全科室醫(yī)學禮包智慧2腫瘤科2-chapter8gene mutation dna repairand transposable elements

XunchaoXiangSouthwestUniversityofScienceandTechnologyChapter8Genemutation,DNArepair,andtransposableelements8.1Classificationofmutations8.2Detectionofmutation8.3Periodandcharacteristicsofgenemutation8.4Identificationofgenemutation8.5Themolecularbasisofmutation8.6Ultravioletandhigh-energyradiation8.7Mutationsinhumans:casestudies8.8CounteractingDNAdamage:repairsystems8.9TransposablegeneticelementsXunchaoXiangSouthwestUniversityofScienceandTechnologyChapter8Genemutation,DNArepair,andtransposableelementsGeneticinformation:ReplicationStorageExpressionvariationbymutation(Mutationisafailuretostorethegeneticinformationfaithfully.)

chromosomalaberrationsMutationsgenemutationsXunchaoXiangSouthwestUniversityofScienceandTechnology◆基因突變(genemutation)：染色體上某一基因位點內(nèi)部發(fā)生了化學性質(zhì)(結(jié)構(gòu))的變化，與原來基因形成對性關系。★例如：植物高稈基因D突變?yōu)榘捇騞。經(jīng)典遺傳學(基因論)認為：基因就是一個“點”，在染色體上具有一定的位置和相互排列關系，而基因突變就是一個點的改變，是以一個整體進行突變。因此從經(jīng)典遺傳學水平看，基因突變又稱為“點突變”(pointmutation)?！裟柛?910年發(fā)現(xiàn)果蠅眼色的突變(Ww)，并進行鑒定與分析，從而明確證實基因突變的存在。XunchaoXiangSouthwestUniversityofScienceandTechnology8.1ClassificationofmutationsA.SpontaneousversusinducedmutationsSpontaneousmutationsarethosethatjusthappeninnature.Inducedmutationsarethosethatresultfromtheinfluenceofanyartificialfactor.XunchaoXiangSouthwestUniversityofScienceandTechnology8.1ClassificationofmutationsB.gameticversussomaticmutationsSomaticmutationsarenottransmittedtofuturegenerations.Mutationsingametesorgamete-formingtissuesarepartofgermlineandareofgreaterconcernbecausetheyaretransmittedtooffspring.XunchaoXiangSouthwestUniversityofScienceandTechnology8.1ClassificationofmutationsDominantautosomalmutationsRecessiveautosomalmutationsX-linkedrecessivemutationsGameticmutations◆顯性突變：突變產(chǎn)生的新基因?qū)υ瓉淼幕虮憩F(xiàn)為顯性?！綦[性突變：

突變產(chǎn)生的新基因?qū)υ瓉淼幕虮憩F(xiàn)為隱性。XunchaoXiangSouthwestUniversityofScienceandTechnology8.1ClassificationofmutationsC.OthercategoriesofmutationMutationsaffectingmorphologicaltraitNutritionalorbiochemicalvariationsBehaviormutationsRegulatorymutationsLethalmutationsConditionalmutationsXunchaoXiangSouthwestUniversityofScienceandTechnology◆根據(jù)突變所引起的表型改變分為：★形態(tài)突變型；★生化突變型；★致死突變型；★條件致死突變型。基因突變的類型◆經(jīng)典遺傳學認為：★基因是染色體上的一個點?！衄F(xiàn)代基因概念認為：★基因是DNA分子帶有遺傳信息的堿基序列區(qū)段；★基因是由眾多堿基對構(gòu)成，此時將一個堿基對稱為基因的一個位點(site)；★而將基因在染色體上的位置則稱為座位(locus)?；蛲蛔兊念愋汀舾鶕?jù)基因結(jié)構(gòu)的改變方式：★點突變(pointmutation)：指單個堿基的突變。點突變有多種形式。點突變會給所編碼蛋白質(zhì)的生物活性帶來不同的后果?！锎笃瓮蛔?grossmutation)：指長片段DNA序列的改變。主要有缺失和插入兩種類型。大片段突變通常導致所編碼蛋白質(zhì)的生物活性完全喪失。點突變(pointmutation)的類型1.錯義突變(missensemutation)：指一個堿基改變引起的密碼子變化，導致所編碼的氨基酸改變。2.無義突變(nonsensemutation)：指編碼氨基酸的密碼子突變變成終止密碼子。3.移碼突變(frameshiftmutation)：指基因的DNA序列中額外堿基插入或現(xiàn)有堿基缺失引起的突變。4.沉默突變(silentmutation)：沉默突變可能存在于密碼子的第三個堿基，由于密碼子的簡并性，氨基酸不會改變。大片段突變(grossmutation)的類型1.缺失(deletion)：指DNA序列部分丟失。2.插入(insertion)：來自染色體其他部分的額外堿基插入引起的突變。3.重排(rearrangement)：由于基因內(nèi)外的DNA序列相互交換位置產(chǎn)生的突變。XunchaoXiangSouthwestUniversityofScienceandTechnology8.2DetectionofmutationA.DetectioninbacteriaandfungiNeurospora(脈孢菌):Lifecycle,haploiddiploidgametophyte(vegetativephase)sporophyte(sexualphase)

Wild-typemutantPrototroph(原養(yǎng)型)auxotroph(營養(yǎng)缺陷型)(minimalmedium,基本培養(yǎng)基)(completemedium)mutationXunchaoXiangSouthwestUniversityofScienceandTechnologyDetectionin

bacteriaand

fungiXunchaoXiangSouthwestUniversityofScienceandTechnology8.2DetectionofmutationB.DetectioninDrosophilaAttached-Xprocedure:XunchaoXiangSouthwestUniversityofScienceandTechnology8.2DetectionofmutationC.DetectioninplantsAnalysisofbiochemicalcompositionTissuecultureD.DetectioninhumansPedigree:tracingthefamilyhistoryasfarbackaspossible.CultureinvitroXunchaoXiangSouthwestUniversityofScienceandTechnologyDominantautosomalmutationsApedigreeofinheritedCataract(白內(nèi)障)XunchaoXiangSouthwestUniversityofScienceandTechnologyX-linkedrecessivemutationHemophilia(血友病，QueenVictoria)XunchaoXiangSouthwestUniversityofScienceandTechnologyX-linkedrecessivemutationCircleswithadotindicatepresumedfemalecarriersheterozygousforthetrait.XunchaoXiangSouthwestUniversityofScienceandTechnology8.3PeriodandCharacteristicsofGeneMutation1.基因突變的時期◆生物個體發(fā)育的任何時期均可發(fā)生：★性細胞(突變)突變配子后代個體；★體細胞(突變)突變體細胞組織器官。體細胞突變的保留與芽變選擇?！粜约毎耐蛔冾l率比體細胞高：★性母細胞與性細胞對環(huán)境因素更為敏感。◆(等位)基因突變常常是獨立發(fā)生的：★在體細胞中如果隱性基因發(fā)生顯性突變，當代就會表現(xiàn)出來，同原來性狀并存，形成鑲嵌現(xiàn)象或稱嵌合體(chimaera)◆突變時期不同，其表現(xiàn)也不相同

XunchaoXiangSouthwestUniversityofScienceandTechnology2.基因突變的一般特征基因突變表現(xiàn)出以下幾個方面的普遍特征：(1)突變的重演性和可逆性(2)突變的多方向性與復等位基因(3)突變的有害性和有利性(4)突變的平行性XunchaoXiangSouthwestUniversityofScienceandTechnology(1)突變的重演性和可逆性◆突變的重演性：

同一突變可以在生物的不同個體上多次發(fā)生?！锿换蛲蛔冊诓煌膫€體上均可能發(fā)生；★不同群體中發(fā)生同一基因突變的頻率相近。XunchaoXiangSouthwestUniversityofScienceandTechnology(1)突變的重演性和可逆性◆突變的可逆性：基因突變的發(fā)生方向是可逆的。★正突變(forwardmutation)：顯性基因A隱性基因a；★反突變(reversemutation)：隱性基因a顯性基因A。★通常認為：野生型基因是正常、有功能基因；而最初基因突變往往是野生型基因突變而喪失功能、發(fā)生功能改變，表現(xiàn)為隱性基因。所以反突變又稱為回復突變(backmutaiton)?！敉ǔＳ胾表示正突變頻率、v表示反突變頻率，則：

正突變u

野生型===========突變型反突變vXunchaoXiangSouthwestUniversityofScienceandTechnology正突變與反突變的頻率◆正突變與反突變發(fā)生的頻率一般都不相同。多數(shù)情況下：正突變率總是高于反突變率。◆原因在于：★正常野生型基因內(nèi)部存在許多可突變部位，其中之一結(jié)構(gòu)改變均會導致其功能改變；★但是一旦突變發(fā)生，要回復正常野生型功能則只能由原來發(fā)生突變的部位恢復原狀。XunchaoXiangSouthwestUniversityofScienceandTechnology(2)突變的多方向性與復等位基因◆突變的多方向性：指基因突變可以多方向發(fā)生，即基因內(nèi)部多個突變部位分別改變后會產(chǎn)生多種等位基因形式?！衾纾篈基因不同部位發(fā)生改變產(chǎn)生突變基因a1、a2、a3等對A均表現(xiàn)為隱性的基因。新基因可能均是無功能的，也可能各具不同功能。◆復等位基因(multipleallele)：位于同一基因位點上的多種各個等位基因?！镌诙扼w與異源多倍體中，同一位點只能有一對基因，最多存在兩種等位基因形式；因此復等位基因的各種形式會存在于生物群體的不同個體中。XunchaoXiangSouthwestUniversityofScienceandTechnology人類ABO血型的復等位基因◆人類紅細胞表面抗原的特異性由3個復等位基因IA，IB，i決定?！锲渲蠭A，IB對i均為顯性；★

IA，IB間為共顯性?！?/p>

3種基因兩兩組合可能形成6種基因型、4種紅細胞表面抗原反應類型，如下表所示(其中用IO表示i)：XunchaoXiangSouthwestUniversityofScienceandTechnology煙草的自交不親和性基因◆自交不親和性(patibility)：植物自花授粉不結(jié)實，而株間授粉可能結(jié)實的現(xiàn)象?！魺煵輰儆袃蓚€野生種(Nicotianaforgationa與N.alata)表現(xiàn)自交不親和性?！镞@一特性由15個復等位基因(S1,S2,…,S15)控制，稱為自交不親和基因?！镅芯勘砻鳎浩湓蚴蔷哂心骋换虻幕ǚ哿２荒茉诰哂邢嗤虻闹^上萌發(fā)、伸長，因而不能完成受精過程。也即：柱頭對具有相同基因的花粉粒具有拮抗作用?！锲錂C理如下圖XunchaoXiangSouthwestUniversityofScienceandTechnology野生煙草交配親和性遺傳機理XunchaoXiangSouthwestUniversityofScienceandTechnology(3)突變的有害性和有利性①突變的有害性：大多數(shù)基因的突變，對生物的生長與發(fā)育往往是有害的?！锷锏囊吧突蚨际钦Ｓ泄δ艿?；★生物細胞內(nèi)現(xiàn)有的基因是通過長期自然選擇進化而來，并且基因間達到某種相對平衡與協(xié)調(diào)狀態(tài)?！镆虼耍蛲蛔兛赡軙е拢夯蜷g及相關代謝過程的協(xié)調(diào)關系被破壞。★基因突變與表現(xiàn)往往會導致當代生物個體：性狀變異、個體發(fā)育異常、生存競爭與生殖能力下降，甚至死亡——致死突變。XunchaoXiangSouthwestUniversityofScienceandTechnology致死突變◆致死突變：指發(fā)生突變后會導致特定基因型個體死亡的基因突變?！锎蠖鄶?shù)致死突變都為隱性致死(recessivelethal)，只有突變后代中的隱性純合體才表現(xiàn)為致死的效應。★少數(shù)致死突變表現(xiàn)為顯性致死(dominantlethal)，帶有突變基因的個體都會死亡。如：人的神經(jīng)膠癥(epiloia)基因?！锶绻滤劳蛔儼l(fā)生在性染色體上，將產(chǎn)生伴性致死(sex-linkedlethal)現(xiàn)象。XunchaoXiangSouthwestUniversityofScienceandTechnology小鼠毛色遺傳的隱性致死突變◆在正常黑色鼠中發(fā)現(xiàn)一種黃色突變型，雜合體(黃色)自群交配、雜合體與黑色鼠交配結(jié)果如下圖所示?！粞芯勘砻鳎狐S色基因(AY)在毛色上表現(xiàn)為顯性，但是同時具有顯性純合致死效應；AYAY個體胚胎階段即死亡，所以雜合體自群交配毛色會表現(xiàn)2:1。XunchaoXiangSouthwestUniversityofScienceandTechnologyXunchaoXiangSouthwestUniversityofScienceandTechnology植物隱性白化突變◆與葉綠體形成有關的基因多達50多對，其中不少基因突變(喪失功能)均可能導致葉綠素不能形成，產(chǎn)生白化苗?！舭谆绮荒苓M行光合作用，子葉或胚乳中養(yǎng)料耗盡時，幼苗就死亡。如下圖所示：XunchaoXiangSouthwestUniversityofScienceandTechnology②突變的有利性◆突變的有害與有利性是相對的：★主要針對突變性狀表現(xiàn)當代個體而言；★同時也主要是對生物本身的生長發(fā)育、繁殖而言?！粼谀承┣闆r下，基因突變的有害與有利性可以轉(zhuǎn)化：★對突變性狀表現(xiàn)當代及后代群體而言：例如：抗逆性(抗生物、非生物協(xié)迫)；★對后代群體在特殊環(huán)境中生存而言：例如：作物矮稈突變型在多風與高肥環(huán)境下；又如：果蠅殘翅突變型在多風海鳥環(huán)境下。★對人類需求與利用而言：如：作物矮稈突變型的利用；又如：作物雄性不育突變型的利用。XunchaoXiangSouthwestUniversityofScienceandTechnology③

中性突變◆中性突變：指突變型的性狀變異對生物個體生活力與繁殖力沒有明顯的影響，在自然條件下不具有選擇差異的基因突變?！锷镞M化過程中自然環(huán)境對生物的選擇主要依據(jù)生物在競爭條件下生活力與繁殖力的差異。在特定環(huán)境下生活力與繁殖力相對較高的類型(各種突變型)被保存下來；反之則淘汰?！餂]有生活力與繁殖力差異的類型則是隨機地保留下來，因此某些性狀在生物群體內(nèi)多種突變型與突變基因共同存在。XunchaoXiangSouthwestUniversityofScienceandTechnology(4)突變的平行性◆指親緣關系相近的物種因為遺傳基礎比較接近，往往會發(fā)生相似的基因突變。★根據(jù)這一學說，如果一個物種或更大的生物分類單位中存在某種類型的變異，與其同類的生物中也可以預期得到這些變異類型。如：禾本科植物籽粒性狀變異、矮稈突變。XunchaoXiangSouthwestUniversityofScienceandTechnology8.4IdentificationofGeneMutation1.植物基因突變的鑒定

(1)突變真實性的鑒定◆原始材料與變異體在一致的環(huán)境條件下種植(培育)；◆對兩類個體進行性狀考察與比較分析(進行方差分析)；◆根據(jù)試驗結(jié)果進行判定：★兩類個體間沒有差異不可遺傳變異(環(huán)境變異)；★差異仍然存在存在真實差異為突變體?！舴肿铀借b定方法：★蛋白質(zhì)產(chǎn)物的差異分析；★

DNA(RFLP、RAPD等方法)。(2)突變顯隱性的鑒定XunchaoXiangSouthwestUniversityofScienceandTechnology(3)突變率的測定①

基因突變的頻率★突變率(mutationrate)指生物在一個世代中在特定條件下發(fā)生某一突變的概率。

也就是突變體占該世代個體的比例?！镉行陨成铮?/p>

用突變配子占總配子比例(配子發(fā)生突變的概率)表示；★(單細胞)無性繁殖生物：

每一世代中細胞發(fā)生突變的頻率。XunchaoXiangSouthwestUniversityofScienceandTechnology②

自然突變頻率自然條件下基因突變率一般較低，并隨生物種類、基因而異：◆不同生物種類的基因突變率：★高等植物： ~1×10-5-1×10-8;★低等生物，如細菌： ~1×10-4-1×10-10;★人： ~1×10-4-1×10-6.◆同一物種的不同基因的天然突變率也明顯不同：XunchaoXiangSouthwestUniversityofScienceandTechnology③

花粉直感法測定突變(誘變)率玉米籽粒胚乳：非甜(Su)甜(su)P: 甜粒親本(susu)×非甜粒親本(SuSu)G: su SusuF1: Susu(非甜) susu(甜粒)

正?；ǚ哿：蟠?突變花粉粒后代誘變處理XunchaoXiangSouthwestUniversityofScienceandTechnology④

體細胞誘變頻率測定◆對種子(胚)進行誘變處理，突變可能發(fā)生于：★葉原基 葉片；★葉腋原基 分蘗(有效分蘗/無效分蘗)；★莖尖生長點 主穗及后發(fā)生分蘗。◆發(fā)生顯性突變：★突變當代M1相應器官表現(xiàn)突變性狀。◆發(fā)生隱性突變：★突變當代M1并不表現(xiàn)突變性狀；★其自交后代M2將有部分個體表現(xiàn)突變性狀?！暨@時往往用M2中突變體比例來表示突變率(例)。XunchaoXiangSouthwestUniversityofScienceandTechnologyXunchaoXiangSouthwestUniversityofScienceandTechnology2.生化突變的鑒定◆Beadle,G.W.(1941)通過紅色面包霉突變研究發(fā)現(xiàn)：基因是通過酶的作用控制性狀表現(xiàn)，提出“一個基因一個酶”假說(如圖所示)。XunchaoXiangSouthwestUniversityofScienceandTechnology生化突變及相關概念◆生化突變：由于誘變因素影響導致生物代謝功能的變異?？梢詫φ€體與變異個體的生化特性研究以分析基因的作用機制?！粢吧?wildtype)與原養(yǎng)型(prototroph)★野生型是指存在于自然界中沒有經(jīng)過基因突變，具有正常生化代謝功能的遺傳類型；★原養(yǎng)型指具有與野生型相同營養(yǎng)需求與表現(xiàn)的遺傳類型，有時特指突變型恢復為與野生型相同的個體。◆營養(yǎng)缺陷型(auxotroph)★因基因突變喪失了某種生活物質(zhì)合成能力，在基本培養(yǎng)基上不能正常生長，需加入相應營養(yǎng)成分的突變型。XunchaoXiangSouthwestUniversityofScienceandTechnology(1)紅色面包霉的生化突變型◆野生型紅色面包霉能在基本培養(yǎng)基上正常生長。水、無機鹽、糖類、微量生物素(酶促合成)必需的復雜有機物◆幾種生化突變型：★突變型a：精氨酸 (精氨酸合成缺陷型)；★突變型c：精氨酸或瓜氨酸 (瓜氨酸合成缺陷型)；★突變型o：精氨酸、瓜氨酸或鳥氨酸

(鳥氨酸合成缺陷型)。◆研究表明，精氨酸是蛋白質(zhì)合成的必需氨基酸，而其合成途徑為：XunchaoXiangSouthwestUniversityofScienceandTechnology(2)紅色面包霉生化突變的鑒定方法◆突變的誘發(fā)：X射線或UV照射分生孢子，再與野生型交配，產(chǎn)生分離的子囊孢子◆突變的鑒定：★

突變的真實性：在基本培養(yǎng)基上培養(yǎng)——

能夠生長未發(fā)生營養(yǎng)缺陷型突變； 不能生長可能發(fā)生營養(yǎng)缺陷型突變。★

突變的類型(哪類型營養(yǎng)缺陷型突變？)： 氨基酸？(加入各種氨基酸)不能生長。 維生素？(加入各種維生素)能夠生長。★進一步鑒定具體類型：

硫胺素(VB1)、吡醇素(VB6)、泛酸、肌醇。XunchaoXiangSouthwestUniversityofScienceandTechnology紅色面包霉生長突變的誘發(fā)和鑒定XunchaoXiangSouthwestUniversityofScienceandTechnology3.人類基因突變的鑒定XunchaoXiangSouthwestUniversityofScienceandTechnology8.5ThemolecularbasisofmutationTransition(轉(zhuǎn)換):Amutationaleventinwhichonepurineisreplacedbyanother,oronepyrimidineisreplacedbyanother.Transversion(顛換):Amutationaleventinwhichapurineisreplacedbyapyrimidine,orapyrimidineisreplacedbyapurine.XunchaoXiangSouthwestUniversityofScienceandTechnology8.5ThemolecularbasisofmutationAtmolecularlevel:Pointmutation:Amutationthatcanbemappedtoasinglelocus.Atthemolecularlevel,amutationthatresultsinthesubstitutionofonenucleotideforanother.Frameshiftmutation:Amutationaleventleadingtotheinsertionordeletionofoneormorebasepairsinagene,shiftingthecodonreadingframeinallcodonsfollowingthemutationalsite.XunchaoXiangSouthwestUniversityofScienceandTechnology8.5ThemolecularbasisofmutationXunchaoXiangSouthwestUniversityofScienceandTechnology8.5ThemolecularbasisofmutationInsertionsanddeletionshavethepotentialtochangeallsubsequenttripletsinagene.OneofmanyalteredtripletswillbeeitherUAA,UAG,orUGA.Whenoneisencounteredduringtranslation,polypeptidesynthesisisterminated.XunchaoXiangSouthwestUniversityofScienceandTechnology8.5ThemolecularbasisofmutationA.Tautomeric(互變異構(gòu)體)shiftAreversibleisomerization(異構(gòu)化)inamoleculebroughtaboutbyashiftinthelocalizationofahydrogenatom.Innucleicacids,tautomericshiftsinthebasesofnucleotidescancausechangesinotherbasesatreplicationandareasourceofmutations.XunchaoXiangSouthwestUniversityofScienceandTechnology酮式

烯醇式

亞氨基XunchaoXiangSouthwestUniversityofScienceandTechnologyXunchaoXiangSouthwestUniversityofScienceandTechnology8.5ThemolecularbasisofmutationB.BaseanalogsAchemicalcompoundstructurallysimilartoanother,butdifferingbyasinglefunctionalgroup,aremoleculesthatcansubstituteforpurinesorpyrimidinesduringnucleicacidbiosynthesis.Forexample,5-bromouracil(5-BU).XunchaoXiangSouthwestUniversityofScienceandTechnologyXunchaoXiangSouthwestUniversityofScienceandTechnology8.5Themolecularbasisofmutation2-aminopurine(2-AP),canservesuccessfullyasananalogofadenine.Inadditiontoitsbase-pairingaffinitywiththymine,2-APcanalsobase-pairwithcytosine.Assuch,transitionsfromA＝

TtoG≡Ccanresultfollowingreplication.XunchaoXiangSouthwestUniversityofScienceandTechnologyMutationaltypesofbaseanalogs①.Transition②.Reversemutation(reversion)Amutationthatrestoresthewild-typephenotype.XunchaoXiangSouthwestUniversityofScienceandTechnology8.5ThemolecularbasisofmutationC.Alkylating(烷基化)agentsThesulfur-containingmustardgaseswereoneofthefirstgroupsofchemicalmutagensdiscovered.Mustardgasesarealkylatingagents;thatis,theydonateanalkylgroupsuchasCH3-orCH3-CH2-toaminoorketogroupsinnucleotides.Forexample,ethylmethanesulfonate(EMS).XunchaoXiangSouthwestUniversityofScienceandTechnologyMutationaltypeofalkylatingagents:transition.

XunchaoXiangSouthwestUniversityofScienceandTechnology8.5ThemolecularbasisofmutationD.Acridine(吖啶)dyesandframeshiftmutationsAcridinedyesareofaboutthesamedimensionasanitrogenousbasepairandareknowntointercalateorwedgebetweenpurinesandpyrimidinesofintactDNA.IntercalationofacridinedyesinducescontortionsintheDNAhelix,causingdeletionsandinsertions.MutationaltypeofAcridine:

frameshiftmutation.XunchaoXiangSouthwestUniversityofScienceandTechnology8.5ThemolecularbasisofmutationE.Apurinic(無嘌呤)sitesanddeamination(去氨基)

Anothertypeofmutationinvolvesthespontaneouslossofoneofthenitrogenousbasesinanintactdouble-helicalDNAmolecule.Mostfrequently,suchaneventinvolveseitherguanineoradenine.Thesesites,createdbythe“breaking”oftheglycosidic(糖苷)bondlinkingthe1′-Cofd-riboseandthenumber9positionofthepurinering,arecalledapurinicsites(APsites).XunchaoXiangSouthwestUniversityofScienceandTechnology8.5ThemolecularbasisofmutationTheabsenceofanitrogenousbaseatanAPsiteswillalterthegeneticcodeifthestrandinvolvedistranscribedandtranslated.Ifreplicationoccurs,theAPsiteisaninadequatetemplateandcancausereplicationtostall.Deamination:anaminogroupisconvertedtoaketogroupincytosineandadenine.Inthesetwocases,cytosineisconvertedtouracilandadenineischangedtohypoxanthine(次黃嘌呤).XunchaoXiangSouthwestUniversityofScienceandTechnology8.6Ultravioletandhigh-energyradiation宇宙射線

XunchaoXiangSouthwestUniversityofScienceandTechnologyA.Ultraviolet(UV)radiationPyrimidinedimersXunchaoXiangSouthwestUniversityofScienceandTechnologyB.IonizingradiationX-rays,gammarays,andcosmicrayshaveshorterwavelengthsthandoesUVradiationandarethereforemoreenergetic.Asaresult,theyarestrongenoughtopenetratedeeplyintotissues,causingionizationofthemoleculesencounteredalongtheway.Thus,stablemoleculesandatomsaretransformedintofreeradicalsandreactiveions.Thetrailofionsleftalongthepathofahigh-energyraycaninitiateavarietyofchemicalreactions.XunchaoXiangSouthwestUniversityofScienceandTechnologyXunchaoXiangSouthwestUniversityofScienceandTechnologyTargettheory:Thereareoneormoresites,ortargets,withincellsandthatasingleeventofirradiationatonesitewillbringaboutadamagingeffect,ormutation.XunchaoXiangSouthwestUniversityofScienceandTechnologyIrradiationeffects:Intensityofthedose(doserate)administeredseemstomakelittledifferenceinthemutageniceffect.Atotalexposureof100roentgens(倫琴

)seemstoproducethesameoverallmutageniceffectwhetheritisadministeredinasingleacutedoseorspreadoutovertimeinseveralsmallerchronicdoses.Certainportionsofthecellcyclearemoresusceptibletoradiationeffects.XunchaoXiangSouthwestUniversityofScienceandTechnology8.7Mutationsinhumans:casestudiesA.ABObloodtypesHAorBHOHismodifiedbytheproductsofIAorIBFailingtomodifytheHsubstance,asaresultoftheIOalleleGlycosyltransferase,糖基轉(zhuǎn)移酶

Becauseofframeshiftmutation,thepolypeptidechainterminatesprematurely,resultinginanonfunctionalproduct.ConvertingLackXunchaoXiangSouthwestUniversityofScienceandTechnologyB.Musculardystrophy(肌肉萎縮癥

)Asevereprogressivemusculardegeneration,resultinginthedeathoftheaffectedindividualsbyearlyadulthood.RecessiveandX-linked,1/3500livemalebirths.Duchennemusculardystrophy(DMD):commonandmoresevere.Beckermusculardystrophy(BMD):lesssevere.XunchaoXiangSouthwestUniversityofScienceandTechnologyB.MusculardystrophyGeneMessengerRNADystrophinprotein,肌營養(yǎng)不良蛋白

(3685a.a.)CanbedetectedinBMDIsrarelyfoundinDMD(2000kb)(14kbinunaffectedindividual)

XunchaoXiangSouthwestUniversityofScienceandTechnologyB.MusculardystrophyDenDunnen’sfinding:InananalysisoftheDNAof194patients(160DMDand34BMD),128(65%)consistedofsubstantialdeletionsorinsertions.Of115deletions,17occurredinBMDcases,andof13insertions,1occurredinBMD,withtheremainderinDMDcases.XunchaoXiangSouthwestUniversityofScienceandTechnologyB.MusculardystrophyHypothesis:MostmutationscausingBMDdonotalterthereadingframe,butthatmostDMDmutationschangethereadingframeearlyinthegene,resultingintheprematureterminationofdystrophintranslation.XunchaoXiangSouthwestUniversityofScienceandTechnologyB.MusculardystrophyDenDunnen’sfindingreflectsthefact:

Amutationcausedbyarandomsingle-nucleotidesubstitutionwithinageneismorelikelytobetoleratedwithoutthedevastatingeffectofmusculardystrophythantheadditionorlossofnumerousnucleotidesthatcanlatertheframeofreading.XunchaoXiangSouthwestUniversityofScienceandTechnologyReasons1.Anucleotidesubstitutionmaynotchangetheencodedaminoacid,sincethecodeisdegenerate.2.Ifanaminoacidsubstitutiondoesresult,thechangemaynotbepresentatalocationwithintheproteinthatiscriticaltoitsfunction.3.Evenifthealteredaminoacidispresentatacriticalregion,itmaystillhavelittleornoeffectonthefunctionoftheprotein.XunchaoXiangSouthwestUniversityofScienceandTechnologyC.Trinucleotiderepeatsinfragile-Xsyndrome,myotonicdystrophy(強直性肌營養(yǎng)不良),andhuntingtondiseaseResultsbythemolecularanalysisoftheDNArepresentingthegenesresponsibleforanumberofinheritedhumandisorders:Mutantgeneswerecharacterizedbyanexpansionofasimpletrinucleotiderepeatsequence,usuallyfromfewerthan15copiesinnormalindividualstoalargenumberinaffectedindividuals.XunchaoXiangSouthwestUniversityofScienceandTechnologyC.Trinucleotiderepeatsinfragile-Xsyndrome,myotonicdystrophy(強直性肌營養(yǎng)不良),andhuntingtondiseaseTheautosomaldisordersmyotonicdystrophyandhuntingtondiseaseandtheX-linkedfragile-Xsyndrome,eachgenewasfoundtocontainauniquetrinucleotideDNAsequencerepeatedmanytimes.XunchaoXiangSouthwestUniversityofScienceandTechnologyTrinucleotide-repeatDisordersTrinucleotideRepeatNumber(normal)Number(affected)InheritancePatternHuntingtonCAG6-3536-120AutosomaldominantMyotonicdystrophyCTG5-35>200AutosomaldominantFragile-XsyndromeCGG6-50>200X-linkeddominantSpinobularmuscularatrophyCAG10-3035-60X-linkeddominantXunchaoXiangSouthwestUniversityofScienceandTechnology8.8CounteractingDNAdamage:repairsystemsA.Photoreactivationrepair:reversalofUVdamageinprokaryotesPhotoreactivationenzyme~PREXunchaoXiangSouthwestUniversityofScienceandTechnologyB.ExcisionrepairinprokaryotesandeukaryotesTheprocess:ⅰ.Thedistortionorerrorisrecognizedandenzymaticallyclippedoutbyanuclease.This“excision”mayaffectonlyasinglebase,asinglenucleotide,orincludeseveralnucleotidesadjacenttotheerroraswell,leavingagapinthehelix.ⅱ.DNApolymeraseⅠfills

thisgapbyinsertingd-ribonucleotidescomplementarytothoseontheintactstrand.Theenzymeaddsthesebasestothe3′-OHendoftheclippedDNA.ⅲ.ThejoiningenzymeDNAligasesealsthefinal“nick”thatremainsatthe3′-OHendofthelastbaseinserted,closingthegap.XunchaoXiangSouthwestUniversityofScienceandTechnologyTypesofExcisionRepairBaseexcisionrepair(BER):

isinvolvedincorrectingdamagetonitrogenousbasescreatedbythespontaneoushydrolysis(水解)ofDNAbases,aswellasbyagentsthatchemicallyalterthem.DNAglycosylases(轉(zhuǎn)葡糖基酶),endonuclease,theexcision-repairsystem,inE.coli.Nucleotideexcisionrepair(NER):

repairs“bulky”lesionsinDNAthatalterordistorttheregularDNAdoublehelix.XunchaoXiangSouthwestUniversityofScienceandTechnologyBaseexcisionrepair(BER)XunchaoXiangSouthwestUniversityofScienceandTechnologyNucleotideexcisionrepair(NER)XunchaoXiangSouthwestUniversityofScienceandTechnologyC.XerodermaPigmentosumandNucleotideExcisionRepairXerodermaPigmentosum(XP,著色性干皮病),araregeneticdisorderthatpredisposesindividualstosevereskinabnormalities.TheseindividualshavelosttheirabilitytoundergoNER.XunchaoXiangSouthwestUniversityofScienceandTechnologyD.ProofreadingandMismatchRepairDNApolymeraseⅢpossessesaproofreadingfunction.Proofreadingisthoughttoincreasefidelityduringsynthesisbytwoordersofmagnitude.Thoseerrorsthatremainafterproofreadingarerepairedbyanothermechanism,calledmismatchrepair.XunchaoXiangSouthwestUniversityofScienceandTechnologyPuzzleHowdoestherepairsystemrecognizewhichstrandiscorrect?Whichcontainsthemismatchedbase?Howtherepairsystemdiscriminatesandrecognizesthe“new”nucleotide?XunchaoXiangSouthwestUniversityofScienceandTechnologyKeyDNAmethylationMethylase(4types)MethylaserecognizestheDNAsequenceasasubstrate.Uponrecognition,amethylgroupisaddedtoeachoftheresidues.Themodificationisstablethroughoutthecellcycle.Followingafurtherroundofreplication,thenewlysynthesizedstrandsremaintemporarilyunmethylated(therepairenzymerecognizesthemismatchandpreferentiallybindstotheunmethylatedstrand).XunchaoXiangSouthwestUniversityofScienceandTechnologyE.Post-ReplicationRepairandtheSOSRepairPost-ReplicationRepairisalsoreferredtoashomologousbinationrepair.XunchaoXiangSouthwestUniversityofScienceandTechnologySOSRepairSystemJeamWeigleetal.found,UVBacteria－h(huán)ighlivability

(beirradiatedbyUV)Bacteriophage

λ

Bacteria－lowlivability

(

notbeirradiatedbyUV)UV-reactivation

（UV-復活）Errorpronerepair

（傾向容錯修復）infectingXunchaoXiangSouthwestUniversityofScienceandTechnologySOSresponse

（SOS反應）TheinductionofenzymestorepairdamagedDNAinE.coli.Theresponseinvolvesactivationofanenzymethatcleavesarepressor,activatingaseriesofgenesinvolvedinDNArepair.Nogapiscreatedandthefidelityofreplicationiscompromised.ErrorproneXunchaoXiangSouthwestUniversityofScienceandTechnologyXunchaoXiangSouthwestUniversityofScienceandTechnologyF.Double-StrandBreakRepair

(DSBRepair)inMammalsDSBrepairisreferredtoashomologousbinationalrepair,becausedamagedDNAisactuallybinedandreplacedwithhomologousundamagedDNA.XunchaoXiangSouthwestUniversityofScienceandTechnology8.9TransposablegeneticelementsADNAsegmentthattranslocatestoothersitesinthegenome,essentiallyindependentofsequencehomology.Usuallysuchelementsareflankedbyshort,invertedrepeatsof20to40basepairsateachend.Insertionintoastructuralgenecanproduceamutantphenotype.Insertionandexcisionoftransposableelementsdependsontwoenzymes,transposase(轉(zhuǎn)座酶

)andresolvase(解離酶

).Suchelementshavebeenidentifiedinbothprokaryotesandeukaryotes.Sometimesreferredtoastransposons(Tn).XunchaoXiangSouthwestUniversityofScienceandTechnologyWemustmemorizeBarbaraMcClintock(June16,1902—September2,1992),thefirstperson,whofoundtransposableelementsinmaizein1932.XunchaoXiangSouthwestUniversityofScienceandTechnologyInsertionSequences(IS)AmobileDNAsegmentthatistransposabletoanyofanumberofsitesinthegenome.Insertingintothechromosome;Affectinggenefunction;beingrelativelyshort,notexceeding2kb.XunchaoXi