DNA损伤与修复-DNA-damage--repair--and-recombination

SECTIONFDNAdamage,repair,andrecombinationF1MutagenesisF2DNAdamageF3DNArepairF4Recombination1.MutationF1MutagenesisMutationareheritablepermanentchangesinthebasesequenceofDNA.1.1Pointmutation:isthesimplestmutation-asinglebasechange.Transition:purineorpyrimidineisreplacedbytheotherAGTCTransversion:apurineisreplacedbyapyrimidineorviceverseATorCTAorGGTorCCAorG1.2Deletionsandinsertions(Frameshiftmutation)Deletionsandinsertionsinvolvethelossoradditionofbasesandcancauseframeshiftsinreadingthegeneticcode.1MutationF1MutagenesisExamplesofdeletionmutations2.ReplicationfidelityThehighaccuracyofDNAreplicationdependsonthreefactors:★acombinationofproperbasepairingoftemplatestrandandincomingnucleotideintheactivesiteoftheDNApolymerase★proofreadingoftheincorporatedbaseby3’→5’exonuclease★mismatchrepairF1Mutagenesis3Mutagens3.1PhysicalmutagensIonizing(e.g.X-andγ-rays)andnonionizing(e.g.UV)radiationproduceavarietyofDNAlesions.PyrimidinedimersarethecommonestproductofUVirradiation.F1Mutagenesis3.2ChemicalmutagensBaseanalogs(5-BrU):derivativesofthenormalbaseswithalteredbasepairingproperties.Nitrousacid:deaminatescytosineandadenine.3MutagensF1MutagenesisAlkylatingandarylatingagentsgenerateavarietyofadductsthatcanblocktranscriptionandreplicationandcausemutationsbydirector,morecommonly,indirectmutagenesis.3.2Chemicalmutagens3MutagensF1MutagenesisMostchemicalmutagensarecarcinogenic.4mutagenesis4.1DirectmutagenesisIfabaseanalogormodifiedbasewhosebasepairingpropertiesaredifferentfromtheparentbaseisnotremovedbyaDNArepairmechanismbeforepassageofareplicationfork,thenanincorrectbasewillbeincorporated.F1Mutagenesis5-BrU:G:AenolformBrOHHOBrKetoformHOAGCTTCCTATCGAAGGATAGCTBCCTATCGAAGGATAGCTBCCTATCGAGGGATAGCTTCCTATCGAAGGATAGCTBCCTATCGAAGGATAGCTCCCTATCGAGGGATE.g.A·TG·CtransitionBaseanalogincorporation1stroundofreplication2ndroundofreplication4.2IndirectmutagenesisF1Mutagenesis4mutagenesisMostlesionsinDNAarerepairedbyerror-freedirectreversalorexcisionrepairmechanismsbeforepassageofareplicationfork.Ifthisisnotpossible,anerror-proneformoftranslesionDNAsynthesismaytakeplaceinvolvingspecializedDNApolymerasesandoneormoreincorrectbasesbecomeincorporatedoppositethelesion.E.g.E.colitranslesionreplication:SOSresponseTranslesionDNAsynthesistomaintaintheDNAintegritybutnotthesequenceaccuracyF2DNAdamageDNAlesionsOxidativedamageAlkylationBulkyadducts1.OccursunderNormalcondition2.Increasedbyionizingradiation(physicalutagens)Alkylatingagents(Chemicalmutagens)UVlight(physicalmutagens)Carcinogen(Chemicalmutagens)DNAlesionisanalterationtothenormalchemicalorphysicalstructureoftheDNA.DNAlesionsmayblockreplicationortranscriptionandsobelethal,ortheymaygeneratemutationsthroughdirectorindirectmutagenesis.1DNAlesionsThechemicalinstabilityofDNAcangeneratespontaneouslesionssuchasdeaminationanddepurination.F2DNAdamageDeamination:CUDepurination:breakoftheglycosylicbond,non-codinglesion.2OxidativedamageReactiveoxygenspeciessuchassuperoxideandhydroxylradicalsproduceavarietyoflesionsincluding8-oxoguanineand5-formyluracil（5-羟甲基尿嘧啶）.Suchdamageoccursspontaneouslybutisincreasedbysomeexogenousagentsincludingγ-ray.F2DNAdamageOxidationproducts3AlkylationElectrophilicalkylatingagentssuchasmethylmethanesulfonate(MMS,甲烷磺酸甲酯)andethylnitrosourea(ENU，乙基亚硝基尿)canmodifynucleotidesinavarietyofpositions.F2DNAdamageMostlesionsareindirectlymutagenic,but06-alkylguanineisdirectlymutagenic4BulkyadductsBulkylesionssuchaspyrimidinedimersandarylatingagentadductsdistortthedoublehelixandcauselocalizeddenaturation.ThisdisruptsthenormalfunctioningoftheDNA.F2DNAdamageCyclobutanepyrimidinedimer(嘧啶二聚体)Guanineadductofbenzo[a]pyreneF3DNArepair1PhotoreactivationCleavageofthecyclobutaneringofpyrimidinedimersbyDNAphotolyasesrestorestheoriginalDNAstructure.Photolyaseshavechromophoreswhichabsorbbluelighttoprovideenergyforthereaction.2AlkyltransferaseAninducibleproteinspecificallyremovesanalkylgroupfromtheO6positionofguanineandtransfersittoitself,causinginactivationoftheprotein.F3DNArepairDirectreversalofalesionandiserror-free3Excisionrepair3.1nucleotideexcisionrepair(NER)Innucleotideexcisionrepair,anendonucleasemakesnicksoneithersideofthelesion,whichisthenremovedtoleaveagap.ThisgapisfilledbyaDNApolymerase,andDNAligasemakesthefinalphosphodiesterbond.F3DNArepair特异核酸内切酶3.1nucleotideexcisionrepair(NER)DNApolymeraseⅠDNAligaseF3DNArepair3.2baseexcisionrepair(BER)Inbaseexcisionrepair,thelesionisremovedbyaspecificDNAglycosylase.TheresultingAPsiteiscleavedandexpandedtoagapbyanAPendonucleaseplusexonuclease.Thereafter,theprocessislikenucleotideescisionrepair.F3DNArepair4MismatchrepairReplicationerrorsisaspecializedformofexcisionrepairthatdealswithanybasemispairsproducedduringreplicationandthathaveescapedproofreading.HemimethylationoftheDNAafterreplicationallowsthemismatchedbaseisremovedfromthedaughterstrandbyanexcisionrepairmechanism.F3DNArepairHemimethylationoftheDNA5RecombinantrepairF3DNArepair（a）(b)6HereditaryrepairdefectsMutaionsinexcisionrepairgenesoratranslesionDNApolymerasecausedifferentformsofxerod-ermapigmentosum