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    Regulation of Gene Expression

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    Overview: Conducting the Genetic Orchestra

    Prokaryotes and eukaryotes alter gene expression inresponse 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

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    Precursor

    Feedbackinhibition

    Enzyme 1

    Enzyme 2

    Enzyme 3

    Tryptophan

    (a) (b)Regulation of enzyme

    activity

    Regulation of enzyme

    production

    Regulationof geneexpression

    trpEgene

    trpD gene

    trpCgene

    trpB gene

    trpA gene

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    Operons: The Basic Concept

    A cluster of functionally related genes can be undercoordinated controlby 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

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    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 regulatorygene

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    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. colican synthesize the amino acid

    tryptophan

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    By default the trp operon is on and the genes fortryptophan 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

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    Promoter

    DNA

    Regulatorygene

    mRNA

    trpR

    53

    Protein Inactiverepressor

    RNApolymerase

    Promoter

    trp operon

    Genes of operon

    Operator

    mRNA 5Start 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

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    Promoter

    DNA

    Regulatorygene

    mRNA

    trpR

    53

    Protein Inactiverepressor

    RNApolymerase

    Promoter

    trp operon

    Genes of operon

    Operator

    mRNA 5Start 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

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    Figure 18.3b-1

    (b) Tryptophan present, repressor active, operon off

    DNA

    mRNA

    Protein

    Tryptophan(corepressor)

    Activerepressor

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    (b) Tryptophan present, repressor active, operon off

    DNA

    mRNA

    Protein

    Tryptophan(corepressor)

    Activerepressor

    No RNAmade

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    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

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    The lac operon is an inducible operon and containsgenes 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

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    (a) Lactose absent, repressor active, operon off

    (b) Lactose present, repressor inactive, operon on

    Regulatorygene

    Promoter

    Operator

    DNA lacZlacI

    lacI

    DNA

    mRNA5

    3NoRNAmade

    RNApolymerase

    ActiverepressorProtein

    lacoperon

    lacZ lacY lacADNA

    mRNA

    53

    Protein

    mRNA 5

    Inactiverepressor

    RNA polymerase

    Allolactose(inducer)

    -Galactosidase Permease Transacetylase

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    (a) Lactose absent, repressor active, operon off

    Regulatorygene

    Promoter

    Operator

    DNA lacZlacIDNA

    mRNA

    5

    3NoRNAmade

    RNApolymerase

    ActiverepressorProtein

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    (b) Lactose present, repressor inactive, operon on

    lacI

    lacoperon

    lacZ lacY lacADNA

    mRNA

    53

    Protein

    mRNA 5

    Inactiverepressor

    RNA polymerase

    Allolactose

    (inducer)

    -Galactosidase Permease Transacetylase

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    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 are switched off bythe active form of the repressor

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    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 ofE. 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

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    When glucose levels increase, CAP detaches from thelac operon, and transcription returns to a normal rate

    CAP helps regulate other operons that encode enzymes

    used in catabolic pathways

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    Promoter

    DNA

    CAP-binding site

    lacZlacI

    RNApolymerase

    binds andtranscribes

    Operator

    cAMPActiveCAP

    InactiveCAP

    Allolactose

    Inactive lacrepressor

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

    Promoter

    DNA

    CAP-binding site

    lacZlacI

    Operator

    RNApolymerase lesslikely to bind

    Inactive lacrepressor

    InactiveCAP

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

    little lacmRNA synthesized

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    Promoter

    DNA

    CAP-binding site

    lacZlacI

    RNApolymerasebinds andtranscribes

    Operator

    cAMP

    ActiveCAP

    Inactive

    CAP

    Allolactose

    Inactive lacrepressor

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

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    Promoter

    DNA

    CAP-binding site

    lacZlacI

    Operator

    RNApolymerase less

    likely to bind

    Inactive lacrepressor

    InactiveCAP

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

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    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

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    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

    Si l

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    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)

    Si l

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    Signal

    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

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    CYTOPLASM

    mRNA in cytoplasm

    TranslationDegradationof mRNA

    Polypeptide

    Protein processing, such

    as cleavage andchemical modification

    Active proteinDegradation

    of protein

    Transport to cellulardestination

    Cellular function (suchas enzymatic activity,structural support)

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    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

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    Histone Modifications

    In histone acetylation, acetyl groups are attached topositively 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

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    Amino acidsavailablefor chemicalmodification

    Histonetails

    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

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    The histone code hypothesis proposes that specificcombinations of modifications, as well as the order in

    which they occur, help determine chromatin

    configuration and influence transcription

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    DNA Methylation

    DNA methylation, the addition of methyl groups tocertain 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

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    Epigenetic Inheritance

    Although the chromatin modifications just discusseddo 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

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    Regulation of Transcription Initiation

    Chromatin-modifying enzymes provide initial control ofgene expression by making a region of DNA either

    more or less able to bind the transcription machinery

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    Organization of a Typical Eukaryotic

    Gene

    Associated with most eukaryotic genes are multiplecontrol 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

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    Enhancer

    (distal controlelements)

    DNA

    UpstreamPromoter

    Proximal

    controlelements

    Transcriptionstart site

    Exon Intron Exon ExonIntron

    Poly-A

    signalsequence

    Transcriptiontermination

    region

    Downstream

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    Enhancer

    (distal controlelements)

    DNA

    UpstreamPromoter

    Proximal

    controlelements

    Transcriptionstart site

    Exon Intron Exon ExonIntron

    Poly-A

    signalsequence

    Transcriptiontermination

    region

    DownstreamPoly-Asignal

    Exon Intron Exon ExonIntron

    Transcription

    Cleaved

    3 end ofprimarytranscript

    5Primary RNA

    transcript(pre-mRNA)

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    Enhancer

    (distal controlelements)

    DNA

    UpstreamPromoter

    Proximal

    controlelements

    Transcriptionstart site

    Exon Intron Exon ExonIntron

    Poly-A

    signalsequence

    Transcriptiontermination

    region

    DownstreamPoly-Asignal

    Exon Intron Exon ExonIntron

    Transcription

    Cleaved

    3 end ofprimarytranscript

    5Primary RNA

    transcript(pre-mRNA)

    Intron RNA

    RNA processing

    mRNA

    Coding segment

    5 Cap 5 UTR Startcodon Stopcodon 3 UTR3

    Poly-Atail

    PPPG AAA AAA

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    The Roles of Transcription Factors

    To initiate transcription, eukaryotic RNA polymeraserequires 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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    Proximal control elements are located close to the

    promoter

    Distal control elements, groupings of which are called

    enhancers, may be far away from a gene or even

    located in an intron

    Enhancers and Specific Transcription Factors

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    An activator is a protein that binds to an enhancer andstimulates 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

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    DNA

    Activationdomain

    DNA-binding

    domain

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    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

    Promoter

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    Activators

    DNA

    EnhancerDistal controlelement

    PromoterGene

    TATA box

    Promoter

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    Activators

    DNA

    EnhancerDistal controlelement

    PromoterGene

    TATA box

    General

    transcriptionfactors

    DNA-bendingprotein

    Group of mediator proteins

    Promoter

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    Activators

    DNA

    EnhancerDistal controlelement

    PromoterGene

    TATA box

    General

    transcriptionfactors

    DNA-bendingprotein

    Group of mediator proteins

    RNApolymerase II

    RNApolymerase II

    RNA synthesisTranscription

    initiation complex

    Enhancer Promoter

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    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

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    Controlelements

    Enhancer Promoter

    Albumin gene

    Crystallingene

    LIVER CELLNUCLEUS

    Availableactivators

    Albumin geneexpressed

    Crystallin genenot expressed

    (a) Liver cell

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    Controlelements

    Enhancer Promoter

    Albumin gene

    Crystallingene

    LENS CELLNUCLEUS

    Availableactivators

    Albumin genenot expressed

    Crystallin geneexpressed

    (b) Lens cell

    Coordinately Controlled Genes in

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    Coordinately Controlled Genes in

    Eukaryotes

    Unlike the genes of a prokaryotic operon, each of theco-expressed eukaryotic genes has a promoter and

    control elements

    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 ofthe genes

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    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

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    Chromosometerritory

    Chromosomes in theinterphase nucleus

    Chromatinloop

    Transcriptionfactory

    10 m

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    Chromosomes in theinterphase nucleus

    10 m

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    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

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    RNA Processing

    In alternative RNA splicing, different mRNA moleculesare produced from the same primary transcript,

    depending on which RNA segments are treated as

    exons and which as introns

    E

    Figure 18.13

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    Exons

    DNA

    Troponin T gene

    PrimaryRNAtranscript

    RNA splicing

    ormRNA

    1

    1

    1 1

    2

    2

    2 2

    3

    3

    3

    4

    4

    4

    5

    5

    5 5

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    mRNA Degradation

    The life span of mRNA molecules in the cytoplasm is akey 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

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    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

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    Protein Processing and Degradation

    After translation, various types of protein processing,including cleavage and the addition of chemical groups,

    are subject to control

    Proteasomes are giant protein complexes that bind

    protein molecules and degrade them

    Figure 18.14

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    Protein tobe degraded

    Ubiquitin

    Ubiquitinatedprotein

    Proteasome

    Protein enteringa proteasome

    Proteasomeand ubiquitinto be recycled

    Proteinfragments(peptides)

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    Noncoding RNAs play multiple roles in

    controlling gene expression

    Only a small fraction of DNA codes for proteins, and avery small fraction of the non-protein-coding DNAconsists of genes for RNA such as rRNA and tRNA

    A significant amount of the genome may betranscribed into noncoding RNAs (ncRNAs)

    Noncoding RNAs regulate gene expression at twopoints: mRNA translation and chromatin configuration

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    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

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    (a) Primary miRNA transcript

    Hairpin

    miRNA

    miRNA

    Hydrogenbond

    Dicer

    miRNA-

    proteincomplex

    mRNA degraded Translation blocked

    (b) Generation and function of miRNAs

    5 3

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    The phenomenon of inhibition of gene expression byRNA 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

    h d l d ff

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    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

    h l i Si ifi f S ll

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    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

    A f diff ti l

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    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 i P f E b i

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    A Genetic Program for Embryonic

    Development

    The transformation from zygote to adult results from

    cell division, cell differentiation, and morphogenesis

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

    1 mm 2 mm

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

    The physical processes that give an organism its shapeconstitute morphogenesis

    Differential gene expression results from genes beingregulated differently in each cell type

    Materials in the egg can set up gene regulation that iscarried out as cells divide

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

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    Cytoplasmic Determinants and Inductive

    Signals

    An eggs 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 contain different

    cytoplasmic determinants, which lead to different geneexpression

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    (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

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    Unfertilized egg

    Sperm

    Fertilization

    Zygote(fertilized egg)

    Mitotic

    cell division

    Two-celledembryo

    Nucleus

    Molecules of twodifferent cytoplasmicdeterminants

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    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 transcriptional changes innearby target cells

    Thus, interactions between cells induce differentiation

    of specialized cell types

    (b) Induction by nearby cells

    Early embryo

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    Early embryo(32 cells)

    NUCLEUS

    Signaltransductionpathway

    Signalreceptor

    Signalingmolecule(inducer)

    Sequential Regulation of Gene

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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

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    Myoblastsproduce muscle-specific proteins and formskeletal 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

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    Embryonicprecursor cell

    DNA

    gene myoD

    OFF OFF

    Other muscle specific genes

    Nucleus Master regulatorygene myoD Other muscle-specific genes

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    Embryonicprecursor cell

    Myoblast

    (determined)

    DNA

    gene myoD

    OFF OFF

    OFFmRNA

    p g

    MyoD protein(transcription

    factor)

    Nucleus Master regulatorygene myoD Other muscle-specific genes

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    Embryonicprecursor cell

    Myoblast

    (determined)

    Part of a muscle fiber(fully differentiated cell)

    DNA

    gene myoD

    OFF OFF

    OFFmRNA

    p g

    MyoD protein(transcription

    factor)

    mRNA mRNA mRNA mRNA

    MyoD Anothertranscriptionfactor

    Myosin, othermuscle proteins,and cell cycleblocking proteins

    Pattern Formation: Setting Up the Body

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    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

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    Pattern formation has been extensively studied in thefruit fly Drosophila melanogaster

    Combining anatomical, genetic, and biochemical

    approaches, researchers have discovered

    developmental principles common to many otherspecies, including humans

    The Life Cycle of Drosophila

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    The Life Cycle ofDrosophila

    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

    2011 Pearson Education, Inc.

    Head Thorax AbdomenEggdeveloping within

    Follicle cellNucleus1

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    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

    Hatching

    0.1 mm

    Larval stage

    (b) Development from egg to larva

    2

    3

    4

    5

    Genetic Analysis of Early Development:

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    Genetic Analysis of Early Development:

    Scientific Inquiry

    Edward B. Lewis, Christiane Nsslein-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

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    Wild type Mutant

    Eye

    Antenna

    Leg

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    Nsslein-Volhard and Wieschaus studied segmentformation

    They created mutants, conducted breeding

    experiments, and looked for corresponding genes

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

    They found 120 genes essential for normal

    segmentation

    Axis Establishment

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    Axis Establishment

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

    body ofDrosophila

    These maternal effect genes are also called egg-

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

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    One maternal effect gene, the bicoidgene, 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

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    Head Tail

    Tail Tail

    Wild-type larva

    Mutant larva (bicoid)

    250 m

    T1 T2 T3

    A1 A2 A3 A4A5

    A6A7

    A8

    A8

    A7A6A7A8

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    Head Tail

    Wild-type larva250 m

    T1 T2 T3

    A1 A2 A3 A4A5

    A6A7

    A8

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    Tail Tail

    Mutant larva (bicoid)

    A8

    A7A6A7A8

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    This phenotype suggests that the product of themothers bicoidgene is concentrated at the future

    anterior end

    This hypothesis is an example of the morphogen

    gradient hypothesis, in which gradients of substancescalled morphogens establish an embryos axes and

    other features

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    BicoidmRNA in mature

    unfertilized egg

    BicoidmRNA in matureunfertilized egg

    Fertilization,translation ofbicoidmRNA

    Anterior end

    100 m

    Bicoid protein in

    early embryo

    Bicoid protein inearly embryo

    RESULTS

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    BicoidmRNA in matureunfertilized egg

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    Bicoid protein inearly embryo

    Anterior end

    100 m

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    The bicoidresearch is important for three reasons It identified a specific protein required for some early

    steps in pattern formation

    It increased understanding of the mothers role in

    embryo development It demonstrated a key developmental principle that a

    gradient of molecules can determine polarity and

    position in the embryo

    Cancer results from genetic changes

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    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

    Types of Genes Associated with Cancer

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    Types of Genes Associated with Cancer

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

    Tumor viruses can cause cancer in animals including

    humans

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    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 canlead to abnormal stimulation of the cell cycle

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    Proto-oncogene

    DNA

    Translocation ortransposition: genemoved to new locus,under new controls

    Gene amplification:multiple copies ofthe gene

    Newpromoter

    Normal growth-stimulatingprotein in excess

    Normal growth-stimulatingprotein in excess

    Point mutation:

    within a controlelement

    withinthe gene

    Oncogene Oncogene

    Normal growth-stimulatingprotein inexcess

    Hyperactive ordegradation-resistantprotein

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    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

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    Tumor Suppressor Genes

    Tumor-suppressor genes help prevent uncontrolledcell 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

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    Interference with Normal Cell Signaling

    Pathways

    Mutations in the ras proto-oncogene andp53 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

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    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 cyclestimulating pathway

    MUTATION

    Ras

    Ras

    GTP

    GTP

    P

    P

    P P

    P

    P

    (b) Cell cycleinhibiting pathway

    Protein kinases

    UVlight

    DNA damagein genome

    Activeformof p53

    DNA

    Protein thatinhibitsthe cell cycle

    Defective or missingtranscription 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

    Hyperactive Ras protein

    MUTATION

    Ras

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    Receptor

    G protein

    Protein kinases(phosphorylationcascade)

    NUCLEUSTranscriptionfactor (activator)

    DNA

    Gene expression

    Protein thatstimulatesthe cell cycle

    yp p(product of oncogene)issues signals on itsown.

    (a) Cell cyclestimulating pathway

    GTP

    PP

    PP

    P

    P

    2

    3

    4

    5

    Ras

    GTP

    Protein kinases2

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    (b) Cell cycleinhibiting 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

    1

    3

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    Suppression of the cell cycle can be important in the

    case of damage to a cells DNA;p53 prevents a cell

    from passing on mutations due to DNA damage

    Mutations in thep53 gene prevent suppression of the

    cell cycle

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    EFFECTS OF MUTATIONS

    (c) Effects of mutations

    Proteinoverexpressed

    Cell cycleoverstimulated

    Increased celldivision

    Protein absent

    Cell cycle notinhibited

    The Multistep Model of Cancer

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    p

    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 and the

    mutation of several tumor-suppressor genes

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    Colon

    Normal colonepithelial cells

    Lossof tumor-

    suppressorgeneAPC(or other)

    1

    2

    3

    4

    5Colon wall

    Small benigngrowth(polyp)

    Activationofrasoncogene

    Lossof tumor-suppressorgene DCC

    Loss

    of tumor-suppressorgenep53

    Additionalmutations

    Malignanttumor(carcinoma)

    Largerbenign growth(adenoma)

    Inherited Predisposition and Other

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    p

    Factors Contributing to Cancer

    Individuals can inherit oncogenes or mutant alleles of

    tumor-suppressor genes

    Inherited mutations in the tumor-suppressor gene

    adenomatous polyposis coliare common in individualswith colorectal cancer

    Mutations in the BRCA1 or BRCA2 gene are found in at

    least half of inherited breast cancers, and tests usingDNA sequencing can detect these mutations