Gene-X-ppt(Gene10-基因十)--Chapter09

Chapter9Chromosomes9.1Introduction•chromosome–Adiscreteunitofthegenomecarryingmanygenes.EachconsistsofaverylongmoleculeofduplexDNAandanapproximatelyequalmassofproteins.–Itisvisibleasamorphologicalentityonlyduringcelldivision.9.1Introduction•nucleoid–Thestructureinaprokaryoticcellthatcontainsthegenome.–TheDNAisboundtoproteinsandisnotenclosedbyamembrane.•chromatin–ThestateofnuclearDNAanditsassociatedproteinsduringtheinterphase(betweenmitoses)oftheeukaryoticcellcycle.•packingratio–TheratioofthelengthofDNAtotheunitlengthofthefibercontainingit.9.2ViralGenomesArePackagedintoTheirCoats•capsid–Theexternalproteincoatofavirusparticle.•ThelengthofDNAthatcanbeincorporatedintoavirusislimitedbythestructureoftheheadshell.•Nucleicacidwithintheheadshellisextremelycondensed.FIGURE02:AhelicalpathforTMVRNAiscreatedbythestackingofproteinsubunitsinthevirion9.2ViralGenomesArePackagedintoTheirCoats•icosahedralsymmetry–Astructurewith60rotationalsymmetries(suchasasoccerball).•FilamentousRNAvirusescondensetheRNAgenomeastheyassembletheheadshellaroundit.•nucleationcenter–AduplexhairpininTMV(tobaccomosaicvirus)inwhichassemblyofcoatproteinwithRNAisinitiated.TopphotoreproducedfromD.CueandM.Feiss,Proc.Natl.Acad.Sci.USA90(1993):9240-9294.Copyright©2004NationalAcademyofSciences,U.S.A.PhotocourtesyofMichaelG.Feiss,UniversityofIowa.BottomphotocourtesyofRobertDuda,UniversityofPittsburgh.FIGURE03:Maturationofphagelambdapassesthroughseveralstages9.2ViralGenomesArePackagedintoTheirCoats•SphericalDNAvirusesinserttheDNAintoapreassembledproteinshell.•terminase–AnenzymecleavesmultimersofaviralgenomeandthenuseshydrolysisofATPtoprovidetheenergytotranslocatetheDNAintoanemptyviralcapsidstartingwiththecleavedend.FIGURE04:Terminaseproteinbindstospecificsitesonamultimerofvirusgenomesgeneratedbyrollingcirclereplication9.3TheBacterialGenomeIsaNucleoid•Thebacterialnucleoidis~80%DNAbymassandcanbeunfoldedbyagentsthatactonRNAorprotein.•TheproteinsthatareresponsibleforcondensingtheDNAhavenotbeenidentified.PhotocourtesyoftheMolecularandCellBiologyInstructionalLaboratoryProgram,UniversityofCalifornia,Berkeley.FIGURE05:Athinsectionshowsthebacterialnucleoidasacompactmassinthecenterofthecell9.4TheBacterialGenomeIsSupercoiled•Thenucleoidhas~400independentnegativelysupercoileddomains.•Theaveragedensityofsupercoilingis~1turn/100bp.FIGURE07:ThebacterialgenomeconsistsofalargenumberofloopsofduplexDNA9.5EukaryoticDNAHasLoopsandDomainsAttachedtoaScaffold•DNAofinterphasechromatinisnegativelysupercoiledintoindependentdomainsof~85kb.•MetaphasechromosomeshaveaproteinscaffoldtowhichtheloopsofsupercoiledDNAareattached.FIGURE09:Histone-depletedchromosomesconsistofaproteinscaffoldtowhichloopsofDNAareanchoredReprintedfromCell,vol.12,J.R.PaulsonandU.K.Laemmli,Thestructureofhistone-depletedmetaphasechromosomes,pp.817-828.Copyright1977,withpermissionfromElsevier[].PhotocourtesyofUlrichK.Laemmli,UniversityofGeneva,Switzerland.9.6SpecificSequencesAttachDNAtoanInterphaseMatrix•DNAisattachedtothenuclearmatrixatspecificsequencescalledMARsorSARs.•TheMARsareA-T-richbutdonothaveanyspecificconsensussequence.•chromosomescaffold–Aproteinaceousstructureintheshapeofasisterchromatidpair,generatedwhenchromosomesaredepletedofhistones.FIGURE10:Identifyingmatrix-associatedregionsofDNA9.7ChromatinIsDividedintoEuchromatinandHeterochromatin•Individualchromosomescanbeseenonlyduringmitosis.•Duringinterphase,thegeneralmassofchromatinisintheformofeuchromatin,whichisslightlylesstightlypackedthanmitoticchromosomes.FIGURE12:RegionsofcompactheterochromatinareclusterednearthenucleolusandnuclearmembranePhotocourtesyofEdmundPuvion,CentreNationaldelaRechercheScientifique9.7ChromatinIsDividedintoEuchromatinandHeterochromatin•Regionsofheterochromatinremaindenselypackedthroughoutinterphase.•chromocenter–Anaggregateofheterochromatinfromdifferentchromosomes.9.8ChromosomesHaveBandingPatterns•Certainstainingtechniquescausethechromosomestohavetheappearanceofaseriesofstriations,whicharecalledG-bands.•ThebandsarelowerinG-Ccontentthantheinterbands.•GenesareconcentratedintheG-C-richinterbands.FIGURE14:ThehumanXchromosomecanbedividedintodistinctregionsbyitsbandingpattern9.9LampbrushChromosomesAreExtended•chromomeres–Denselystaininggranulesvisibleinchromosomesundercertainconditions,especiallyearlyinmeiosis,whenachromosomemayappeartoconsistofaseriesofchromomeres.•Sitesofgeneexpressiononlampbrushchromosomesshowloopsthatareextendedfromthechromosomalaxis.9.9LampbrushChromosomesAreExtendedFIGURE16:Alampbrushchromosomeisameioticbivalentinwhichthetwopairsofsisterchromatidsareheldtogetheratchiasmata(indicatedbyarrows)PhotocourtesyofJosephG.Gall,CarnegieInstitution.9.10PolyteneChromosomesFormBands•Polytenechromosomesofdipteranshaveaseriesofbandsthatcanbeusedasacytologicalmap.•insituhybridization–HybridizationperformedbydenaturingtheDNAofcellssquashedonamicroscopeslidesothatreactionispossiblewithanaddedsingle-strandedRNAorDNA;theaddedpreparationisradioactivelylabeledanditshybridizationisfollow