DNA损伤和修复

1、Chapter5DNAdamageandrepair1 .主要内容1）诱变2 ）DNA损伤3 ）DNA修复4 .教学要求：1）熟悉DNA损伤的原因、类型2）掌握DNA修复的方式5.1 Mutagenesis5.2 DNAdamage5.3 DNARepair5.1 Mutagenesis例变）?5.1.1Thereasonsofmutation?5.1.2Typesofmutations?5.1.3Mutagens傍变剂）?5.1.4mutagenesis诱变）5.1 Mutagenesis（诱变）?Mutation（突变）=Permanent,heritablealterationsinth

2、ebasesequenceofaDNAmolecule（是DNA碱基序列水平上永久性的、可遗传的变化）5.1.1 Thereasonsofmutation?Spontaneouserrors自发性错误）:inDNAreplicationormeioticrecombination?Mutagen（诱变物）:AconsequenceofthedamagingeffectsofphysicalorchemicalmutagensonDNA?Mutagen=mutationcausingagent?Essentiallyallmutagensarecarcinogen敛癌物）?Mostcarcino

3、gensaremutagens5.1.2 Typesofmutations?Multisite（多位点）：- Causegross（重的）chromosomeabnormalities（奇形）；-InvolvelargeregionsofDNA；-Ariseduringmeiosis（在减数分裂期发生）?Pointmutations（点突变）-Involveonlyoneorafewnucleotides；- AriseduringDNAreplication- Requiretwoerrors:1）AnerrorduringDNAreplication；2）Failuretocorrectt

4、haterrorTypesofpointmutationsInversions:ACBDEFDuplicationsABCDEEFDeletions:ABCD-FInsertions:ABCDSEFSubstitutions:ATCDEFTypesofpointmutationsThephenotypiceffectofpointmutation（点突变的表型效应）?Missensemutation（昔义突变）：碱基序列的改变引起了产物氨基酸序列的改变。?Nonsensemutation（无义突变）：某个碱基的改变使代表某种氨基酸的密码子变为蛋白质合成的终止密码子（stopcodon,TAG,

5、TAA,TGA）。?Samesensemutation（同义突变）：是指未改变产物氨基酸序列的密码子变化。Noeffect（silentmutation沉默突变）Readingframe（阅读框）isoneofthethreepossiblewaysinwhichanmRNAsequenceofnucleotidescanbereadasaseriesofbasetriplets三个一组tospecifytheaminoacidsinaproteinchain.ORF（openreadingfram/放性I阅读框）:fromstartcodontostopcodonWhatisthefirst

6、defenseagainstmutations?Ontheonehand,theactualerrorrateofthepolymerase,beforeediting,isoftheorderof0.1%to1.0%.However,theoverallerrorrateforDNAreplicationis1errorin109to1010basepairs.Thisphenomenalfidelityisachievedinthreeways.Replicationfidelity（复制的保真度）First,Watson-CrickbasepairingtoSecond,DNApolym

7、eraseshavetheabilitytoedit（"proofread"）theirwork（3exonucleaseactivityofthepolymerases）.Third,post-replicationrepairofDNA（mismatchrepair）.5.1.3 Mutagens例变剂）Physicalmutagens（物理诱变齐J）?High-energyionizingradiation（电离辐射）：X-raysand-raysstrandbreaks断链），base/sugardestruction（碱基/核糖损伤）?Nonionizingrad

8、iation（非电离辐射）：UVlightpyrimidinedimers（示呢二聚体）Chemicalmutagens（化学诱变剂）?Baseanalogs（碱基类彳以物）:mispair,directmutagenesis?Nitrousacid（亚硝酸）：deaminatesCtoproduceU（脱氨基作用）?Alkylatingagents（烷化剂）?Arylatingagents（芳基化剂）5.1.4 Mutagenesis例变）?Directmutagenesis直接诱变）：resultsfromthepresenceofstable,unrepairedbasewithalte

9、redbasepairingpropertiesintheDNA.Indirectmutagenesis（同接诱变）:Themutationisintroducedasaresultofanerror-pronerepair（倾向差错的修复）.一Translesion的越损伤）DNAsynthesis:Insertionofbasesoppositetheunrepairedlesionregardlessoftheoriginalsequence（不顾原始序列如何，在未修复的损伤序列对面插入碱基）-tomaintaintheDNAintegritybutnotthesequenceaccur

10、acy（只彳於证DNA序列的完整性，不保证序列的精确性）5.2 DNAdamageDNAlesions损伤：oxidativedamages化性损伤Alkylation烷基化bulkyadducts聚化力口合物5.2.1 DNAlesions（DNA损伤）?DNAlesions（DNA损伤）：AnalterationtothenormalchemicalorphysicalstructureoftheDNA（DNA正常的化学或物理结构的改变）5.2.3 Alkylation（烷基化作用）5.2.4 Bulkyadducts（聚化加合物）Bulkylesionssuchaspyrimidined

11、imersandarylating芳基化agentadductsdistortthedoublehelixandcauselocalized局部denaturation.ThisdisruptsthenormalfunctioningoftheDNA.口密噬二聚体的形成喀呢二聚体是如何造成DNA损伤的？?由于相邻的胸腺喀呢产生二聚体，两个碱基平面被环丁基所扭转，引起双螺旋构型的局部变化，同时氢键结合力也显著减弱。这样，当有胸腺喀呢二聚体的DNA作为模板进行复制时，Polin将两个腺喋吟核甘酸加上去，但由于不能很好地形成氢键，然后，又由3'-5'校对功能而将之水解。如此反复发生，

12、而产生了一个空耗的过程，即大量的dATP被分解，而DNA复制却毫无进展。喀呢二聚体是如何造成DNA损伤的？?由于蛋白质仍在不断合成，而DNA不能复制，细胞也就不能分裂，结果出现细丝状的所谓蛇形细胞，最后导致细胞死亡。5.3 DNARepair（DNA修复）?DirectRepair：-Photoreactivation（光复活）-Alkyltransferase（烷基转移酶）?Exisionrepair（切除修复）?Mismatchrepair（错配彳复）?Post-ReplicationRepair（复制后修复）-recombinationalrepair（重组修复）-SOSrepair5.

13、3.1 Photoreactivation洸复活）在可见光的存在下，DNA光解酶（photolyase,光复活酶）可将环丁烷二聚体再分解为单体。这些酶含有可吸收蓝光并将能量转移到待切环丁烷环中的辅基。E.coli的光解酶含有2个色素分子，N5,N10-次甲基四氢叶酸和还原性的黄素腺喋吟二核甘酸（FAD）0光复活对喀呢二聚体是专一性的。是损伤被直接修复”的一种例子，是无差错的。PhotoreactivationofThymineDimers?Processiscatalyzed®化inaprocesssimilartophotosynthesis-harvestingenergyfro

14、mlight?Humancellsdonotcontainphotolyase5.3.2 Alkyltransferase皖基转移酶）?AlkylationofDNABases:?Canblock®碍DNAreplicationbecauseofmodifiedbasesthatareformed?Sometimeusedinchemotherapy（化学疗法）toblockcelldivision?Usuallypurinesarealtered-spectrurtt谱ofproductsvariesMosthighlymutagenicoftheseproducts:Guanin

15、eO6-alkylguanine错配的O6-methylguanin琲口thymine一起造成GC替换为AT。无差错直接修复?烷基转移酶特异性地转移O6耳基鸟口S吟或O6-乙基鸟喋吟上的甲基或乙基基团到酶分子的半胱氨酸上，从而保护DNA免受烷基化诱变。5.3.3 Exisionrepair（切除彳复）?Including一NucleotideExcisionRepair（NER,核甘酸切除修复）E.coliUvrABCendonuclease系统一Baseexcisionrepair（BER碱基切除修复）-Uracil-DNANglycosylasesystem（糖基化酶系统）?Isaubiq

16、uitousmechanismrepairingavarietyoflesions是修复多种损伤的普遍性机制?Error-freerepai吮差错修复核甘酸切除修复ExcisionrepairsystemsinE.colil.uvrA和uvrB蛋白识别损伤部位。2 .发现二聚体后，uvrA解离，uvrC加入。3 .uvrB和uvrC复合体在损伤部位的两端造成单链缺口。4 .损伤部位的DNA被DNA螺旋酶（uvrD）移走。5 .DNA上的缺口由DNA聚合酶I和连接酶填补）。核甘酸切除修复ExcisionrepairsystemsineukaryotesNucleotideexcisionrepa

17、irxerodermapigmentosum（XP,着色性干皮病）是一种常染色体隐性紊乱，在表型上表现为对阳光极为敏感，极易产生皮肤癌。XP患者缺乏对包括由紫外线引起的大块DNA损伤的核甘酸切除功能，至少有7种不同基因的缺陷可导致XP。Baseexcisionrepair（BER,碱基切除修复）5.3.4 Mismatchrepair仔音配彳修复）?用于修复在复制中错配并漏过校正检验的任何碱基。可使复制的保真性提高102-103倍。?occursjustafterreplication?mustdistinguishtheparentfromthedaughterstrand5.3.5 Rec

18、ombinationalRepair（重组修复）?Presentinprokaryoticandeukaryoticcells?Onlypoorlyunderstood?WeknowitexistsbecauseUvrA-andRecA-cellsaremuchmoresensitivetoUVthancellscontainingonlyonemutation.RecombinationalRepair?子链在母链损伤处形成缺口?DependsonRecAproteinimportantforrecombinationandrepair;itcatalyzesstrandpairing.?以

19、子链为模板填补母链的缺口?复制重新开始，损伤位点被切除修复5.3.6 SOSrepair（SOS修复）?UVreactivation（紫外激活反应）或Wreactivation（W激活反应）：-50年代J.Weigle发现，用经紫外线照射后的噬菌体感染用低剂量UV照射过的大肠杆菌时，噬菌体的存活率比感染未用低剂量UV照射的大肠杆菌明显增加，突变率也随之增加，这一效应称为UVreactivation（紫外激活反应）或Wreactivation（W激活反应）。?TheincreasedsurvivalintheUVirradiatedhostisduetotheinductionoftheSOS-

20、repairsysteminthehost.（存活率上升是由于紫外线照射引起了寄主SOS修复系统的感应）?SOSt复是一种旁路系统,它允许新生的DNA链越过胸腺喀呢二聚体而生长，其代价是保真度的极大降低，这是一种易错修复系统。SOS反应机制?Metabolicsystemthathelpsthecellsurviveinperiodsofpotentiallylethalstresse的潜在的致死压力下，细胞的新陈代谢系统帮助细胞存活）-Inducedby（诱导因素）：?UVirradiation（紫外照射）?Thyminestarvation（胸腺喀呢饥饿）?TreatmentwithDNAmodifyingenzymes（DNA修饰酶处理）?Inactivationofgenesessentialtoreplication（DNA复制必须的基因失活）TheSOSgenesSOSrepairDiseasescausedbydefectsinRecQhelicasegenesWernersyndrome?prematureagin（gi衰老,beginningin20's;lifeexp.45?Predisposition患病白体质tomalignanciesDiseasescausedbydefectsinRecQhelicasegen