18 ge gene expression lecture presentation

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Control of Gene Expression

BIOLOGYA Global Approach

Campbell Reece Urry Cain Wasserman Minorsky Jackson 2015 Pearson Education Ltd TENTH EDITION

Global Edition

Lecture Presentation by Nicole Tunbridge andKathleen Fitzpatrick18

2015 Pearson Education Ltd

Differential Expression of GenesProkaryotes and eukaryotes precisely regulate gene expression in response to environmental conditionsIn multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell typesRNA molecules play many roles in regulating gene expression in eukaryotes


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Figure 18.1

2015 Pearson Education Ltd3Figure 18.1 How can this fishs eyes see equally well in both air and water?

Figure 18.1a

2015 Pearson Education Ltd4Figure 18.1a How can this fishs eyes see equally well in both air and water? (part 1: full view)

Concept 18.1: Bacteria often respond to environmental change by regulating transcriptionNatural selection has favored bacteria that produce only the products needed by that cellA cell can regulate the production of enzymes by feedback inhibition or by gene regulationOne mechanism for control of gene expression in bacteria is the operon model


Figure 18.2Precursor

FeedbackinhibitionTryptophan(b) Regulation of enzymeproduction(a) Regulation of enzymeactivityRegulationof geneexpressiontrpEtrpDtrpCtrpBtrpAEnzyme 1Enzyme 2Enzyme 3

2015 Pearson Education Ltd6Figure 18.2 Regulation of a metabolic pathway

Operons: The Basic ConceptA cluster of functionally related genes can be coordinately controlled by a single on-off switchThe switch is a segment of DNA called an operator usually positioned within the promoterAn 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 repressorThe repressor prevents gene transcription by binding to the operator and blocking RNA polymeraseThe repressor is the product of a separate regulatory gene


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The repressor can be in an active or inactive form, depending on the presence of other moleculesA corepressor is a molecule that cooperates with a repressor protein to switch an operon offFor example, E. coli can synthesize the amino acid tryptophan when it has insufficient tryptophan


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By default the trp operon is on and the genes for tryptophan synthesis are transcribedWhen 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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Figure 18.3PromoterDNAtrpRRegulatory geneRNApolymerasemRNA53ProteinInactiverepressormRNA 5(a) Tryptophan absent, repressor inactive, operon onDNAmRNAProteinActiverepressorNoRNAmade

Promotertrp operonGenes of operontrpEtrpDtrpCtrpBtrpAOperatorStart codonStop codontrpRtrpETryptophan(corepressor)(b) Tryptophan present, repressor active, operon off35Polypeptide subunitsthat make up enzymesfor tryptophan synthesisEDCB A

2015 Pearson Education Ltd11Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes

Figure 18.3a

PromoterDNAtrpRRegulatory geneRNApolymerasemRNA53ProteinInactiverepressormRNA 5(a) Tryptophan absent, repressor inactive, operon onPromotertrp operonGenes of operontrpEtrpDtrpCtrpBtrpAOperatorStart codonStop codonPolypeptide subunits that make upenzymes for tryptophan synthesisEDCB A

2015 Pearson Education Ltd12Figure 18.3a The trp operon in E. coli: regulated synthesis of repressible enzymes (part 1: tryptophan absent)

Figure 18.3bDNAmRNAProteinActiverepressortrpRtrpETryptophan(corepressor)(b) Tryptophan present, repressor active, operon off35NoRNAmade

2015 Pearson Education Ltd13Figure 18.3b The trp operon in E. coli: regulated synthesis of repressible enzymes (part 2: tryptophan present)

Repressible and Inducible Operons:Two Types of Negative Gene RegulationA repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcriptionThe trp operon is a repressible operonAn 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 and contains genes that code for enzymes used in the hydrolysis and metabolism of lactoseBy itself, the lac repressor is active and switches the lac operon offA molecule called an inducer inactivates the repressor to turn the lac operon on


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Figure 18.4

PromoterDNARegulatory genemRNA53

OperatorRNApolymeraseActiverepressorNo RNAmadeIacZlacZlacYlacA(a) Lactose absent, repressor active, operon off(b) Lactose present, repressor inactive, operon on53DNAlac operonRNA polymeraseStart codonStop codonmRNA3ProteinProteinInactiverepressorAllolactose(inducer)mRNA 5

PermeaseTransacetylase-GalactosidaselacIlacI

2015 Pearson Education Ltd16Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes

Figure 18.4aPromoterDNARegulatory genemRNA53OperatorRNApolymeraseActiverepressorNo RNAmadeIacZ(a) Lactose absent, repressor active, operon offProteinlacI

2015 Pearson Education Ltd17Figure 18.4a The lac operon in E. coli: regulated synthesis of inducible enzymes (part 1: lactose absent)

Figure 18.4b

lacZlacYlacA(b) Lactose present, repressor inactive, operon on5DNARNA polymerasemRNA3ProteinInactiverepressorAllolactose(inducer)mRNA 5

lacIStart codonStop codonPermeaseTransacetylase-Galactosidaselac operon

2015 Pearson Education Ltd18Figure 18.4b The lac operon in E. coli: regulated synthesis of inducible enzymes (part 2: lactose present)

Video: Cartoon Rendering of the lac Repressor from E. coli


Inducible enzymes usually function in catabolic pathways; their synthesis is induced by achemical signalRepressible enzymes usually function in anabolic pathways; their synthesis is repressed by high levels of the end productRegulation of the trp and lac operons involves negative control of genes because operons are switched off by the active form of the repressor


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Positive Gene RegulationSome operons are also subject to positive control through a stimulatory protein, such as catabolite activator protein (CAP), an activator of transcriptionWhen 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


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When glucose levels increase, CAP detaches from the lac operon, and transcription returns to a normal rateCAP helps regulate other operons that encode enzymes used in catabolic pathways


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Figure 18.5PromoterDNAOperator

PromoterDNACAP-binding sitecAMPActiveCAPInactiveCAPRNApolymerasebinds and transcribeslacIlacIAllolactoseInactive lacrepressor(a) Lactose present, glucose scarce (cAMP level high):abundant lac mRNA synthesizedlacZlacZCAP-binding siteRNApolymerase lesslikely to bindOperatorInactiveCAPInactive lacrepressor(b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized

2015 Pearson Education Ltd23Figure 18.5 Positive control of the lac operon by catabolite activator protein (CAP)

Figure 18.5aDNA

PromoterOperatorCAP-binding sitecAMPActiveCAPInactiveCAPRNApolymerasebinds and transcribeslacIAllolactoseInactive lacrepressor(a) Lactose present, glucose scarce (cAMP level high):abundant lac mRNA synthesizedlacZ

2015 Pearson Education Ltd24Figure 18.5a Positive control of the lac operon by catabolite activator protein (CAP) (part 1: glucose scarce)

Figure 18.5b

PromoterDNAlacZCAP-binding siteRNApolymerase lesslikely to bindOperatorInactiveCAPInactive lacrepressor(b) Lactose present, glucose present (cAMP level low):little lac mRNA synthesizedlacI

2015 Pearson Education Ltd25Figure 18.5b Positive control of the lac operon by catabolite activator protein (CAP) (part 2: glucose present)

Concept 18.2: Eukaryotic gene expression is regulated at many stagesAll organisms must regulate which genes are expressed at any given timeIn multicellular organisms regulation of gene expression is essential for cell specialization


Differential Gene ExpressionAlmost all the cells in an organism are genetically identicalDifferences between cell types result from differential gene expression, the expression of different genes by cells with the same genomeAbnormalities in gene expression can lead to diseases including cancerGene expression is regulated at many stages


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Figure 18.6SignalChromatinDNAGene available for transcriptionRNAExonIntronCapPrimarytranscriptTailmRNA in nucleusNUCLEUSTranscriptionRNA processingTransport tocytoplasmChromatinmodification:DNA unpacking

CYTOPLASMmRNA in cytoplasmTranslationDegradationof mRNAPolypeptideProtein processingActive proteinDegradationof proteinTransport to cellulardestinationCellular function(such as enzymaticactivity or structuralsupport)

2015 Pearson Education Ltd28Figure 18.6 Stages in gene expression that can be regulated in eukaryotic cells

Figure 18.6a

SignalChromatinDNAGene available for transcriptionRNAExonIntronCapPrimarytranscriptTailmRNA in nucleusNUCLEUSTranscriptionRNA processingTransport tocytoplasmChromatinmodification:DNA unpackingCYTOPLASM

2015 Pearson Education Ltd29Figure 18.6a Stages in gene expression that can be regulated in eukaryotic cells (part 1: nucleus)

Figure 18.6bCYTOPLASMmRNA in cytoplasmTranslationDegradationof mRNAPolypeptideProtein processingActive proteinDegradationof proteinTransport to cellulardestinationCellular function(such as enzymaticactivity or structuralsupport)

2015 Pearson Education Ltd30Figure 18.6b Stages in gene expression that can be regulated in eukaryotic cells (part 2: cytoplasm)

Animation: Protein Degradation


Animation: Protein Processing


Animation: Blocking Translation


Regulation of Chromatin StructureThe structural organization of chromatin helps regulate gene expression in several waysGenes within highly packed heterochromatin are usually not expressedChemical modifications to histones and DNA of chromatin influence both chromatin structure and gene expression


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Histone Modifications and DNA MethylationIn histone acetylation, acetyl groups are attached to positively charged lysines in histone tailsThis loosens chromatin structure, thereby promoting the initiation of transcriptionThe addition of methyl groups (methylation) can condense chromatin; the addition of phosphate groups (phosphorylation) next to a methylated amino acid can loosen chromatin


Figure 18.7HistonetailsDNA doublehelixAmino acidsavailablefor chemicalmodificationNucleosome(end view)(a) Histone tails protrude outward from a nucleosome(b) Acetylation of histone tails promotes loose chromatinstructure that permits transcriptionAcetylated histonesUnacetylated histones(side view)AcetylgroupsDNA

2015 Pearson Education Ltd36Figure 18.7 A simple model of histone tails and the effect of histone acetylation

DNA methylation, the addition of methyl groups to certain bases in DNA, is associated with reduced transcription in some speciesDNA methylation can cause long-term inactivation of genes in cellular differentiationIn 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 InheritanceAlthough the chromatin modifications just discussed do not alter DNA sequence, theymay be passed to future generations of cellsThe inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence is called epigenetic inheritance


Regulation of Transcription InitiationChromatin-modifying enzymes provide initial control of gene expression by making a region of DNA either more or less able to bind the transcription machinery


Organization of a Typical Eukaryotic GeneAssociated with most eukaryotic genes are multiple control elements, segments of noncoding DNA that serve as binding sites for transcription factors that help regulate transcriptionControl elements and the transcription factors they bind are critical to the precise regulation of gene expression in different cell types


Figure 18.8Enhancer (group ofdistal control elements)Proximalcontrol elementsTranscriptionstart site

PromoterExonExonPrimary RNAtranscript(pre-mRNA)IntronIntronExonExonIntronIntronExonExonPoly-A signalsequenceTranscriptionterminationregionDownstreamPoly-AsignalCleaved 3 endof primarytranscript53TranscriptionUpstreamDNAIntron RNA

RNA processingCoding segmentStartcodonStopcodon3 UTRPoly-AtailAAAAAAmRNA5 Cap5 UTRGPPP

2015 Pearson Education Ltd41Figure 18.8 A eukaryotic gene and its transcript

Figure 18.8a

Enhancer (group ofdistal control elements)Proximalcontrol elementsTranscriptionstart sitePromoterExonIntronExonIntronExonPoly-A signalsequenceTranscriptionterminationregionDownstreamUpstreamDNA

2015 Pearson Education Ltd42Figure 18.8a A eukaryotic gene and its transcript (part 1)

Figure 18.8b-1

Proximalcontrol elementsTranscriptionstart sitePromoterExonIntronExonIntronExonPoly-A signalsequenceDNA

2015 Pearson Education Ltd43Figure 18.8b-1 A eukaryotic gene and its transcript (part 2, step 1)

Figure 18.8b-2

Proximalcontrol elementsTranscriptionstart sitePromoterExonIntronExonIntronExonPoly-A signalsequenceDNAExonPrimary RNAtranscript(pre-mRNA)IntronExonIntronExonPoly-AsignalCleaved 3 endof primarytranscript5Transcription


Figure 18.8b-3

Proximalcontrol elementsTranscriptionstart sitePromoterExonIntronExonIntronExonPoly-A signalsequenceDNAExonPrimary RNAtranscript(pre-mRNA)IntronExonIntronExonPoly-AsignalCleaved 3 endof primarytranscript5Transcription

AAAAAA

3Intron RNARNA processingCoding segmentStartcodonStopcodon3 UTRPoly-AtailmRNA5 Cap5 UTRGPPP


Animation: mRNA Degradation


The Roles of Transcription FactorsTo initiate transcription, eukaryotic RNA polymerase requires the assistance of transcription factorsGeneral transcription factors are essential for the transcription of all protein-coding genesIn eukaryotes, high levels of transcription of particular genes depend on control elements interacting with specific transcription factors


Enhancers and Specific Transcription FactorsProximal control elements are located close tothe promoterDistal control elements, groupings of which are called enhancers, may be far away from a geneor even located in an intron


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Figure 18.9Activationdomain

DNADNA-bindingdomain

2015 Pearson Education Ltd49Figure 18.9 The structure of MyoD, an activator

An activator is a protein that binds to an enhancer and stimulates transcription of a geneActivators have two domains, one that binds DNA and a second that activates transcriptionBound activators facilitate a sequence of protein-protein interactions that result in transcription of a given gene


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Figure 18.10-1DNA

ActivatorsPromoterEnhancerDistal controlelementTATAboxGene

2015 Pearson Education Ltd51Figure 18.10-1 A model for the action of enhancers and transcription activators (step 1)

Figure 18.10-2DNA

ActivatorsPromoterEnhancerDistal controlelementTATAboxGeneDNA-bending proteinGroup of mediator proteinsGeneraltranscriptionfactors


Figure 18.10-3DNA

ActivatorsPromoterEnhancerDistal controlelementTATAboxGeneDNA-bending proteinGroup of mediator proteinsGeneraltranscriptionfactorsRNApolymerase IIRNA polymerase IIRNA synthesisTranscriptioninitiation complex


Animation: Initiation of Transcription


Some transcription factors function as repressors, inhibiting expression of a particular gene by a variety of methodsSome activators and repressors act indirectly by influencing chromatin structure to promote or silence transcription


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Combinatorial Control of Gene ActivationA particular combination of control elements can activate transcription only when the appropriate activator proteins are present


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Figure 18.11Controlelements

DNA in both cellscontains the albumingene and thecrystallin gene:Enhancer foralbumin gene PromoterAlbumin geneCrystallin genePromoterEnhancer forcrystallin gene AvailableactivatorsAvailableactivatorsAlbumingeneexpressedAlbumin genenot expressedCrystallingene expressedCrystallin gene not expressedLIVER CELL NUCLEUSLENS CELL NUCLEUS(a) Liver cell(b) Lens cell

2015 Pearson Education Ltd57Figure 18.11 Cell typespecific transcription

Figure 18.11aControlelementsDNA in both cellscontains the albumingene and thecrystallin gene:Enhancer foralbumin gene PromoterAlbumin geneCrystallin genePromoterEnhancer forcrystallin gene

2015 Pearson Education Ltd58Figure 18.11a Cell typespecific transcription (part 1: DNA)

Figure 18.11bAvailableactivatorsAlbumingeneexpressedCrystallin gene not expressedLIVER CELL NUCLEUS(a) Liver cell

2015 Pearson Education Ltd59Figure 18.11b Cell typespecific transcription (part 2: liver cell nucleus)

Figure 18.11cAvailableactivatorsAlbumin genenot expressedCrystallingene expressedLENS CELL NUCLEUS(b) Lens cell

2015 Pearson Education Ltd60Figure 18.11c Cell typespecific transcription (part 3: lens cell nucleus)

Coordinately Controlled Genes in EukaryotesCo-expressed eukaryotic genes are not organized in operons (with a few minor exceptions)These genes can be scattered over different chromosomes, but each has the same combination of control elementsCopies of the activators recognize specific control elements and promote simultaneous transcription of the genes


Nuclear Architecture and Gene ExpressionLoops of chromatin extend from individual chromosome territories into specific sites in the nucleusLoops from different chromosomes may congregate at particular sites, some of which are rich in transcription factors and RNA polymerasesThese may be areas specialized for a common function


Figure 18.12Chromatinloop5 mTranscriptionfactoryChromosometerritoryChromosomes in theinterphase nucleus(fluorescence micrograph)

2015 Pearson Education Ltd63Figure 18.12 Chromosomal interactions in the interphase nucleus

Figure 18.12a5 mChromosomes in theinterphase nucleus(fluorescence micrograph)

2015 Pearson Education Ltd64Figure 18.12a Chromosomal interactions in the interphase nucleus (part 1: micrograph)

Mechanisms of Post-Transcriptional RegulationTranscription alone does not account for gene expressionRegulatory mechanisms can operate at various stages after transcriptionSuch mechanisms allow a cell to fine-tune gene expression rapidly in response to environmental changes


RNA ProcessingIn 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


Figure 18.13ExonsRNA splicing

123451234512351245ORTroponin T geneDNAPrimaryRNAtranscriptmRNA

2015 Pearson Education Ltd67Figure 18.13 Alternative RNA splicing of the troponin T gene

Animation: RNA Processing


Initiation of Translation and mRNA DegradationThe initiation of translation of selected mRNAs can be blocked by regulatory proteins that bind to sequences or structures of the mRNAAlternatively, translation of all mRNAs in a cell may be regulated simultaneouslyFor example, translation initiation factors are simultaneously activated in an egg following fertilization


The life span of mRNA molecules in the cytoplasm is a key to determining protein synthesisEukaryotic mRNA is more long lived than prokaryotic mRNANucleotide sequences that influence the lifespan of mRNA in eukaryotes reside in the untranslated region (UTR) at the 3 end of the molecule


Protein Processing and DegradationAfter translation, various types of protein processing, including cleavage and the additionof chemical groups, are subject to controlThe length of time each protein function is regulated by selective degradationCells mark proteins for degradation by attaching ubiquitin to themThis mark is recognized by proteasomes, which recognize and degrade the proteins


Concept 18.3: Noncoding RNAs play multiple roles in controlling gene expressionOnly a small fraction of DNA codes for proteins, and a very small fraction of the non-protein-coding DNA consists of genes for RNA such as rRNAand tRNAA significant amount of the genome may be transcribed into noncoding RNAs (ncRNAs)Noncoding RNAs regulate gene expression attwo points: mRNA translation and chromatin configuration


Effects on mRNAs by MicroRNAs and Small Interfering RNAsMicroRNAs (miRNAs) are small single-stranded RNA molecules that can bind to mRNAThese can degrade mRNA or block its translationIt is estimated that expression of at least half of all human genes may be regulated by miRNAs


Figure 18.14miRNAmiRNA-proteincomplexThe miRNA bindsto a target mRNA.mRNA degradedTranslation blockedORIf bases are completely complementary, mRNA is degraded.If match is less than complete, translation is blocked.

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2015 Pearson Education Ltd74Figure 18.14 Regulation of gene expression by miRNAs

Small interfering RNAs (siRNAs) are similar to miRNAs in size and functionThe blocking of gene expression by siRNAs is called RNA interference (RNAi)RNAi is used in the laboratory as a means of disabling genes to investigate their function


Chromatin Remodeling by ncRNAsSome ncRNAs act to bring about remodeling of chromatin structureIn some yeasts siRNAs re-form heterochromatin at centromeres after chromosome replication


Figure 18.15RNA transcripts (red) produced.Yeast enzyme synthesizes strandscomplementary to RNA transcripts.Double-stranded RNA processed intosiRNAs that associate with proteins.The siRNA-protein complexes recruithistone-modifying enzymes.The siRNA-protein complexes bindRNA transcripts and become tetheredto centromere region.Chromatin condensation is initiatedand heterochromatin is formed.

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Centromeric DNARNA polymeraseRNA transcriptSister chromatids(two DNAmolecules)siRNA-proteincomplex

Centromeric DNAChromatin-modifyingenzymes

Heterochromatin atthe centromere region

2015 Pearson Education Ltd77Figure 18.15 Condensation of chromatin at the centromere

Figure 18.15a

RNA transcripts (red) produced.Yeast enzyme synthesizes strandscomplementary to RNA transcripts.Double-stranded RNA processed intosiRNAs that associate with proteins.The siRNA-protein complexes bindRNA transcripts and become tetheredto centromere region.Centromeric DNARNA polymeraseRNA transcriptSister chromatids(two DNAmolecules)siRNA-proteincomplex

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2015 Pearson Education Ltd78Figure 18.15a Condensation of chromatin at the centromere (part 1: siRNA-protein complexes)

Figure 18.15b

The siRNA-protein complexes recruithistone-modifying enzymes.Chromatin condensation is initiatedand heterochromatin is formed.Centromeric DNAChromatin-modifyingenzymesHeterochromatin atthe centromere region

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2015 Pearson Education Ltd79Figure 18.15b Condensation of chromatin at the centromere (part 2: chromatin-modifying enzymes)

Small ncRNAs called piwi-associated RNAs (piRNAs) induce heterochromatin, blocking the expression of parasitic DNA elements in the genome, known as transposonsRNA-based regulation of chromatin structure is likely to play an important role in gene regulation


The Evolutionary Significance of Small ncRNAsSmall ncRNAs can regulate gene expressionat multiple stepsAn increase in the number of miRNAs in a species may have allowed morphological complexity to increase over evolutionary timesiRNAs may have evolved first, followed by miRNAs and later piRNAs


Concept 18.4: A program of differential gene expression leads to the different cell types in a multicellular organismDuring embryonic development, a fertilized egg gives rise to many different cell typesCell types are organized successively into tissues, organs, organ systems, and the whole organismGene expression orchestrates the developmental programs of animals


A Genetic Program for Embryonic DevelopmentThe 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 tadpole1 mm2 mm

2015 Pearson Education Ltd84Figure 18.16 From fertilized egg to animal: what a difference four days makes

Figure 18.16a(a) Fertilized eggs of a frog1 mm

2015 Pearson Education Ltd85Figure 18.16a From fertilized egg to animal: what a difference four days makes (part 1: eggs)

Figure 18.16b(b) Newly hatched tadpole2 mm

2015 Pearson Education Ltd86Figure 18.16b From fertilized egg to animal: what a difference four days makes (part 2: tadpole)

Cell differentiation is the process by which cells become specialized in structure and functionThe physical processes that give an organism its shape constitute morphogenesisDifferential gene expression results from genes being regulated differently in each cell typeMaterials in the egg set up gene regulation that is carried out as cells divide


Cytoplasmic Determinants and Inductive SignalsAn eggs cytoplasm contains RNA, proteins, and other substances that are distributed unevenly in the unfertilized eggCytoplasmic determinants are maternal substances in the egg that influence early developmentAs the zygote divides by mitosis, cells contain different cytoplasmic determinants, which lead to different gene expression


The other important source of developmental information is the environment around the cell, especially signals from nearby embryonic cellsIn the process called induction, signal molecules from embryonic cells cause transcriptional changes in nearby target cellsThus, interactions between cells induce differentiation of specialized cell types


Figure 18.17(a) Cytoplasmic determinants in the egg(b) Induction by nearby cellsEarly embryo(32 cells)SignaltransductionpathwaySignalreceptorSignalingmoleculeTwo-celledembryoNUCLEUSZygote(fertilized egg)Mitotic celldivisionFertilizationSpermUnfertilizedeggNucleusMolecules of two differentcytoplasmic determinants

2015 Pearson Education Ltd90Figure 18.17 Sources of developmental information for the early embryo

Figure 18.17a(a) Cytoplasmic determinants in the eggTwo-celledembryoZygote(fertilized egg)Mitotic celldivisionFertilizationSpermUnfertilizedeggNucleusMolecules of two differentcytoplasmic determinants

2015 Pearson Education Ltd91Figure 18.17a Sources of developmental information for the early embryo (part 1: cytoplasmic determinants)

Figure 18.17b(b) Induction by nearby cellsEarly embryo(32 cells)SignaltransductionpathwaySignalreceptorSignalingmoleculeNUCLEUS

2015 Pearson Education Ltd92Figure 18.17b Sources of developmental information for the early embryo (part 2: induction)

Animation: Cell Signaling


Sequential Regulation of Gene Expression During Cellular DifferentiationDetermination irreversibly commits a cell to its final fateDetermination precedes differentiationCell differentiation is marked by the production of tissue-specific proteins


Myoblasts are cells determined to produce muscle cells and begin producing muscle-specific proteinsMyoD is a master regulatory gene encodes a transcription factor that commits the cell to becoming skeletal muscleThe MyoD protein can turn some kinds of differentiated cellsfat cells and liver cellsinto muscle cells


Figure 18.18-1NucleusEmbryonicprecursor cellOther muscle-specific genesOFFOFFDNAMaster regulatory gene myoD

2015 Pearson Education Ltd96Figure 18.18-1 Determination and differentiation of muscle cells (step 1)

Figure 18.18-2NucleusEmbryonicprecursor cellOther muscle-specific genesOFFOFFDNAMaster regulatory gene myoDMyoblast(determined)mRNAMyoD protein(transcriptionfactor)OFF


Figure 18.18-3NucleusEmbryonicprecursor cellOther muscle-specific genesOFFOFFDNAMaster regulatory gene myoDMyoblast(determined)mRNAMyoD protein(transcriptionfactor)OFFPart of a muscle fiber(fully differentiated cell)mRNAmRNAmRNAmRNAAnothertranscriptionfactorMyoDMyosin, othermuscle proteins,and cell cycleblocking proteins


Pattern Formation: Setting Up the Body PlanPattern formation is the development of a spatial organization of tissues and organsIn animals, pattern formation begins with the establishment of the major axesPositional 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 in the fruit fly Drosophila melanogasterCombining anatomical, genetic, and biochemical approaches, researchers have discovered developmental principles common to many other species, including humans


The Life Cycle of DrosophilaIn Drosophila, cytoplasmic determinants in the unfertilized egg determine the axes before fertilizationAfter fertilization, the embryo develops into a segmented larva with three larval stages


Figure 18.19Head

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3ThoraxAbdomenDorsalRightPosteriorVentralAnteriorLeft(a) AdultBODYAXES0.5 mmLarva(b) Development from egg to larvaSegmentedembryoBody segments0.1 mmHatchingEmbryonicdevelopmentFertilized eggDepletednurse cellsFertilizationLaying of eggEggshellMature,unfertilized eggDeveloping eggwithin ovarian follicleEggNucleusNurse cellFollicle cell

2015 Pearson Education Ltd102Figure 18.19 Key developmental events in the life cycle of Drosophila

Figure 18.19aHeadThoraxAbdomenDorsalRightPosteriorVentralAnteriorLeft(a) AdultBODYAXES0.5 mm

2015 Pearson Education Ltd103Figure 18.19a Key developmental events in the life cycle of Drosophila (part 1)

Figure 18.19b-1Nurse cellDeveloping egg within ovarian follicleEggNucleusFollicle cell(b) Development from egg to larva

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2015 Pearson Education Ltd104Figure 18.19b-1 Key developmental events in the life cycle of Drosophila (part 2, step 1)

Figure 18.19b-2Depletednurse cellsEggshellMature,unfertilized egg

2Nurse cellDeveloping egg within ovarian follicleEggNucleusFollicle cell

1(b) Development from egg to larva


Figure 18.19b-3Fertilized eggFertilizationLaying of egg

3Depletednurse cellsEggshellMature,unfertilized egg

2Nurse cellDeveloping egg within ovarian follicleEggNucleusFollicle cell

1(b) Development from egg to larva


Figure 18.19b-4SegmentedembryoBody segmentsEmbryonicdevelopment

4Fertilized eggFertilizationLaying of eggDepletednurse cellsEggshellMature,unfertilized eggNurse cellDeveloping egg within ovarian follicleEggNucleusFollicle cell(b) Development from egg to larva0.1 mm

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Figure 18.19b-5LarvaHatching

5SegmentedembryoBody segmentsEmbryonicdevelopmentFertilized eggFertilizationLaying of eggDepletednurse cellsEggshellMature,unfertilized eggNurse cellDeveloping egg within ovarian follicleEggNucleusFollicle cell(b) Development from egg to larva0.1 mm

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Genetic Analysis of Early Development: Scientific InquiryEdward B. Lewis, Christiane Nsslein-Volhard, and Eric Wieschaus won a Nobel Prize in 1995 for decoding pattern formation in DrosophilaLewis discovered the homeotic genes, which control pattern formation in late embryo, larva, and adult stages


Figure 18.20Wild typeEyeAntennaLegMutant

2015 Pearson Education Ltd110Figure 18.20 Abnormal pattern formation in Drosophila

Figure 18.20aWild typeEyeAntenna

2015 Pearson Education Ltd111Figure 18.20a Abnormal pattern formation in Drosophila (part 1: wild type)

Figure 18.20bLegMutant

2015 Pearson Education Ltd112Figure 18.20b Abnormal pattern formation in Drosophila (part 2: mutant)

Nsslein-Volhard and Wieschaus studied segment formationThey created mutants, conducted breeding experiments, and looked for corresponding genesMany of the identified mutations were embryonic lethals, causing death during embryogenesisThey found 120 genes essential for normal segmentation


Axis EstablishmentMaternal effect genes encode cytoplasmic determinants that initially establish the axes of the body of DrosophilaThese maternal effect genes are also called egg-polarity genes because they control orientation of the egg and consequently the fly


Bicoid: A Morphogen That Determines Head StructuresOne maternal effect gene, the bicoid gene, affects the front half of the bodyAn embryo whose mother has no functional bicoid gene lacks the front half of its body and has duplicate posterior structures at both ends


Figure 18.21HeadTailTailTailWild-type larvaMutant larva (bicoid )T1T2T3A1A2A3A4A5A6A7A8A8A7A7A8A6250 m

2015 Pearson Education Ltd116Figure 18.21 Effect of the bicoid gene on Drosophila development

Figure 18.21aHeadTailWild-type larvaT1T2T3A1A2A3A4A5A6A7A8250 m

2015 Pearson Education Ltd117Figure 18.21a Effect of the bicoid gene on Drosophila development (part 1: wild type)

Figure 18.21bTailTailMutant larva (bicoid )A8A7A7A8A6

2015 Pearson Education Ltd118Figure 18.21b Effect of the bicoid gene on Drosophila development (part 2: mutant)

This phenotype suggests that the product of the mothers bicoid gene is essential for setting up the anterior end of the embryoThis hypothesis is an example of the morphogen gradient hypothesis, in which gradients of substances called morphogens establish an embryos axes and other features of its formExperiments showed that bicoid protein is distributed in an anterior to posterior gradient in the early embryo


Figure 18.22Anterior end100 mBicoid protein inearly embryoFertilization,translation ofbicoid mRNABicoid mRNA in matureunfertilized eggResults

2015 Pearson Education Ltd120Figure 18.22 Inquiry: Could Bicoid be a morphogen that determines the anterior end of a fruit fly?

Figure 18.22a100 mBicoid mRNA in matureunfertilized egg

2015 Pearson Education Ltd121Figure 18.22a Inquiry: Could Bicoid be a morphogen that determines the anterior end of a fruit fly? (part 1: Bicoid mRNA)

Figure 18.22bAnterior end100 mBicoid protein inearly embryo

2015 Pearson Education Ltd122Figure 18.22b Inquiry: Could Bicoid be a morphogen that determines the anterior end of a fruit fly? (part 2: Bicoid protein)

Animation: Development of Head-Tail Axis in Fruit Flies


The bicoid research was ground breaking for three reasonsIt identified a specific protein required for some early steps in pattern formationIt increased understanding of the mothers role in embryo developmentIt demonstrated a key developmental concept that a gradient of molecules can determine polarity and position in the embryo


Evolutionary Developmental Biology(Evo-Devo)The fly with legs emerging from its head in Figure 18.20 is the result of a single mutation in one geneSome scientists considered whether these types of mutations could contribute to evolution by generating novel body shapesThis line of inquiry gave rise to the field of evolutionary developmental biology, evo-devo


Concept 18.5: Cancer results from genetic changes that affect cell cycle controlThe gene regulation systems that go wrong during cancer are the very same systems involved in embryonic development


Types of Genes Associated with CancerCancer can be caused by mutations to genes that regulate cell growth and divisionMutations in these genes can be caused by spontaneous mutation or environmental influences such as chemicals, radiation, and some viruses


Oncogenes are cancer-causing genes in some types of virusesProto-oncogenes are the corresponding normal cellular genes that are responsible for normal cell growth and divisionConversion of a proto-oncogene to an oncogene can lead to abnormal stimulation of the cell cycle


Proto-oncogenes can be converted tooncogenes byMovement of DNA within the genome: if it ends up near an active promoter, transcription may increaseAmplification of a proto-oncogene: increases the number of copies of the genePoint mutations in the proto-oncogene or its control elements: cause an increase in gene expression


Figure 18.23Proto-oncogene

Proto-oncogeneProto-oncogenePoint mutation:Gene amplification:multiple copies ofthe geneTranslocation ortransposition: genemoved to new locus,under new controlsNew promoterOncogeneOncogeneOncogenewithin the genewithin a controlelementNormal growth-stimulatingprotein in excessNormal growth-stimulatingprotein in excessNormal growth-stimulating protein in excessHyperactive ordegradation-resistantprotein

2015 Pearson Education Ltd130Figure 18.23 Genetic changes that can turn proto-oncogenes into oncogenes

Figure 18.23aProto-oncogene

Translocation ortransposition: genemoved to new locus,under new controlsNew promoterOncogeneNormal growth-stimulatingprotein in excess

2015 Pearson Education Ltd131Figure 18.23a Genetic changes that can turn proto-oncogenes into oncogenes (part 1: translocation or transposition)

Figure 18.23b

Proto-oncogeneGene amplification:multiple copies ofthe geneNormal growth-stimulatingprotein in excess

2015 Pearson Education Ltd132Figure 18.23b Genetic changes that can turn proto-oncogenes into oncogenes (part 2: gene amplification)

Figure 18.23c

Proto-oncogenePoint mutation:OncogeneOncogenewithin the genewithin a controlelementNormal growth-stimulating protein in excessHyperactive ordegradation-resistant protein

2015 Pearson Education Ltd133Figure 18.23c Genetic changes that can turn proto-oncogenes into oncogenes (part 3: point mutation)

Tumor-Suppressor GenesTumor-suppressor genes normally help prevent uncontrolled cell growthMutations that decrease protein products of tumor-suppressor genes may contribute to cancer onsetTumor-suppressor proteinsRepair damaged DNAControl cell adhesionAct in cell-signaling pathways that inhibit thecell cycle


Interference with Normal Cell-Signaling PathwaysMutations in the ras proto-oncogene and p53 tumor-suppressor gene are common in human cancersMutations in the ras gene can lead to production of a hyperactive Ras protein and increased cell division


Figure 18.24G protein

Growth factorReceptorProteinkinasesTranscriptionfactor (activator)NUCLEUSProtein thatstimulatesthe cell cycleTranscriptionfactor (activator)NUCLEUSOverexpressionof proteinRasRasMUTATIONGTPGTPRas protein activewith or withoutgrowth factor.PPPPPP

1

3

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5

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2015 Pearson Education Ltd136Figure 18.24 Normal and mutant cell cyclestimulating pathway

Suppression of the cell cycle can be importantin the case of damage to a cells DNA; p53 prevents a cell from passing on mutations dueto DNA damageMutations in the p53 gene prevent suppression of the cell cycle


Figure 18.25Protein kinases

DNA damagein genomeActive formof p53TranscriptionDNA damagein genomeUVlightUVlightDefective ormissingtranscriptionfactor.InhibitoryproteinabsentProtein thatinhibits thecell cycleNUCLEUSMUTATION

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2015 Pearson Education Ltd138Figure 18.25 Normal and mutant cell cycleinhibiting pathway

The Multistep Model of Cancer DevelopmentMultiple mutations are generally needed for full-fledged cancer; thus the incidence increaseswith ageAt the DNA level, a cancerous cell is usually characterized by at least one active oncogene and the mutation of several tumor-suppressor genes


Figure 18.26Colon

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5Colon wallLoss of tumor-suppressor geneAPC (or other)Activation ofras oncogeneAdditionalmutationsLossof tumor-suppressorgene SMAD4Largerbenigngrowth(adenoma)Malignanttumor(carcinoma)Small benigngrowth (polyp)Normal colonepithelial cellsLoss oftumor-suppressorgene p53

2015 Pearson Education Ltd140Figure 18.26 A multistep model for the development of colorectal cancer

Figure 18.26a

1Colon wallLoss of tumor-suppressor geneAPC (or other)Normal colonepithelial cells

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3

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5Activation ofras oncogeneAdditionalmutationsLoss of tumor-suppressorgene SMAD4Larger benigngrowth (adenoma)Malignant tumor(carcinoma)Small benigngrowth (polyp)Loss oftumor-suppressorgene p53

2015 Pearson Education Ltd141Figure 18.26a A multistep model for the development of colorectal cancer (part 1: detail)

Routine screening for some cancers, such as colorectal cancer, is recommendedIn such cases, any suspicious polyps may be removed before cancer progressesBreast cancer is a heterogeneous disease that is the commonest form of cancer in women in the United StatesA genomics approach to profiling breast tumors has identified four major types of breast cancer


Figure 18.27MAKE CONNECTIONS:Genomics, Cell Signaling, and CancerNormal Breast Cells in a Milk DuctBreast Cancer Subtypes ER+ PR+ HER2+

Estrogenreceptoralpha (ER)DuctinteriorProgesteronereceptor (PR)HER2(a receptortyrosinekinase)SupportcellExtracellularmatrix ER+++ PR++ HER2 40% of breast cancers Best prognosis

ER++ PR++ HER2 (shown); some HER2++ 1520% of breast cancers Poorer prognosis thanluminal A subtype

ER PR HER2++ 1015% of breast cancers Poorer prognosis thanluminal A subtype

ER PR HER2 1520% of breast cancers More aggressive; poorerprognosis than other subtypes

Luminal ALuminal BHER2Basal-like

2015 Pearson Education Ltd143Figure 18.27 Make connections: genomics, cell signaling, and cancer

Figure 18.27aMAKE CONNECTIONS:Genomics, Cell Signaling, and CancerNormal Breast Cells in a Milk Duct ER+ PR+ HER2+

Estrogenreceptoralpha (ER)DuctinteriorProgesteronereceptor (PR)HER2(a receptortyrosinekinase)SupportcellExtracellularmatrix

2015 Pearson Education Ltd144Figure 18.27a Make connections: genomics, cell signaling, and cancer (part 1: normal breast cells)

Luminal BMAKE CONNECTIONS:Genomics, Cell Signaling, and CancerBreast Cancer SubtypesFigure 18.27b ER+++ PR++ HER2 40% of breast cancers Best prognosis

ER++ PR++ HER2 (shown); some HER2++ 1520% of breast cancers Poorer prognosis thanluminal A subtype

Luminal A

2015 Pearson Education Ltd145Figure 18.27b Make connections: genomics, cell signaling, and cancer (part 2: breast cancer subtypes, luminal A and luminal B)

Figure 18.27c ER PR HER2++ 1015% of breast cancers Poorer prognosis thanluminal A subtype

ER PR HER2 1520% of breast cancers More aggressive; poorerprognosis than other subtypes

HER2Basal-likeMAKE CONNECTIONS:Genomics, Cell Signaling, and CancerBreast Cancer Subtypes

2015 Pearson Education Ltd146Figure 18.27c Make connections: genomics, cell signaling, and cancer (part 3: breast cancer subtypes, HER2 and Basal-like)

Inherited Predisposition and Environmental Factors Contributing to CancerIndividuals can inherit oncogenes or mutant alleles of tumor-suppressor genesInherited mutations in the tumor-suppressor gene adenomatous polyposis coli are common in individuals with colorectal cancerMutations in the BRCA1 or BRCA2 gene are found in at least half of inherited breast cancers, and tests using DNA sequencing can detect these mutations


The Role of Viruses in CancerA number of tumor viruses can also cause cancer in humans and animalsViruses can interfere with normal gene regulation in several ways if they integrate into the DNA ofa cellViruses are powerful biological agents


Figure 18.UN01CHROMATIN MODIFICATIONTRANSCRIPTIONRNA PROCESSINGmRNADEGRADATIONTRANSLATIONPROTEIN PROCESSINGAND DEGRADATION

2015 Pearson Education Ltd149Figure 18.UN01 In-text figure, chromatin modification, p. 366


2015 Pearson Education Ltd150Figure 18.UN02 In-text figure, transcription, p. 369

Figure 18.UN03aEnhancer with possiblecontrol elements

PromoterReportergene123050100150200Relative level of reportermRNA (% of control)

2015 Pearson Education Ltd151Figure 18.UN03a Skills exercise: analyzing DNA deletion experiments (part 1)

Figure 18.UN03b

2015 Pearson Education Ltd152Figure 18.UN03b Skills exercise: analyzing DNA deletion experiments (part 2)


2015 Pearson Education Ltd153Figure 18.UN04 In-text figure, RNA processing, p. 372


2015 Pearson Education Ltd154Figure 18.UN05 In-text figure, mRNA degradation and translation, p. 374

Figure 18.UN06Operon

PromoterGenesRNApolymeraseOperatorPolypeptidesABCABC

2015 Pearson Education Ltd155Figure 18.UN06 Summary of key concepts: operon, general

Figure 18.UN07Repressible operon:Genes expressedPromoterGenesOperatorInactive repressor:no corepressor presentGenes not expressedCorepressorActive repressor:corepressor bound

2015 Pearson Education Ltd156Figure 18.UN07 Summary of key concepts: operon, repressible

Figure 18.UN08Inducible operon:Genes expressedPromoterGenesOperatorInactive repressor:inducer boundGenes not expressedInducerActive repressor:no inducer present

2015 Pearson Education Ltd157Figure 18.UN08 Summary of key concepts: operon, inducible

Figure 18.UN09CHROMATIN MODIFICATIONTRANSCRIPTIONRNA PROCESSINGmRNADEGRADATIONTRANSLATIONPROTEIN PROCESSINGAND DEGRADATIONChromatin modificationTranscriptionRNA processingmRNA degradationTranslationProtein processing and degradation

Each mRNA has acharacteristic life span,determined in part bysequences in the 5 and 3UTRs. Regulation of transcription initiation:DNA controlelements inenhancers bindspecific tran-scription factors.Bending of the DNA enablesactivators to contact proteins at the promoter,initiating transcription. Coordinate regulation:Enhancer forliver-specific genesEnhancer forlens-specific genes Alternative RNA splicing:Primary RNAtranscriptmRNAOR Initiation of translation can be controlled viaregulation of initiation factors. Protein processing and degradation aresubject to regulation. Genes in highly compactedchromatin are generally not transcribed. Histone acetylationseems to loosenchromatin structure, enhancing transcription. DNA methylation generallyreduces transcripton.

2015 Pearson Education Ltd158Figure 18.UN09 Summary of key concepts: eukaryotic regulation of gene expression

Figure 18.UN09aCHROMATIN MODIFICATIONTRANSCRIPTIONRNA PROCESSINGmRNADEGRADATIONTRANSLATIONPROTEIN PROCESSINGAND DEGRADATION

2015 Pearson Education Ltd159Figure 18.UN09a Summary of key concepts: eukaryotic regulation of gene expression (part 1)

Figure 18.UN09bChromatin modification Genes in highly compactedchromatin are generally not transcribed. Histone acetylationseems to loosenchromatin structure,enhancing transcription. DNA methylation generallyreduces transcription.RNA processingmRNA degradationTranslationProtein processing and degradation Each mRNA has acharacteristic life span,determined in part bysequences in the 5 and 3UTRs. Alternative RNA splicing:Primary RNAtranscriptmRNAOR Initiation of translation can be controlled viaregulation of initiation factors. Protein processing and degradation aresubject to regulation.

2015 Pearson Education Ltd160Figure 18.UN09b Summary of key concepts: eukaryotic regulation of gene expression (part 2)

Figure 18.UN09cTranscription

Regulation of transcription initiation:DNA controlelements inenhancers bindspecific tran-scription factors.Bending of the DNA enablesactivators to contact proteins at the promoter,initiating transcription. Coordinate regulation:Enhancer forliver-specific genesEnhancer forlens-specific genes

2015 Pearson Education Ltd161Figure 18.UN09c Summary of key concepts: eukaryotic regulation of gene expression (part 3)

Figure 18.UN10CHROMATIN MODIFICATIONTRANSCRIPTIONRNA PROCESSINGmRNADEGRADATIONTRANSLATIONPROTEIN PROCESSINGAND DEGRADATIONChromatin modification Small and/or large noncoding RNAscan promote heterochromatin formationin certain regions, which can blocktranscription.mRNA degradation miRNA or siRNA can block thetranslation of specific mRNAs. miRNA or siRNA can target specific mRNAs for destruction.Translation

2015 Pearson Education Ltd162Figure 18.UN10 Summary of key concepts: noncoding RNAs

Figure 18.UN11EFFECTS OF MUTATIONSProteinoverexpressedCell cycleoverstimulatedIncreased celldivisionProtein absentCell cycle notinhibited

2015 Pearson Education Ltd163Figure 18.UN11 Summary of key concepts: effects of mutations

Figure 18.UN12Enhancer

PromoterGene 1Gene 2Gene 3Gene 4Gene 5

2015 Pearson Education Ltd164Figure 18.UN12 Test your understanding, question 11 (activator proteins)

Figure 18.UN13

2015 Pearson Education Ltd165Figure 18.UN13 Test your understanding, question 16 (flashlight fish)

18 ge gene expression lecture presentation

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