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Chapter 3 The nucleus（第1页）

Chapter3Thenucleus

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Althoughthehelargestandmostahecell，theprocesseswithinithaveprovedrathermoredifficulttostudythahesurrounding.Thismayhavebeehediffiicalseparationofitsts.LargelyasaresultofimprovedteologyoverthelastdeowknowthatthehemostspatiallyanizedandprobablythemostdynamicallyactivepartoftheeheproduofDNA，RNAandtheassemblyofribosomesinvolvesamassivelevelofiiooplasmandalsotrepositioningoftheents.Weshalluclearstrutheoutsideinwards，beginningwiththebouweennud—thenuvelope.

Thenuvelopeseparatestheentsfromthe，alsatandmassivemoletergebetweewoents.Sowhydotothistroublewhenprokaryoteslikebacteriahavenyetreproducethemselvesatsugrates?Successfulastheyare，atleastintermsoferiasideredessentially‘oriies’.Theyhavereachedtheirlimitassimplesinglecells，despitereprodugprolifidretainingsuffieticvariabilitytotly（andunfortunately）produtibiotitstraialnumberanismsoisequatedwithsucbaeoutontop.versely，intermsofbiologiplexity，theyarealsothemostsimplealy‘primitive’anismso.Bacteriaalsoaretheoldestatsomefourbillionyears，andassuchprovidedtherawmaterialforallsubsequehelargeststepiionoflivingthihwastheswitprokaryotictoeukaryoticizatioheaofaaierialinsideanisolatihishasledtotheproliferationaionoflifeasweknowittoday.Justhowthenucleuswasacquiredisuain，butitrobablyasaresultofphagoallbacteriumbyalargerohesmallerbacteriumthen‘tookover’troleroneorendosymbiosisoccurredwiththepartitioningoftheDNAinsideamembrahoughcellbiologyisgenerallyitheinofmitodriaandchloroplastsbye，nosususexistsfinofthenucleus.

Engthegeofthecellwithinitsownenthasfosteredthediversityweseeinbothunicellularandmulticellulareukaryoticcreatures.Eaprodud150，000differeiacrossthevariousspecializedtissuesthroughoutthebody.Thisispossibleihefactthatweonlyhave23，600gehegeicmessagebemodifiediheertrans（thetransferofinformationfromDNAtoRNA），aheheadditionofsimplechemicalstructuressuchasfatsandsugars），thusiheoverallnumberofpossibleproteinproduparisobacteriumisprobablyMyagenitalium（fenitalia）whichhasarouheongutbacteriumEscherichiacolihas4，300gehesmallestfluvirus（whieedstohijackthemaeryofthecellsitioreproduceitself）hasbut11genes.

&ioentsandhasresultediicuchlargerandmoreplexinparisontoprokaryotes.ThecircularmoleAiatotheihecellmembravariouspointsandmaystretdthewholecell.Thisisfinefeh4.6millioidebasepairs（idesequehegehecaseforE.coli.Witha1000-foldihfortheDNAinhumancellshowever，agaspeeofaroundafewhundredbasepairsfromtheoverall3.1billionbasepairshastobeeasierwhentheDNAistratedwithintheherestriofDNAtothenueukaryotecellsalsoavoidsaerferehesophisticatedwsofthecytoskeletonandielles.Therearenosusinprokaryotes，wheretheDNAisshort（andcirdthereislittleifaructure.

Thenuvelopeandporeplexes

Thenuvelopesistsoftwodistines，theoutermostbeingformedbyendoplasmicreticulum，whichisseparatedfromtheinnernuclearmembranebyaperinuclearspace（Figure5a）.Theinnernuclearmembrahaworkoffibrousproteinswhiastruownasthenuclearlamina.Boththenuclearmembrahelamiarepieruplexes，whitroltheflintoandoutofthefromverysmallmoleculesassdirectlythroughthenuvelope.Therearearound5，000poreplexesdistributedoverthesurfaceofthenumammalianuplexesaremadefrom50proteins（nus），amolecularmatheuclearporeseerandouternuclearmembranes，andalshtcable-likeproteinsioplasm，aherfibresintothenuingastructureratherlikeabasket（Figure　7d，e）.Inboesofmoleculesattachtothefibresextendingoutwardsfromthepore，ahehroughthemembraofthebasketintothenucleus.

7.Thenucleus.（a）Sethroughanintaucleus，（b）surfaceview，

withtheinternalatinexposedbyremovalofpartoftheporeuvelope，and（uuucleoskeletonafterDNAremoval，（d），（e）nuclearporesviewedfromtheoutsideahenucleus，（f）leukaemiatainingdistortednuclei

Ascaled-upanalogyofporetrafficactivitywouldilengthofdraiheporeel）throughwhichamixtureoftennisballs，golfballs，andmarbleswouldpassinbothdiresatarateof1000journeyspersed.Theflowistrolledbynuproteinswhichprojetotheel，sahevariousmolethecorrectdire.

&eincargois‘tagged’byanamihatactslikealuggagelabeltoeheyfinishosideoftheheactualpassagethroughtheporerequiresattatof‘e’proteiiins，ahroughtheporebutarethenchoppedoffasthecargoexitstheporeaao.RibosomesareassembledfromRhenudproteihe），alygeeahighlevelardlessofotheroplasmige.InaHeLamillionribosomesareproducedeachday.Seventhousandareproducedeaute，eagaround80prtheproduofhalfamillionproteieioplasm.Theseproteiedintothearateof100perporeperminute，passing（amoraffic）threeribosomalsubunitsontheirwayoutoftheaindiseasesaredirectlyassociatedwithnuproteins.Inprimarybiliarycirrhosis，proteins（autoimmuibodies）areproducedthatattaus，eveopletecirrhosisoftheliver.

Althoughthenuvelopeclearlyseparatesnud，italsophysiksthem.Proteinsesprins，whitheinnernuclearmembrane，reachacrosstheperinuclearspace，passthroughtheouternuclearmembraendforsomedistahe，wheretheyattachtothecytoskeleton.Nesprinsaresomeestproteihisattaolewithintheiheoskeletalelements（whichthemselvesareliheplasmamembrathereisapotentialmedirethecellsurfacethroughtothenuiasyetunexplainedlinkage.

Thenuclearlamina

Thenuclearlaminawasinallyvisualizedironmicroscopeasafibrousmatrixontheiheinnerheseproteisresiststretdformthe‘high-tensilecables’，closelyrelatedtotheiefilameoskeleton.Thus，thenuclearlamiheentsfrommeicalstress，andalsoaionofthehecell，providiattothecytoskeletooplasm.Astructurecalledthee，whichisthemainmicrtreofthecell，isalsokeptclosetothenuclearsurfacebyattatstothenuclearlamihesemeis，thenuclearlaminaalsoplaysamajorroleintheanizatioehgeionandthepassageofgeiatiooplasm.Geleadtodisruptionofthenuvelopeandlamiinsevereces，termed‘hies’or‘lamihesareusuallyied，aregenerallyincurable，aremelyraremusculardystrophiditioyofgeiditifrommalfungbuildingbloyightmakethemappeartrivial，butmorelikely，theseargrequirementsofbuildingananismwithoutallthefullentofcorrectpartsislikelytostopdevelmuchfartherthanafewdivisioe.

&istituentsofthenucleus

AlthoughthehavebeenreizedbyAntonievanLeeuwehelate17thtury，itwasnotuntil1831thatitortedasaspecificstruorchidepidermalcellsbyaScottishbotaBrowerknnizing‘Brow’ofpollengrainsinwater）.In1879，WaltherFlemmihenucleusbrokedownintmentsatcelldivision，followedbyre-formatioscalledakehedaughtercells.Itwasnotuntil1902thatWalterSuttonandTheodorBoveriilylinkedesdirealiaahan’sworkwithfruitflies（Drosophila）atthestartofthe20thturyshowedspecificcharacterspositiohelengthofthees，followedbytherealizationbyOswaldAveryin1944thatthegeerialwasDNA.Someer，JamesWatsonandFrancisCrickshowedthestructureofDNAtobeadoublehelix，forwhichtheysharedtheNobelPrizein1962withMauris，whoselaboratoryhadprovidedtheevideledtothediscovery.RosalindFranklin，whoseX-raydiffraagesofDNAfromtheWilkinslabhadbeeoDNAstructure，dieded37in1958，andNobelPrizesarehumously.WatsonandCrickpublishedtheclassicdoublehelixmodelin1953.ThefihejigsawofDNAstructureroducedbyWatsoionthatthepairiidebases，adehymineaosionlyprovidedtherungsholdiingladderofDalsoprovidedacodeforaccuratereplidatemplateforproteinassembly.tiudyaethebasepairingrequiredfproteins，ahefual‘dogma’that‘DNAmakesRNAandRein’.ThediscoveryofDNAstructuremarkedanenormousadvanbiology，probablythemostsignifi’spubliof　inofSpecies.

WehavealotofDNA

Ifthedouble-strandedDNAineaucleuswaslaidoutasasiwouldmeasurearoundoneandahalfmetresihegeiationitcarriesisstoredintheorderoffouridebases—e（e（G），adenine（A），a）—aloh.GroupsofthreebaseseniAeheamiT—phenylalanine）.AsinglegenemayrequirehuhousandsofbasestogeeasiermsoftheinformatithislengthofDNA，itwouldtake200telephooprihreebillionbasesequences.

Allthe23，600humangeintoaround2cmofourDNA，whichleaves98.5%uhiswasinallysideredtobe‘junk’Dermjunkisperhapsmoreiheignoranceofearlyreseardthus‘nong’（i.e.notgfenes）isabetterdes.AsitseemsuhecellshouldgotothetroubleofrepligmorethahsofitsDimeitdividesfornoreason，wewoulddobesttosiderthisvastmajorityofourDNAtohaveanunkhannofun.Atleastsomenon-Aislyimportanttothecell，asdamagerestrion-gregioobejustaseffectiveatgcelldeathasthatingregions.NonAspseudogeareomakeproteiheremainsofinformationaccumulatedoveraioime，whichmaybesilentformilliobereactivatedaransenon-AalmostlyrepresentstheincorporationofviralDNAfrompastiioed，individualsrarelypletelylosevirusDionarytimesscales，thesescnifits，estimatedat8%forthehumangenome.

&hemselvesareplexstructures，havingastartier）builtintothebeginningofeadainator）attheend.Amidthegsequehereareinterveningnon-gsequerons），whiovedbeforeuse.Ingeneral，ifaprimitiveanismhasapartie，thenanismsplexitywillanumberenesinproportioioiohissuggeststhatwithtime，geenduplidtheheirsequeely.

Hoackaged?

Toaodateoneandahalfmetresofdouble-strahinaspheriucleusroughlyohousandthofthislength，itisclearthattheDNAhastobepaafairlysophistier.Pagsuoleculemustalleobeaccessible，andalsoforthewholeoftheDobeduplicatedsothatexactbepassedtoeachdaughtercell.AtcelldivisioebloAwhichexistwithinthearenotvisibleasdistinctbodies，uherlevelsofdsuperg，aprocessdensation.Thisproducesthediseswhicharethefamiliarimageeer（Figure　8c，d，e）.Duringfinalesation，thenuvelopeisbrokendown，andtheesaredistributedtoeachdaughtercell（seeChapter4formoredetails）.heineachdaughtercell，duringwhichtherigidandrod-likeesappeartolosetheirindividualidentityastheydeseaotheoverallstructureofthedaughteruclei.Thequestionofwhereesgoinnon-dividing（interphase）nucleiwasauryaftertheirinitialdiscoverythankstoateiquecalledfluoressituhybridization（FISH），edbyJoeGallandMary-LouParduein1969.Arelatedteiquecalledepaintingiiplefluorestprobes，allowingindividualesthierphasenucleus.epaintingshowsthateaeoccupiesadistiorywithinthenucleus，usuallywithattatsatthenuclearlamina.Interphaseesoccupyabouthalfoftheinternalnuclearspace，therestbeingfilledbyahostofothernupos，suudCajalbodies（seelater）.Thetsofthenucleusarebynomeahereisatfiuxaofallnupohlongandshortdistances，whichrequiresenergy.

8.DNAandes.（a）NakedDNAandnuake‘beads’，（b）atihinthenucleus，（anes（knoe），（d）esduringfiion，and（e）humaaphasees

Althoughitis‘naked’inprokaryotes，Diccellsisalwaysassociatedwithothermoledispackagedviaaseriesofstages.HumanDNAisfirstbihgroupsofstructuralproteiohefirststageofpag，DNAbeesedtwidgroupsofeighthistooformastruownasanue，leadingtoa‘beadsappearance（Figure　8a）.AdjauestheachedtoeachotherbyaonecalledH1，inazig-zagfashiafibreteresihisfibreistheoasolenoidfiguration（ahollowtube30resier）calledatin.atiandardfiguratioeDNApag（Figure　8b），asintwoformscalledheteroatiiinismoredenselypackaged，produgdarkerstainiobeperipherallydistributedwithinthenucleus（Figure　7a）.MuchoftheDinhasshortidebasesequearerepeatedthousandsoftimes（repetitiveDNA）andmayhaveastructuralratherthaion，toanAwithinthenutrast，euatinismuchlesssedandained，andprisesalmostallofthegeicallyactivepartoftheDerphaseesundergofiionjustbeforedivisioiinfblogthelengthofthees，whibestaioprodusistentbandihislengthwiseseriesofsubdivisionsprovidesa‘roadmap’thathasallowedindividualgeelypositioonlyonspees，buttospecifisalohee.Duringfinalesatioinisfurtherlooped，folded，dsuperassivelyredugtheoverallDoapoihepagratioofDNAinaeatdivisionreaches10，000to1（Figure　8c，d，e）.Agoodanalogytoappreciatethisamazinganizatioakeaskippihelengthofafootballfieldandfolditintthofabouthalfaeologiceshaveshowofthehumanes（e1）hasDNAwith246millionbasepairs，anddisruptionsintheirsequencehavebeeoover350humandiseasesingdneurologiddevelopmentaldisorders.

iregenomehasbeensequesuspectthattheselvesmightbeelessrelevantbeindividual’sDNAalysed，paredroblemsdiagiallybyputer.ItisworthpointingoutthatatthetimeonlysevenindividualsohavehadtheirDheseincludeter，pioneerofDNAdeesWatson（fittingly），twoKoreans，aese，aYoruban（amemberofahniNigeria），andaleukaemiavictim.Thecostofsequehumangenomein2003wasaround$500millioreearer$250，000.Forsequengtobeafeasiblediagnostie，aethousanddollarsisthetarget，whichmaybeteologicallyfeasibleiure.However，themaihemediesisthatdiseasessucer，diabetes，orAlzheimer’sareinvariablycausedbymanyDNAvariations，makingitdifficulttoidesfiinostididtlylimitingtheideaofpersonalizedmediebasedonanindividualgehetimebeiheWelleFoundationhaveannouncedaprojecttosequenes，fromamixtureofhealthypeopleandthfromavarietyofmediditiastatistiifiparison.

Thenucleolus

&ebodieswithinthereizedbyFeliain1774andnamed‘nuucleoliarethelargestofthedistihinthenudeaucleustofive，clearlyvisiblebylightmicroscopywithoutneedforspecifig.Nucleoliareformedfromamixtureofproteinsandnucleicacids，whichareshowronmicroscopy（Figure　7c）taoa‘tripartite’iructureofafibrillartre，adensefibrillarpohegranularpo.Nucleoliaredevotedtotheprodues，aitestructurereflectsthethreeeventsthattakeplacethere:transalRer4），proalRNA，andribosomalassembly.TheDNArespoheseprocessesissituatedonfivedifferenthumanesatdivisionatsitesucleregions（NORs）.Theseregioherafterdivisiontoformthreeorfournucleoli，wheretheribosomalgeransdribosomalsubunitspartiallyassembled，readyforexportfromthehistratioransaery，prog，aooeallowsamazingratesofprodu—dividinghumancellsmaketenmillionribosomesihanaday，sothenucleolusisessentiallyaribosomefactory，withawouldhavebeentheenvyofHenryFord.

Beandsofribosomeprodu，thenucleolusisalsoremarkablyrespooaressthatacellmayexperie，oticstress，aysallucleolarstructure，asdoviraliionaarvatiooseewhetheracellishealthyorhenucleolusfirst.

Cajalbodies，snurposomes，andsplies

In1906，RamonyadridandPaviasharedtheNobelPrizefortheirworkoureofthenervoussystem.Golgidiscoveredtheapparatusorplexthattookhisname，whileddeaiothenucleolus，whichheinallycalledaccessorybodies，andweresubsequentlynamedcoiledbodiesbecauseoftheatureoftheirmainprotein，1999，JoeGallsuggestedtheyshouldbecalledCajalbodies.CajalbodiesalsobodiescalledGemsandGall’swoitledspliesandsnurposomes（similartocoiledbodiesbutrestriphibianoouclei），alliheprogofRNAintheertrans.Sainsmallnuclearriboeins（snRNPs，pronounurps’），andspliesaresiteswherespliAtakesplathelastfewyears，maranuclearbodieshavebeeified，althoughiisstilllimited.JusttomeherearePMLbodies（alsocalledKremerbodies），speckles，paraspedes.

theerior

Fromrelativelyretresearch，ithasbeeapparentthatfarfrombeioryforDNA，thenucleusisjustasvariedanddynamicastheintermsofdactivity.Thesegregatiooplasmbymembraoindividualanellesallowsroutinebiochemicalseparationandanalysis.Teasingoutdiffereherickier，astherearenosuch‘bordered’sub-ents，althoughthehighdensityofthenuititsisolationfromsfragmentedbyasohisstartingpoint，usingmassspee700humaeiifiedsofarinaEuropeancollaborationledbyAngusLamondatDuy.

Interphaseesocdhalfthetotalnue，aedfromeachotherbytheinterespace，whichisfilledwithnu，aviscousliquid，equivaleoplasmoutsidethenucleus.that，aswellashavingtheirown‘domains’，interphaseovearoundtheeriene-ries（whichhavemoreeuatinandcarrythemajorityofaes）tendtobefouralpartsofthenucleus，wheremosttransalactivitytakesplasequentlythegene-poores（whichhavemoreheteroatin）aremoreperipherallypositioned，aheihenuvelopewheretheproteinsofthenuclearlaminaprovideafibrousworkidealfortheangofents.Ifthenuclearlamihenthenormallyaiaightstrayintoatransallyactiveregionofthenudbeeinappropriatelyexpressed，ashappehediseasescalledlaminopathies，suchasDuemusculardystrophy.

Althougheleicroscopyhasproducedvastamountsofinformationsofthe，ithasbeeivelylesssuccessfulforthehisisduetotheintensepadfibrousheents，whichmakesitvirtuallyimpossibletofollowalengthofatinoveranydistahiioransmissioronmicroscopy.Addthesheersizeofthenuesthevolumeofamito）anditwouldrequire200–300serialsestobecut，colledphotographedataround100imagespersebeforeanythree-dimeioed，whichistlyask.Novelapproachessuchastheselectiveremovalofpossimplifythings.AfibrousstrubeseenbysgEMafterthebiochemicalremovalofDNAandatiworkoffibresrunningthroughthehickersesiscalledtherixofscaffold（Figure　7c）.Althoughthistypeofapproaitiallytroversialbecausetheextensivebiochemicalprotpreparationmightewstructures（artefacts）ratherthaheinalanization，theideaofafibrwthroughouttheerior（theon）isnoted.