�\u0018��Darepreventedfromrotatingthemselvesbythesidearm,which239生物化學簡明雙語教程actsasastator.ThismovementofthetipoftheγsubunitwithintheballofαandβsubunitsprovidestheenergyfortheactivesitesintheβsubunitstoundergoacycleofmovementsthatproducesandthenreleasesATP.ATP合成反應稱為結合ThisATPsynthesisreactioniscalledthebindingchange變構機製,涉及亞基活βmechanism(結合變構機製)andinvolvestheactivesiteofaβ性中心在個狀態之間循3subunitcyclingbetweenthreestates.Inthe“open”state,ADP環轉變。
andphosphateentertheactivesite(showninbrowninthediagram).Theproteinthenclosesuparoundthemoleculesandbindsthemloosely—the“loose”state(鬆散狀態).Theenzymethenchangesshapeagainandforcesthesemoleculestogether,withtheactivesiteintheresulting“tight”state(緊密狀態)bindingthenewlyproducedATPmoleculewithveryhighaffinity(高度親和力).Finally,theactivesitecyclesbacktotheopenstate,releasingATPandbindingmoreADPandphosphate,readyforthenextcycle.MechanismofATPsynthase.在一些細菌和古生菌中,Insomebacteriaandarchaea,ATPsynthesisisdrivenbyATP的合成由鈉離子通themovementofsodiumions(鈉離子)throughthecell過細胞膜的運動來驅動,membrane,ratherthanthemovementofprotons.Archaeasuch而不是質子運動。
asMethanococcus(甲烷球菌)alsocontaintheA1Aosynthase,aformoftheenzymethatcontainsadditionalproteinswithlittlesimilarityinsequencetootherbacterialandeukaryoticATPsynthasesubunits.Itispossiblethat,insomespecies,the240PartIIMetabolism新陳代謝A1AoformoftheenzymeisaspecializedsodiumdrivenATPsynthase,butthismightnotbetrueinallcases.55OxidativestressandInhibitors氧化應激與抑製劑551Reactiveoxygenspecies活性氧類Molecularoxygenisanidealterminalelectronacceptor氧分子是理想的電子終becauseitisastrongoxidizingagent.Thereductionofoxygen端接受體,因為它是強氧化劑。氧的還原反應包含doesinvolvepotentiallyharmfulintermediates.Althoughthe有害的中間產物。
transferoffourelectronsandfourprotonsreducesoxygentowater,whichisharmless,transferofoneortwoelectronsproducessuperoxide(過氧化物)orperoxideanions(過氧化陰離子),whicharedangerouslyreactive.--。
e-e2-關於氧的化學反應O2→O2→O2SuperoxidePeroxideThesereactiveoxygenspeciesandtheirreactionproducts,活性氧類及其反應產物suchasthehydroxylradical(羥基),areveryharmfultocells,的危害。
astheyoxidizeproteinsandcausemutations(突變)inDNA.Thiscellulardamagemightcontributetodiseaseandisproposedasonecauseofaging.ThecytochromeCoxidasecomplexishighlyefficientat細胞色素C氧化酶複合reducingoxygentowater,anditreleasesveryfewpartly物的作用。
reducedintermediates;howeversmallamountsofsuperoxideanionandperoxideareproducedbytheelectrontransportchain.ParticularlyimportantisthereductionofcoenzymeQin輔酶Q自由基可以形成complexIII,asahighlyreactiveubisemiquinonefreeradicalisQ循環的中間產物。
formedasanintermediateintheQcycle.Thisunstablespeciescanleadtoelectron“leakage”(滲透)whenelectronstransferdirectlytooxygen,formingsuperoxide.Astheproductionofreactiveoxygenspeciesbytheseprotonpumpingcomplexesisgreatestathighmembranepotentials,ithasbeenproposedthat241生物化學簡明雙語教程膜電位的變化。mitochondriaregulatetheiractivitytomaintainthemembranepotentialwithinanarrowrangethatbalancesATPproductionagainstoxidantgeneration.Forinstance,oxidantscanactivateuncouplingproteinsthatreducemembranepotential(膜電位).為了抵消活性氧,細胞含Tocounteract(抵消)thesereactiveoxygenspecies,cells有大量的抗氧化係統。containnumerousantioxidantsystems(抗氧化係統),including抗氧化係統的組成。antioxidantvitaminssuchasvitaminCandvitaminE,andantioxidantenzymessuchassuperoxidedismutase(超氧化物歧化酶),catalase(過氧化氫酶),andperoxidases(氧化物酶),whichdetoxifythereactivespecies,limitingdamagetothecell.552Inhibitors抑製劑幾種眾所周知的藥物和Thereareseveralwellknowndrugsandtoxinsthatinhibit毒物對氧化磷酸化的抑oxidativephosphorylation.Althoughanyoneofthesetoxins製。
inhibitsonlyoneenzymeintheelectrontransportchain,inhibitionofanystepinthisprocesswillhalttherestofthe抑製物產生的危害和process.Forexample,ifoligomycin(寡黴素)inhibitsATP例子。synthase,protonscannotpassbackintothemitochondrion.Asaresult,theprotonpumpsareunabletooperate,asthegradientbecomestoostrongforthemtoovercome.NADHisthennolongeroxidizedandthecitricacidcycleceasestooperatebecausetheconcentrationofNAD+fallsbelowtheconcentrationthattheseenzymescanuse.氧化磷酸化中不是所有Notallinhibitorsofoxidativephosphorylationaretoxins的抑製物都是有毒的,有(毒素).Inbrownadiposetissue(脂肪組織),regulatedproton的是解偶聯劑。
channelscalleduncouplingproteins(解偶聯蛋白)canuncouplerespirationfromATPsynthesis.Thisrapidrespirationproducesheat,andisparticularlyimportantasawayofmaintainingbodytemperatureforhibernatinganimals,althoughtheseproteinsmayalsohaveamoregeneralfunctionincellsresponsestostress.242PartIIMetabolism新陳代謝CompoundsUseEffectonoxidativephosphorylationCyanideCarbonInhibittheelectrontransportchainbybindingmorestronglythanmonoxideAzidePoisonsoxygentotheFe—CucenterincytochromeCoxidase,preventingtheHydrogensulfidereductionofoxygen.InhibitsATPsynthasebyblockingtheflowofprotonsthroughtheF0OligomycinAntibioticsubunit.IonophoresthatdisrupttheprotongradientbycarryingprotonsacrossCCCPamembrane.ThisionophoreuncouplesprotonpumpingfromATPPoisons2,4Dinitrophenolsynthesisbecauseitcarriesprotonsacrosstheinnermitochondrialmembrane.PreventsthetransferofelectronsfromcomplexItoubiquinonebyRotenonePesticideblockingtheubiquinonebindingsite.MalonateandPoisonsCompetitiveinhibitorsofsuccinatedehydrogenase(complexⅡ)oxaloacetateBindstotheQisiteofcytochromeCreductase,therebyinhibitingtheAntimycinAPiscicideoxidationofubiquinol【KeyWords】pyruvatedehydrogenasecomplex丙酮酸脫氫酶複oxidativephosphorylation氧化磷酸化合體electrontransportchain電子傳遞鏈NADHQoxidoreductaseNADHQ氧化還原酶ironsulfurprotein鐵硫蛋白succinateQreductase琥珀酸輔酶Q還原酶QcytochromeCoxidoreductaseQ細胞色素Clavoprotein黃素蛋白氧化還原酶ubiquinone泛醌cytochromeCoxidase細胞色素C氧化酶ATPsynthaseATP合酶glycerol3phosphateshuttle磷酸甘油穿梭superoxidedismutase超氧化物歧化酶malateaspartateshuttle蘋果酸天冬氨酸穿梭flavinmononucleotide(FMN)黃素單核苷酸redoxreactions氧化還原反應Questions1BrieflydefinetheComplexI,II,III,IVandV.2WritedownthelocationofspecificATPproducingsitesintheelectrontransportchain.3Brieflydefinethefollowingterms:(a)Electrontransportchain.(b)OxidativephosphorylationandP/Oratio.(c)Redoxreaction.243生物化學簡明雙語教程4Abiologicalredoxreactionalwaysinvolves:(a)Alossofelectron.(b)Againofelectron.(c)Areducingagent.(d)Anoxidizingagent.(e)Alloftheabove.References[1]McKeeTrudy,RMcKeeJames.Biochemistry:AnIntroduction(SecondEdition)[M].NewYork:McGrawHillCompanies,1999.[2]NelsonDL,CoxMM.LehningerPrinciplesofBiochemistry(ThirdEdition)[M].Derbyshire:WorthPublishers,2000.[3]DingW,JiaHT.Biochemistry(FirstEdition)[M].Beijing:HigherEducationPress,2012.[4]鄭集,陳鈞輝.普通生物化學(第四版)[M].北京:高等教育出版社,2007.[5]BergJM,TymoczkoJL,StyerL.Biochemistry(SeventhEdition)[M].NewYork:W.H.FreemanandCompany,2012.244PartIIIInformationalMacromolecules信息大分子PartIIIInformationalMacromolecules信息大分子PartIIIInformationalMacromolecules信息大分子Chapter1DNAsynthesisandrepairDNA合成與修複11DNAreplicationDNA複製NAreplicationisabiologicalprocessthatoccursinalllivingorganisms(生物體)andcopiestheirDNA;itistheDbasisforbiologicalinheritance(生物遺傳).TheprocessDNA分子複製的概念以startswithonedoublestrandedDNAmolecule(雙鏈DNA分及原理。
子)andproducestwoidentical(完全相同的)copiesofthemolecule.EachstrandoftheoriginaldoublestrandedDNAmoleculeservesastemplate(模板)fortheproductionofthecomplementarystrand(互補鏈).Cellularproofreading(校對)anderrorcheckingmechanisms(錯誤檢查機製)ensurenearperfectfidelity(忠實性)forDNAreplication.Inacell,DNAreplicationbeginsatspecificlocations(特定的位置)inthegenome(基因組),called“origins”.UnwindingofDNA(解旋的DNA)attheorigin(起始點),andsynthesisofDNA分子複製的過程及newstrands,formsareplicationfork(複製叉).Inadditionto特點。
DNApolymerase(DNA聚合酶),theenzymethatsynthesizes(合成)thenewDNAbyaddingnucleotides(核苷酸)matchedtothetemplatestrand(模板鏈),anumberofotherproteinsareassociatedwiththeforkandassistintheinitiation(起始)andcontinuationofDNAsynthesis.DNAreplicationcanalsobeperformedinvitro(體外)DNA複製的應用。
(artificially,outsideacell).DNApolymerases(DNA聚合酶),isolatedfromcells,andartificialDNAprimers(人工合成的引物)areusedtoinitiateDNAsynthesisatknownsequences(已知序列)inatemplatemolecule.Thepolymerasechainreaction(PCR,聚合酶鏈式反應),acommonlaboratorytechnique,247生物化學簡明雙語教程employssuchartificialsynthesisinacyclicmanner(循環方式)toamplify(放大)aspecifictargetDNAfragment(特定目標DNA片段)fromapoolofDNA.DNAreplication.Thedoublehelixisunwoundandeachstrandactsasatemplateforthenextstrand.Basesarematchedtosynthesizethenewpartnerstrands.(1)DNAstructureDNA結構DNA分子的雙螺旋結構DNAusuallyexistsasadoublestrandedstructure,with與分子組成。bothstrandscoiledtogethertoformthecharacteristicdoublehelix(雙螺旋).EachsinglestrandofDNAisachainoffourtypesofnucleotides(核苷酸)havingthebases(堿基):adenine(腺嘌呤),cytosine(胞嘧啶),guanine(鳥嘌呤),andthymine(胸腺嘧啶).Anucleotideisamono,di,ortriphosphatedeoxyribonucleoside(單磷酸脫氧核苷、二磷酸脫氧核苷或者三磷酸脫氧核苷);thatis,adeoxyribosesugar(脫氧核糖)isattachedtoone,two,orthreephosphates.Chemicalinteractionofthesenucleotidesformsphosphodiester(磷酸二酯鍵)linkages,creatingthephosphatedeoxyribosebackbone(磷酸脫氧核糖骨架)oftheDNAdoublehelixwiththebasespointinginward.Nucleotides(bases)arematchedbetweenstrandsthroughhydrogenbonds(氫鍵)toformbasepairs(堿基對).248PartIIIInformationalMacromolecules信息大分子Adenine(腺嘌呤)pairswiththymine(胸腺嘧啶),andcytosine堿基互補配對。
(胞嘧啶)pairswithguanine(鳥嘌呤).DNAstrandshaveadirectionality(方向性),andthedifferentendsofasinglestrandarecalledthe“3′(threeprime)end”andthe“5′(fiveprime)end”.Thesetermsrefertothecarbonatomindeoxyribose(脫氧核糖)towhichthenextDNA分子合成的特點。
phosphateinthechainattaches.Inadditiontobeingcomplementary(互補的),thetwostrandsofDNAareantiparallel(反相平行的):Theyareorientatedinoppositedirections.ThisdirectionalityhasconsequencesinDNAsynthesis(DNA合成),becauseDNApolymerase(DNA聚合酶)cansynthesizeDNAinonlyonedirectionbyaddingnucleotides(核苷酸)tothe3′endofaDNAstrand.ThechemicalstructureofDNA.ThepairingofbasesinDNAthroughhydrogenbonding(氫鍵)meansthattheinformationcontainedwithineachstrandisredundant(多餘的).Thenucleotidesonasinglestrandcanbe堿基之間的連接鍵。
249生物化學簡明雙語教程usedtoreconstructnucleotidesonanewlysynthesized(新合成的)partnerstrand.(2)DNApolymeraseDNA聚合酶DNApolymerasesareafamilyofenzymesthatcarryoutallDNA聚合酶的概念及作formsofDNAreplication(DNA複製).However,aDNA用特點。polymerasecanonlyextendanexistingDNA(現有的DNA鏈)strandpairedwithatemplatestrand(模板鏈);itcannotbeginthesynthesisofanewstrand.Tobeginsynthesis,ashortfragment(短片段)ofDNAorRNA,calledaprimer(引物),mustbecreatedandpairedwiththetemplateDNAstrand(模板鏈).圖示DNA聚合酶作用過程。
DNApolymerasesaddsnucleotidestothe3′endofastrandofDNA.Ifamismatchisaccidentallyincorporated,thepolymeraseisinhibitedfromfurtherextension.Proofreadingremovesthemismatchednucleotideandextensioncontinues.250PartIIIInformationalMacromolecules信息大分子DNApolymerasethensynthesizesanewstrandofDNAbyDNA聚合酶參與DNAextending(延伸)the3′endofanexistingnucleotidechain,複製的過程。
addingnewnucleotidesmatchedtothetemplatestrandoneatatimeviathecreationofphosphodiesterbonds(磷酸二酯鍵).TheenergyforthisprocessofDNApolymerization(DNA聚合)comesfromtwoofthethreetotalphosphates(磷酸)attachedtoDNA聚合的能量來源。
eachunincorporatedbase(未結合的堿基).Freebases(遊離堿基)withtheirattachedphosphategroupsarecallednucleosidetriphosphates(三磷酸核苷).Whenanucleotide(核苷酸)isbeingaddedtoagrowingDNAstrand,twoofthephosphatesareremovedandtheenergyproducedcreatesaphosphodiesterbond(磷酸二酯鍵)thatattachestheremainingphosphatetothegrowingchain.Theenergeticsofthisprocessalsohelpexplainthedirectionality(方向性)ofsynthesis,ifDNAweresynthesizedinthe3′to5′direction,theenergyfortheprocesswouldcomefromthe5′endofthegrowingstrandratherthanfromfreenucleotides.111Replicationprocess複製過程(1)Origins起點Foracelltodivide(細胞分裂),itmustfirstreplicate(複起始DNA合成的“起點”製)itsDNA.Thisprocessisinitiated(開始)atparticular概念。
pointsintheDNA,knownas“origins”(起點),whicharetargetedbyproteins(蛋白質)thatseparatethetwostrandsandinitiateDNAsynthesis.OriginscontainDNAsequences(DNA序列)recognizedbyreplicationinitiatorproteins(e.g.,DnaAinE.coli(大腸杆菌)andtheoriginrecognitioncomplex(起點識別複合物)inyeast(酵母菌)).Theseinitiatorproteinsrecruit(招募)otherproteinstoseparatethetwostrandsandinitiatereplicationforks(複製叉).Initiatorproteinsrecruitotherproteinstoseparatethe起始蛋白質。
DNAstrandsattheorigin,formingabubble.Originstendtobe“ATrich”(richinadenineandthyminebases,豐富的腺嘌呤和胸腺嘧啶)toassistthisprocess,becauseATbasepairs豐富的腺嘌呤和胸腺嘧havetwohydrogenbonds(氫鍵)(ratherthanthethreeformed啶能協助DNA複製過程的原因。
inaCGpair)—ingeneral,strandsrichinthesenucleotides(核251生物化學簡明雙語教程苷酸)areeasiertoseparatebecauseagreaternumberofhydrogenbondsrequiresmoreenergytobreakthem.Oncestrandsareseparated,RNAprimersarecreatedonthetemplatestrands(模板鏈).Tobemorespecific(具體的),theleadingstrand(前導鏈)receivesoneRNAprimerperactive(激活)originofreplicationwhilethelaggingstrand(後隨鏈)receivesseveral;theseseveralfragments(幾個片段)ofRNAprimersfoundonthelaggingstrandofDNAarecalledOkazaki岡崎片段的定義。fragments(岡崎片段),namedaftertheirdiscoverer.DNApolymerase(DNA聚合酶)extendstheleadingstrand(前導鏈)inonecontinuousmotionandthelaggingstrandina前導鏈和後隨鏈複製時discontinuous(不連續的)motion(duetotheOkazaki的區別。fragments).RNase(核糖核酸酶)removestheRNAfragmentsusedtoinitiatereplicationbyDNAPolymerase(DNA聚合酶),連接酶的作用。andanotherDNAPolymeraseenterstofillthegaps(缺口).Whenthisiscomplete,asinglenick(缺口)ontheleadingstrandandseveralnicksonthelaggingstrandcanbefound.Ligase(連接酶)workstofillthesenicksin,thuscompletingthenewlyreplicatedDNAmolecule(DNA分子).複製叉的形成。AsDNAsynthesiscontinues,theoriginalDNAstrandscontinuetounwind(解旋)oneachsideofthebubble,formingareplicationforkwithtwoprongs(叉子齒).Inbacteria(細菌),whichhaveasingleoriginofreplicationontheircircularchromosome(環形染色體),thisprocesseventuallycreatesa“thetastructure”(resemblingtheGreekletter(希臘字母)theta:細菌和真核生物複製的θ).Incontrast(相反),eukaryotes(真核生物)havelongerlinear區別。chromosomes(較長的線性染色體)andinitiatereplicationatmultipleorigins(多起點開始複製)withinthese.(2)Replicationfork複製叉Thereplicationforkisastructurethatformswithinthe複製叉的由來。nucleus(細胞核)duringDNAreplication.Itiscreatedbyhelicases(解旋酶),whichbreakthehydrogenbonds(氫鍵)holdingthetwoDNAstrandstogether.Theresultingstructurehastwobranching“prongs”(分支),eachonemadeupofasinglestrandofDNA.Thesetwostrandsserveasthetemplatefortheleadingandlaggingstrands(前導鏈和後隨鏈),which252PartIIIInformationalMacromolecules信息大分子willbecreatedasDNApolymerase(DNA聚合酶)matchescomplementary(互補的)nucleotidestothetemplates;Thetemplatesmaybeproperlyreferredtoastheleadingstrandtemplateandthelaggingstrandtemplate.圖示DNA複製叉上的多種酶的參與。
ManyenzymesareinvolvedintheDNAreplicationfork.Leadingstrand前導鏈TheleadingstrandisthetemplatestrandoftheDNA前導鏈上DNA的複製過doublehelix(雙螺旋結構)sothatthereplicationfork(複製叉)程和特點。
movesalongitinthe3′to5′direction.Thisallowsthenewstrandsynthesizedcomplementary(互補的)toittobesynthesized5′to3′inthesamedirectionasthemovementofthereplicationfork.Ontheleadingstrand,apolymerase(聚合酶)“reads”the原核生物和真核生物複DNAandaddsnucleotides(核苷酸)toitcontinuously.This製時DNA聚合酶的不同。
polymeraseisDNApolymeraseIII(DNA聚合酶III)inprokaryotes(原核生物)andpresumablyPolεineukaryotes(真核生物).Laggingstrand後隨鏈ThelaggingstrandisthestrandofthetemplateDNAdoublehelixthatisoriented(標定方向的)sothatthereplication後隨鏈上的DNA複製過forkmovesalongitina5′to3′manner.Becauseofits程及特點。
orientation(方向),oppositetotheworkingorientationofDNApolymeraseIII,whichmovesonatemplateina3′to5′manner,replicationofthelaggingstrandismorecomplicated253生物化學簡明雙語教程thanthatoftheleadingstrand.Onthelaggingstrand(後隨鏈),primase“reads”theDNAandaddsRNAtoitinshort,separatedsegments.Ineukaryotes(真核生物),primaseisintrinsictoPolα.DNApolymeraseIIIorPolδlengthenstheprimedsegments,formingOkazakifragments(岡崎片段).Primer(引物)removalineukaryotesisalsoperformedbyPolδ.Inprokaryotes(原核生物),DNApolymeraseI“reads”thefragments,removestheRNAusingitsflapendonuclease(核酸內切酶)domain.RNAprimersareremovedby5′3′exonuclease(核酸外切酶)activityofpolymeraseI,andreplacestheRNAnucleotides(RNA核苷酸)withDNAnucleotides(DNA核苷酸)(thisisnecessarybecauseRNAandDNAuseslightlydifferentkindsofnucleotides).DNAligase(DNA連接酶)joinsthefragmentstogether.Dynamicsatthereplicationfork複製叉動力學Ashelicase(解旋酶)unwindsDNAatthereplicationfork,theDNAaheadisforcedtorotate(旋轉).Thisprocessresultsinabuildup(形成、積累)oftwistsintheDNAahead.Thisbuildupwouldformaresistancethatwouldeventuallyhalt(停拓撲異構酶I與II的作用止、終止)theprogressofthereplicationfork.DNA以及區別。topoisomerases(拓撲異構酶)areenzymes(酶)thatsolvethesephysicalproblemsinthecoilingofDNA(DNA螺旋).TopoisomeraseIcutsasinglebackbone(主幹)ontheDNA,enablingthestrandstoswivelaround(轉動)eachothertoremovethebuildupoftwists.TopoisomeraseIIcutsbothbackbones,enablingonedoublestrandedDNAtopassthroughanother,therebyremovingknots(結)andentanglements(纏繞)thatcanformwithinandbetweenDNAmolecules.為了防止二級結構的形BaresinglestrandedDNAtendstofoldbackonitselfand成,單鏈結合蛋白結合到formsecondarystructures(二級結構);thesestructurescan直到第二鏈合成。
DNAinterfere(幹擾)withthemovementofDNApolymerase(DNA聚合酶).Topreventthis,singlestrandbindingproteinsbindtotheDNAuntilasecondstrandissynthesized(合成),preventingsecondarystructure(二級結構)formation.鉗蛋白的作用過程與特點。Clampproteins(鉗蛋白)formaslidingclamparound254PartIIIInformationalMacromolecules信息大分子DNA,helpingtheDNApolymerase(聚合酶)maintaincontactwithitstemplate(模板),therebyassistingwithprocessivity(持續合成能力).TheinnerfaceoftheclampenablesDNAtobethreadedthroughit.Oncethepolymerasereachestheendofthetemplateordetects(發現)doublestrandedDNA,theslidingclampundergoesaconformationalchange(構象變化)thatreleasestheDNApolymerase.Clamploadingproteinsareusedtoinitiallyloadtheclamp,recognizingthejunction(連接)betweentemplateandRNAprimers(引物).(3)Termination終止Eukaryotes(真核生物)initiateDNAreplicationatmultiple真核生物複製有多個pointsinthechromosome(染色體),soreplicationforksmeet起點。
andterminateatmanypointsinthechromosome;thesearenotknowntoberegulatedinanyparticularway.Becauseeukaryoteshavelinear(線性的)chromosomes,DNAreplicationisunabletoreachtheveryendofthechromosomes,butendsatthetelomere(端粒)regionofrepetitiveDNAclosetotheend.Thisshortensthetelomere(端粒)ofthedaughterDNAstrand(子代DNA鏈).Thisisanormalprocessinsomaticcells(體細胞).Asaresult,cellscanonlydivideacertainnumberoftimesbeforetheDNAlosspreventsfurtherdivision.ThisisknownastheHayflick(海弗利克)limit.Withinthegermcell(生殖細胞)海弗利克極限:細胞隻能line,whichpassesDNAtothenextgeneration,telomeraseextends分裂到一定次數。
therepetitivesequencesofthetelomereregiontopreventdegradation.Telomerasecanbecomemistakenlyactiveinsomaticcells(體細胞),sometimesleadingtocancerformation(癌症的形成).Becausebacteria(細菌)havecircularchromosomes(環狀染色體),terminationofreplicationoccurswhenthetworeplicationforksmeeteachotherontheoppositeendoftheparentalchromosome.E.coli(大腸杆菌)regulatethisprocessthroughtheuseofterminationsequencesthat,whenboundbytheTusprotein,enableonlyonedirectionofreplicationforktopassthrough.Asaresult,thereplicationforksareconstrained(約束)toalwaysmeetwithintheterminationregionofthechromosome.255生物化學簡明雙語教程112Polymerasechainreaction聚合酶鏈式反應ResearcherscommonlyreplicateDNAinvitro(體外)usingthepolymerasechainreaction(PCR).PCRusesapairofprimers(引物)tospanatargetregionintemplateDNA,andPCR技術的原理。thenpolymerizes(使聚合)partnerstrandsineachdirectionfromtheseprimersusingathermostable(熱穩定)DNApolymerase.RepeatingthisprocessthroughmultiplecyclesproducesamplificationofthetargetedDNAregion.Atthestartofeachcycle,themixtureoftemplateandprimersisheated,separatingthenewlysynthesizedmoleculeandtemplate.Then,asthemixturecools,bothofthesebecometemplatesforannealingofnewprimers,andthepolymeraseextendsfromthese.Asaresult,thenumberofcopiesofthetargetregiondoubleseachround,increasingexponentially(指數地).12DNArepairDNA修複DNA修複的概念。DNArepairreferstoacollectionofprocessesbywhichacellidentifiesandcorrectsdamagetotheDNAmoleculesthatencode(編碼)itsgenome(基因組).Inhumancells,bothnormalmetabolicactivities(代謝活動)andenvironmental導致DNA損傷的因素。factorssuchasUVlight(紫外線)andradiation(輻射)cancauseDNAdamage,resultinginasmanyas1millionindividualDNA損傷導致的結果。molecularlesions(損傷)percellperday.ManyoftheselesionscausestructuraldamagetotheDNAmoleculeandcanalter(改變)oreliminate(消除)thecellsabilitytotranscribe(轉錄)thegenethattheaffectedDNAencodes(DNA編碼).Otherlesions(損傷)inducepotentially(潛在的)harmfulmutations(突變)inthecellsgenome,whichaffectthesurvivalofitsdaughtercellsafteritundergoesmitosis(有絲分裂).Asaconsequence,theDNArepairprocessisconstantlyactiveasitrespondstodamageintheDNAstructure.Whennormalrepairprocessesfail,andwhencellularapoptosis(細胞凋亡)doesnotoccur,irreparable(無法挽回的)DNAdamagemayoccur,includingdoublestrandbreaksandDNAcrosslinkages(交聯).256PartIIIInformationalMacromolecules信息大分子TherateofDNArepairisdependentonmanyfactors,including影響DNA修複速率的thecelltype,theageofthecell,andtheextracellular(細胞外)因素。
environment.AcellthathasaccumulatedalargeamountofDNAdamage,oronethatnolongereffectivelyrepairsdamage大量的DNA損傷或者不incurredtoitsDNA,canenteroneofthreepossiblestates:能有效修複的DNA損傷會使細胞進入三種可能1anirreversible(不可逆的)stateofdormancy(休眠狀的狀態。
態),knownassenescence(衰老);2cellsuicide,alsoknownasapoptosis(細胞凋亡)orprogrammedcelldeath(程序性細胞死亡);3unregulatedcelldivision(細胞分裂),whichcanleadtotheformationofatumor(腫瘤)thatiscancerous.TheDNArepairabilityofacellisvitaltotheintegrityofitsgenomeandthustoitsnormalfunctioningandthatoftheorganism(器官).ManygenesthatwereinitiallyshowntoinfluencelifespanhaveturnedouttobeinvolvedinDNA糾正DNA損傷失敗可導damagerepairandprotection.Failuretocorrectmolecular致子代基因的突變並且影響進化的速度。
lesions(損傷)incellsthatformgametes(配子)canintroducemutations(突變)intothegenomesoftheoffspring(子代)andthusinfluencetherateofevolution(進化).121DNAdamageDNA損傷DNAdamage,duetoenvironmentalfactorsandnormalmetabolic(代謝)processesinsidethecell,occursatarateof1,000to1,000,000molecularlesions(損傷)percellperday.Whilethisconstitutesonly0.000165%ofthehumangenomesapproximately6billionbases(3billionbasepairs),unrepairedlesionsincriticalgenes(suchastumorsuppressorgenes(腫瘤抑製基因))canimpede(阻止)acellsabilitytocarryoutitsfunctionandappreciablyincreasethelikelihoodoftumorformation(腫瘤形成).ThevastmajorityofDNAdamageDNA損傷影響DNA雙affectstheprimarystructureofthedoublehelix(雙螺旋);that螺旋的一級結構。
is,thebasesthemselvesarechemicallymodified.Thesemodificationscaninturndisruptthemoleculesregularhelicalstructure(螺旋結構)byintroducingnonnativechemicalbondsorbulkyadducts(加合物)thatdonotfitinthestandarddoubleDNA損傷對DNA高級helix.UnlikeproteinsandRNA,DNAusuallylackstertiary結構的影響。
257生物化學簡明雙語教程structure(三級結構)andthereforedamageordisturbancedoesnotoccuratthatlevel.DNAis,however,supercoiled(超螺旋結構)andwoundaround“packaging”proteinscalledhistones(組蛋白)(ineukaryotes(真核生物)),andbothsuperstructures(高級結構)arevulnerabletotheeffectsofDNAdamage.(1)Sourcesofdamage損傷的來源DNA損傷的來源有兩種DNAdamagecanbesubdividedintotwomaintypes:主要類型:內源性損傷與1endogenousdamage(內源性損傷)suchasattackby外源性損傷。