to the precise regulation of gene

expression in different cell types


Figure 18.8-1



Enhancer

(distal controlelements)

DNA

UpstreamPromoter

Proximal

controlelements

Transcriptionstart site

Exon Intron Exon ExonIntron

Poly-A

signalsequence

Transcriptiontermination

region

Downstream

Figure 18.8-2



Enhancer


DNA

UpstreamPromoter

Proximal

controlelements



Poly-A

signalsequence


region

DownstreamPoly-Asignal


Transcription

Cleaved

3′ end of primarytranscript

5′

Primary RNA

transcript(pre-mRNA)

Figure 18.8-3



Enhancer


DNA

UpstreamPromoter

Proximal

controlelements



Poly-A

signalsequence


region

DownstreamPoly-Asignal


Transcription

Cleaved

3′ end of primarytranscript

5′

Primary RNA

transcript(pre-mRNA)

Intron RNA

RNA processing

mRNA

Coding segment

5′ Cap 5′ UTRStart

codonStop

codon 3′ UTR

3′

Poly-Atail

PPPG AAA ⋅ ⋅ ⋅ AAA



The Roles of Transcription Factors

• To initiate transcription, eukaryotic RNApolymerase 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




• Proximal control elements are located close to the

promoter

•

Distal control elements, groupings of which arecalled enhancers, may be far away from a gene

or even located in an intron

Enhancers and Specific Transcription Factors




• An activator is a protein that binds to an enhancer and stimulates transcription of a gene

• Activators have two domains, one that binds DNA

and a second that activates transcription

• Bound activators facilitate a sequence of protein-

protein interactions that result in transcription of a

given gene





Animation: Initiation of Transcription


Figure 18.9



DNA

Activationdomain

DNA-binding

domain



• Some transcription factors function as repressors,inhibiting expression of a particular gene by a

variety of methods

• Some activators and repressors act indirectly by

influencing chromatin structure to promote or

silence transcription


A i PromoterFigure 18.10-1



Activators

DNA

Enhancer Distal controlelement

Promoter Gene

TATA box

A ti t PromoterFigure 18.10-2



Activators

DNA


Promoter Gene

TATA box

General

transcriptionfactors

DNA-bendingprotein

Group of mediator proteins

A ti t PromoterFigure 18.10-3



Activators

DNA


Promoter Gene

TATA box

General

transcriptionfactors

DNA-bendingprotein

Group of mediator proteins

RNApolymerase II

RNApolymerase II

RNA synthesis

Transcription

initiation complex



• A particular combination of control elements can

activate transcription only when the appropriate

activator proteins are present

Combinatorial Control of Gene Activation


Figure 18.11 Enhancer Promoter



Controlelements Albumin gene

Crystallingene

LIVER CELLNUCLEUS

Availableactivators

Albumin geneexpressed

Crystallin genenot expressed

(a) Liver cell

LENS CELLNUCLEUS

Availableactivators

Albumin genenot expressed

Crystallin geneexpressed

(b) Lens cell

Figure 18.11a



Controlelements

Enhancer Promoter

Albumin gene

Crystallingene

LIVER CELLNUCLEUS

Availableactivators

Albumin geneexpressed

Crystallin genenot expressed

(a) Liver cell

Figure 18.11b



Controlelements

Enhancer Promoter

Albumin gene

Crystallingene

LENS CELLNUCLEUS

Availableactivators

Albumin genenot expressed

Crystallin geneexpressed

(b) Lens cell



Coordinately Controlled Genes in Eukaryotes

•Unlike the genes of a prokaryotic operon, each of the co-expressed eukaryotic genes has a promoter

and control elements

• These genes can be scattered over different

chromosomes, but each has the samecombination of control elements

• Copies of the activators recognize specific control

elements and promote simultaneous transcriptionof the genes




Nuclear Architecture and Gene Expression

•Loops of chromatin extend from individualchromosomes into specific sites in the nucleus

• Loops from different chromosomes may

congregate at particular sites, some of which are

rich in transcription factors and RNA polymerases

• These may be areas specialized for a common

function


Figure 18.12



Chromosometerritory

Chromosomes in theinterphase nucleus

Chromatinloop

Transcriptionfactory

10 µm

Figure 18.12a



Chromosomes in theinterphase nucleus

10 µm



Mechanisms of Post-Transcriptional

Regulation

• Transcription alone does not account for gene

expression

• 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




RNA Processing

•In alternative RNA splicing, different mRNAmolecules are produced from the same primary

transcript, depending on which RNA segments are

treated as exons and which as introns





Animation: RNA Processing


E

Figure 18.13



Exons

DNA

Troponin T gene

PrimaryRNAtranscript

RNA splicing

or mRNA

1

1

1 1

2

2

2 2

3

3

3

4

4

4

5

5

5 5



mRNA Degradation

•

The life span of mRNA molecules in the cytoplasmis a key to determining protein synthesis

• Eukaryotic mRNA is more long lived than

prokaryotic mRNA

• Nucleotide sequences that influence the lifespan

of mRNA in eukaryotes reside in the untranslated

region (UTR) at the 3′ end of the molecule





Animation: mRNA Degradation




Initiation of Translation

•

The initiation of translation of selectedmRNAs can be blocked by regulatory proteins that

bind to sequences or structures of the mRNA

• Alternatively, translation of all mRNAs

in a cell may be regulated simultaneously

• For example, translation initiation factors are

simultaneously activated in an egg following

fertilization





Animation: Blocking Translation




Protein Processing and Degradation

•

After translation, various types of proteinprocessing, including cleavage and the addition of

chemical groups, are subject to control

• Proteasomes are giant protein complexes that

bind protein molecules and degrade them





Animation: Protein Processing





Animation: Protein Degradation


Figure 18.14



Protein tobe degraded

Ubiquitin

Ubiquitinatedprotein

Proteasome

Protein enteringa proteasome

Proteasomeand ubiquitinto be recycled

Proteinfragments(peptides)



Concept 18.3: Noncoding RNAs play multiple

roles in controlling gene expression

• Only a small fraction of DNA codes for proteins,and a very small fraction of the non-protein-codingDNA consists of genes for RNA such as rRNA and

tRNA• A significant amount of the genome may be

transcribed into noncoding RNAs (ncRNAs)

• Noncoding RNAs regulate gene expression at two

points: mRNA translation and chromatinconfiguration




Effects on mRNAs by MicroRNAs and

Small Interfering RNAs

• MicroRNAs (miRNAs) are small single-stranded

RNA molecules that can bind to mRNA

• These can degrade mRNA or block its translation


Hairpin

Figure 18.15



(a) Primary miRNA transcript

p

miRNA

miRNA

Hydrogenbond

Dicer

miRNA-

proteincomplex

mRNA degraded Translation blocked

(b) Generation and function of miRNAs

5′ 3′



•

The phenomenon of inhibition of gene expressionby RNA molecules is called RNA interference

(RNAi)

• RNAi is caused by small interfering RNAs

(siRNAs)

• siRNAs and miRNAs are similar but form from

different RNA precursors


Ch i R d li d Eff



Chromatin Remodeling and Effects on

Transcription by ncRNAs

• In some yeasts siRNAs play a role in

heterochromatin formation and can block large

regions of the chromosome

• Small ncRNAs called piwi-associated RNAs

(piRNAs) induce heterochromatin, blocking the

expression of parasitic DNA elements in the

genome, known as transposons• RNA-based mechanisms may also block

transcription of single genes


Th E l i Si ifi f S ll



The Evolutionary Significance of Small

ncRNAs

• Small ncRNAs can regulate gene expression at

multiple steps

• An increase in the number of miRNAs in a species

may have allowed morphological complexity to

increase over evolutionary time

• siRNAs may have evolved first, followed by

miRNAs and later piRNAs


C 18 4 A f diff i l



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


A G ti P f E b i



A Genetic Program for Embryonic

Development

• The transformation from zygote to adult results

from cell division, cell differentiation, and

morphogenesis


Figure 18.16



(a) Fertilized eggs of a frog (b) Newly hatched tadpole

1 mm 2 mm

Figure 18.16a



(a) Fertilized eggs of a frog

1 mm

Figure 18.16b



(b) Newly hatched tadpole

2 mm



•

Cell differentiation is the process by which cellsbecome specialized in structure and function

• The physical processes that give an organism itsshape constitute morphogenesis

• Differential gene expression results from genesbeing regulated differently in each cell type

• Materials in the egg can set up gene regulationthat is carried out as cells divide


C t l i D t i t d I d ti



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

• As the zygote divides by mitosis, cells containdifferent cytoplasmic determinants, which lead to

different gene expression


Figure 18.17



(a) Cytoplasmic determinants in the egg (b) Induction by nearby cells

Unfertilized egg

Sperm

Fertilization

Zygote(fertilized egg)

Mitoticcell division

Two-celledembryo

Nucleus

Molecules of twodifferent cytoplasmicdeterminants

Early embryo(32 cells)

NUCLEUS

Signaltransductionpathway

Signalreceptor

Signalingmolecule(inducer)

Figure 18.17a (a) Cytoplasmic determinants in the egg

Unfertilized egg



Unfertilized egg

Sperm

Fertilization

Zygote(fertilized egg)

Mitotic

cell division

Two-celledembryo

Nucleus

Molecules of twodifferent cytoplasmicdeterminants



•

The other important source of developmentalinformation is the environment around the cell,

especially signals from nearby embryonic cells

• In the process called induction, signal molecules

from embryonic cells cause transcriptionalchanges in nearby target cells

• Thus, interactions between cells induce

differentiation of specialized cell types





Animation: Cell Signaling


Figure 18.17b (b) Induction by nearby cells

Early embryo



Early embryo(32 cells)

NUCLEUS

Signaltransductionpathway

Signalreceptor

Signalingmolecule(inducer)

Sequential Regulation of Gene Expression



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




•

Myoblasts produce muscle-specific proteins andform skeletal muscle 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


Nucleus Master regulatorygene myoD Other muscle-specific genes

Figure 18.18-1



Embryonicprecursor cell

DNA

g y

OFF OFF


Figure 18.18-2




Myoblast

(determined)

DNA

g y

OFF OFF

OFFmRNA

MyoD protein(transcription

factor)


Figure 18.18-3




Myoblast

(determined)

Part of a muscle fiber (fully differentiated cell)

DNA

g y

OFF OFF

OFFmRNA

MyoD protein(transcription

factor)

mRNA mRNA mRNA mRNA

MyoD Another transcriptionfactor

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

• Positional information, the molecular cues that

control pattern formation, tells a cell its location

relative to the body axes and to neighboring cells




•

Pattern formation has been extensively studied inthe fruit fly Drosophila melanogaster

• Combining anatomical, genetic, and biochemical

approaches, researchers have discovered

developmental principles common to many other species, including humans


The Life Cycle of Drosophila



The Life Cycle of Drosophila

•

In Drosophila, cytoplasmic determinants in theunfertilized egg determine the axes before

fertilization

• After fertilization, the embryo develops into a

segmented larva with three larval stages


Head Thorax AbdomenEggdeveloping withinovarian follicle

Follicle cellNucleus

Egg

1Figure 18.19



0.5 mm

BODYAXES

Anterior

LeftVentral

Dorsal Right

Posterior

(a) Adult

ovarian follicle

Nurse cell

Egg

Unfertilized egg

Depletednurse cells

Eggshell

Fertilization

Laying of egg

Fertilized egg

Embryonicdevelopment

Segmentedembryo

Bodysegments

Hatching0.1 mm

Larval stage

(b) Development from egg to larva

2

3

4

5

Figure 18.19a



Head Thorax Abdomen

0.5 mm

BODYAXES

Anterior

LeftVentral

DorsalRight

Posterior

(a) Adult

Figure 18.19b

Eggdeveloping withinovarian follicle

Follicle cellNucleus

Egg

1



ovarian follicle

Nurse cell

Egg

Unfertilized egg

Depletednurse cells

Eggshell

Fertilization

Laying of egg

Fertilized egg

Embryonicdevelopment

Segmentedembryo

Body segments Hatching0.1 mm

Larval stage

(b) Development from egg to larva

5

4

3

2

Genetic Analysis of Early Development:



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 discovered the homeotic genes, which

control pattern formation in late embryo, larva,

and adult stages


Figure 18.20



Wild type Mutant

Eye

Antenna

Leg

Figure 18.20a



Wild type

Eye

Antenna

Figure 18.20b



Mutant

Leg



•

Nüsslein-Volhard and Wieschaus studied segmentformation

• They created mutants, conducted breedingexperiments, and looked for corresponding genes

• Many of the identified mutations were embryoniclethals, causing death during embryogenesis

• They found 120 genes essential for normalsegmentation


Axis Establishment



Axis Establishment

•

Maternal effect genes encode for cytoplasmicdeterminants that initially establish the axes of the

body of Drosophila

• These maternal effect genes are also called egg-

polarity genes because they control orientation of the egg and consequently the fly





Animation: Development of Head-Tail Axis in Fruit Flies




• One maternal effect gene, the bicoid gene, affects

the front half of the body

• An embryo whose mother has no functional bicoid

gene lacks the front half of its body and has

duplicate posterior structures at both ends

Bicoid: A Morphogen Determining HeadStructures


Figure 18.21

Head Tail



Head Tail

Tail Tail

Wild-type larva

Mutant larva (bicoid )

250 µm

T1 T2T3

A1 A2 A3 A4 A5A6

A7A8

A8

A7A6A7A8

Figure 18.21a



Head Tail

Wild-type larva250 µm

T1 T2T3

A1 A2 A3 A4 A5A6

A7 A8

Figure 18.21b



Tail Tail

Mutant larva (bicoid )

A8

A7A6A7A8



•

This phenotype suggests that the product of themother’s bicoid gene is concentrated at the future

anterior end

• This hypothesis is an example of the morphogen

gradient hypothesis, in which gradients of substances called morphogens establish an

embryo’s axes and other features


Figure 18.22



Bicoid mRNA in mature

unfertilized egg

Bicoid mRNA in matureunfertilized egg

Fertilization,translation of bicoid mRNA

Anterior end

100 µm

Bicoid protein in

early embryo

Bicoid protein inearly embryo

RESULTS

Figure 18.22a



Bicoid mRNA in matureunfertilized egg

Figure 18.22b



Bicoid protein inearly embryo

Anterior end

100 µm



•

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 – It demonstrated a key developmental principle that

a gradient of molecules can determine polarity and

position in the embryo


Concept 18.5: Cancer results from genetic



p g

changes that affect cell cycle control

• The gene regulation systems that go wrong during

cancer are the very same systems involved in

embryonic development


Types of Genes Associated with Cancer



yp

•

Cancer can be caused by mutations to genes thatregulate cell growth and division

• Tumor viruses can cause cancer in animals

including humans




• 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 oncogenecan lead to abnormal stimulation of the cell cycle


Figure 18.23



Proto-oncogene

DNA

Translocation or transposition: genemoved to new locus,under new controls

Gene amplification:multiple copies of the gene

Newpromoter

Normal growth-stimulating

protein in excess

Normal growth-stimulatingprotein in excess

Point mutation:

within a controlelement

withinthe gene

Oncogene Oncogene

Normal growth-stimulating

protein inexcess

Hyperactive or degradation-resistantprotein



• 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: cause an increase in gene expression


Tumor-Suppressor Genes



pp

• Tumor-suppressor genes help preventuncontrolled cell growth

• 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


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 productionof a hyperactive Ras protein and increased cell

division


Figure 18.24



Growthfactor

1

2

3

4

5

1

2

Receptor

G protein

Protein kinases(phosphorylationcascade)

NUCLEUS

Transcriptionfactor (activator)

DNA

Gene expression

Protein thatstimulatesthe cell cycle

Hyperactive Ras protein(product of oncogene)

issues signals on itsown.

(a) Cell cycle–stimulating pathway

MUTATION

Ras

Ras

GTP

GTP

P

P

P P

P

P

(b) Cell cycle–inhibiting pathway

Protein kinases

UVlight

DNA damagein genome

Activeformof p53

DNA

Protein thatinhibitsthe cell cycle

Defective or missing

transcription factor,

such as

p53, cannot

activate

transcription.

MUTATION

EFFECTS OF MUTATIONS

(c) Effects of mutations

Proteinoverexpressed

Cell cycleoverstimulated Increased celldivision

Protein absent

Cell cycle notinhibited

3

Growthfactor

1

G i

Hyperactive Ras protein(product of oncogene)

MUTATION

Ras

Figure 18.24a



Receptor

G protein

Protein kinases(phosphorylationcascade)

NUCLEUSTranscriptionfactor (activator)

DNA

Gene expression

Protein thatstimulatesthe cell cycle

(product of oncogene)issues signals on itsown.

(a) Cell cycle–stimulating pathway

GTP

PP

PP

P

P

2

3

4

5

Ras

GTP

Figure 18.24b

Protein kinasesMUTATION

2



(b) Cell cycle–inhibiting pathway

UVlight

DNA damagein genome

Activeformof p53

DNA

Protein thatinhibitsthe cell cycle

Defective or missing

transcription factor,such as

p53, cannot

activate

transcription.

MUTATION

1

3



• 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


Figure 18.24c



EFFECTS OF MUTATIONS

(c) Effects of mutations

Proteinoverexpressed

Cell cycleoverstimulated

Increased celldivision

Protein absent

Cell cycle notinhibited

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 usuallycharacterized by at least one active oncogene and

the mutation of several tumor-suppressor genes


Figure 18.25



Colon

Normal colonepithelial cells

Lossof tumor-

suppressor gene APC (or other)

1

2

3

4

5Colon wall

Small benigngrowth(polyp)

Activationof rasoncogene

Lossof tumor-suppressor gene DCC

Loss

of tumor-suppressor gene p53

Additionalmutations

Malignanttumor (carcinoma)

Larger benign growth(adenoma)

Figure 18.25a

Colon



Colon

Normal colonepithelial cells

Colon wall

Figure 18.25b



Small benigngrowth (polyp)

Loss of tumor-suppressor gene APC (or other)

1

Figure 18.25c



Activation of ras oncogene

Loss of tumor-suppressor gene DCC

Larger benigngrowth (adenoma)

2

3

Figure 18.25d



Loss of tumor-suppressor gene p53

Additionalmutations

Malignant tumor (carcinoma)

4

5

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 inindividuals with colorectal cancer

• Mutations in the BRCA1 or BRCA2 gene are found

in at least half of inherited breast cancers, andtests using DNA sequencing can detect these

mutations


Figure 18.26



Figure 18.UN01



Operon

Promoter Genes

RNApolymerase

Operator

Polypeptides

A B C

A B C

Figure 18.UN02



Genes expressed Genes not expressed

Promoter

Genes

Operator

Corepressor Inactive repressor:no corepressor present

Active repressor:corepressor bound

Figure 18.UN03



Genes expressedGenes not expressedPromoter

GenesOperator

Active repressor:no inducer present

Inactive repressor:inducer bound

Figure 18.UN04Chromatin modification

• Genes in highly compactedchromatin are generally nottranscribed.

• Histone acetylation seemst l h ti t t

• Regulation of transcription initiation:DNA control elements in enhancers bindspecific transcription factors.

Transcription



to loosen chromatin structure,enhancing transcription.

• DNA methylation generallyreduces transcription.

mRNA degradation

• Each mRNA has acharacteristic life span,determined in part bysequences in the 5′ and3′ UTRs.

Bending of the DNA enables activators tocontact proteins at the promoter, initiatingtranscription.

• Coordinate regulation:

Enhancer for liver-specific genes

Enhancer for lens-specific genes

RNA processing

• Alternative RNA splicing:

Primary RNAtranscript

mRNA or

• Initiation of translation can be controlled

via regulation of initiation factors.

• Protein processing anddegradation by proteasomesare subject to regulation.

Translation

Protein processing and degradation

Chromatin modification

Transcription

RNA processing

mRNAdegradation

Translation

Protein processingand degradation


• Genes in highly compactedchromatin are generally nottranscribed

• Regulation of transcription initiation:DNA control elements in enhancers bindspecific transcription factors

Transcription

Figure 18.UN04a



transcribed.

• Histone acetylation seemsto loosen chromatin structure,

enhancing transcription.

• DNA methylation generallyreduces transcription.

specific transcription factors.

Bending of the DNA enables activators tocontact proteins at the promoter, initiatingtranscription.

• Coordinate regulation:

Enhancer for liver-specific genes Enhancer for lens-specific genes

RNA processing

• Alternative RNA splicing:

Primary RNAtranscript

mRNA or


Transcription

RNA processing

mRNAdegradation

Translation



Figure 18.UN04b



mRNA degradation

• Each mRNA has acharacteristic life span,determined in part bysequences in the 5′ and3′ UTRs.

• Initiation of translation can be controlledvia regulation of initiation factors.

• Protein processing anddegradation by proteasomesare subject to regulation.

Translation

Protein processing and degradation

Transcription

RNA processing

mRNAdegradation

Translation


Figure 18.UN05



• Small or large noncoding RNAs canpromote the formation of heterochromatin



Transcription

RNA processing

mRNAdegradation

Translation


Translation

mRNA degradation

• miRNA or siRNA can target specificmRNAs for destruction.

• miRNA or siRNA can block the translationof specific mRNAs.

pin certain regions, blocking transcription.

Figure 18.UN06

Enhancer Promoter



Gene 1

Gene 2

Gene 3

Gene 4

Gene 5

Figure 18.UN07

E h P t



Enhancer Promoter

Gene 1

Gene 2

Gene 3

Gene 4

Gene 5

Figure 18.UN08



Enhancer Promoter

Gene 1

Gene 2

Gene 3

Gene 4

Gene 5

18 lecture presentation pc

Documents

repressor active

protein repressor

repressor inactive

lac repressor

repressible operon

trp repressor protein

operon model

trpa gene tryptophan