4DNAdamage,repairandrecombination

4．DNAdamage,repairandrecombinationIncluding4.1Mutagenesis(诱变)4.2DNAdamage4.3DNArepair4.4DNARecombination4.5DNATransposition4.1Mutagenesis4.1.1Mutation4.1.2Physicalmutagens4.1.3Chemicalmutagens4.1.1MutationMutationsarepermanent,heritablealterationsinthebasesequenceoftheDNA.Piontmutation—asinglebasechange.Transition(转换)–onepurine(orpyrimidine)isreplacedbytheotherTransversion（颠换）--apurinereplacesapyrimidineorviceversaSilentmutation–noeffectontheaminoacidincorporatedintoaprotein.（突变对表达结果不产生影响）Missensemutation–analteredaminoacidinageneproduct.Nonsensemutation–generatenewstopcodons.（终止表达）Frameshiftmutation–thetranslatedproteinsequenceischangedcompletely4.1.2PhysicalmutagensIonizingradiation–x-rays,γ-rayscausesthetargetmoleculestoloseelectrons–strandbreakofbaseandsugardestructionNoninonizing–causesmolecularviberationsofpromotionofelectronstohigherenergylevels---leadtotheformationofnewchemicalbonds.UVlight–pyrimidinedimers4.1.3ChemicalmutagensBaseanalogs碱基类似物–alterbasepairingproperties--directNitrousacid亚硝酸–deaminates(脱胺基)CandAAlkylatingagents烷化剂,Arylatingagents芳基化剂-generateadductsthatcanblocktranscriptionandreplicationandcuasesmutations4.1.4DirectmutagenesisIfabaseanalog（类似物）ormodifiedbasewhosebasepairingpropertiesaredifferentfromtheparentbaseisnotremovedbyaDNArepairmechanismbeforepassageofareplicationfork,thenanincorrectbasewillbeincorporated.AsecondroundofreplicationfixesthemutationpermanentlyintheDNA.4.1.5IndirectmutagenesisMostlesionsinDNAarerepairedbyrepairmechanismsbeforepassageofareplicationfork.Ifthisisnotpossible,anerror-proneformoftranslesion(转移损伤)DNAsynthesismaytakeplaceinvolvingspecializedDNApolymerasesandoneormoreincorrectbasesbecomeincorporatedoppositethelession.4.2DNAdamageDNAlesions损伤–analterationtothenormalchemicalorphysicalstructureoftheDNA.Oxidativedamage–reactiveoxygenspecies活性氧自由基canattacktheDNAandproduceoxidationproducts.Alkylatingagents烷化剂--Methylmethanesulfonate(MMS)甲基磺酸甲酯;Ethylnitrosourea(ENU)乙基亚硝基脲Bulkyadducts大型化合物--areDNAlesionsgeneratedinresponsetoenvironmentalagentsincludingbenzo[a]pyrene(苯并[a]芘)andsolarultravioletradiation.4.3DNArepair4.3.1Excisionrepair（切除修复）4.3.2Mismatchrepair（错配修复）4.3.3Directrepair（DNA的直接修复）4.3.4Decombinationrepair（重组修复）4.3.5SOSresponse(易错修复和应急反应)4.3.1Excisionrepair（切除修复）Nucleotideexcisionrepair(NER):anendonulcease(核酸内切酶)cleavestheDNAandanoligonucleotide(寡核苷酸)isremovedleavingagap.ThegapisfilledbyDNApolymeraseandthefinalphosphodiesterbondmadebyDNAligase.Baseexcisionrepair(BER):modifiedbasesarerecognizedbyDNAglycosylases(糖苷水解酶),whichcleavetheN-glycosylicbondbetweenthealteredbaseandsugar,leavinganapurinicorapyrimidinic(AP)site.AnAPendonucleasecleavestheDNAatthissite.Thereafter,theprocessislikeNER.4.3.2Mismatchrepair（错配修复）Inareplicationalmispair,thewrongbaseisinthedaughterstrand.NewlyreplicatedDNAishemimethylated---theparentalstrandsaremethylatedataGATCsequencesiteandthedaughterstrandsarenot,sotheycanbeeasilydistinguished.Themismatchedbasepairisrecognizedandboundbyacomplexofproteins,andthenthedaughterstandwasnickedatanearbyGATCsite.4.3.3Directrepair（DNA的直接修复）•直接修复是把损伤的碱基回复到原来状态的一种修复。有以下几种方式：•光复活作用；O6-甲基鸟嘌呤-DNA甲基转移酶（MGMT）；单链断裂修复。4.3.4Decombinationrepair（重组修复）•机体细胞对在复制起始时尚未修复的DNA损伤部位可以先复制再修复，这种方式称为重组修复。4.3.5SOSresponse(易错修复和应急反应)•许多能造成DNA损伤或抑制的DNA复制的过程能引起一系列复杂的诱导效应，这种效应称为SOS应急反应。SOS反应是细胞DNA受到损伤或复制系统受到抑制的紧急情况下，细胞为生存而产生的一种应急措施。•SOS反应诱导的修复系统包括：避免差错的修复和易产生差错的修复两类。4.4DNARecombinationIncluding:4.4.1Homologousrecombination(同源重组)4.4.2Site-specificrecombination(位点专一性重组)4.4.3TranspositionorIllegitimaterecombination(转座重组/异常重组)•DNA分子内或分子间发生遗传信息的重新组合,称为遗传重组,或基因重排。•DNA重组对生物进化起着关键性的作用。生物进化以不断产生可遗传的变异为基础。突变和重组—遗传的变异—遗传漂变和自然选择—进化。•DNA重组的意义是能迅速增加群体的遗传多样性，使有利突变与不利突变分开，通过优化组合积累有意义的遗传信息。4.4.1Homologousrecombination（同源重组）•Alsoknownasgeneralrecombination,thisprocessinvolvestheexchangeofhomologousregionsbetweentwoDNAmolecules.(也称一般性重组，由两条同源区的DNA分子，通过配对、链断裂和再连接，而产生的片段间交换的过程)•最初证据来自细胞遗传学对真核生物减数分裂时染色体行为的研究。染色体复制1次-核分裂2次-四联体。•TheexchangeofhomologousbetwwenDNAmoleculesoccursextensivelyineukaryotesduringmeiosis.•InE.coli,thetwohomologousDNAduplexesalignwitheachotherandnicksaremadeinapairofsisterstrandsnearaspecificsequencebyanucleaseassociatedwiththeRecBCDproteincomplex.•ThessDNAcarryingthe5-endsofthenicksbecomescoatedinRecAproteintoformRecA-ssDNAfilaments.ThesecrossoverandsearchtheoppositeDNAduplexforthecorrespondingsequence,afterthatthenicksaresealedandafour-branchedHollidaystructureisformed.•Thesestructureisdynamicandthecross-overpointcanmove(branchmigration).•TheHollidayintermediatecanberesolvedintotwoDNAduplexesinoneoftwoways.–Twoinvadingstrandsarecut------similartooriginal–Thenoninvadingstrandsarecut------heteroduplex,withahybridsectionHolliday于1964年提出了同源重组模型。有4个关键步骤：①两个同源染色体DNA排列整齐；②一个DNA的一条链断裂并与另一个DNA对应的链连接，形成的连接分子，称为Holliday中间体；③通过分支移动产生异源双链DNA；④Holliday中间体切开并修复，形成两个双链重组体DNA。•HomologousrecombinationisalsoimportantforDNArepair.Post-replicationrepair.•Whenareplictionforkencountersanunrepaired,noncodinglesionitcanskipthedamagesectionandleavingadaughterstrandgap.Thisgapca