大二上遗传学jc ch 14 gene mutationdna repair and transposable elements-m

1、Gene Mutation, DNA Repair andTransposable ElementWhat will we learn today?Gene mutation: definition and classificationMolecular basis ofmutationDNA repair and detection of mutationTransposable elementsGene mutationAny change in the base-pair sequence of DNA,which occurs either within the coding sequ

2、ence or within noncoding regions.Gene mutation Chromosome mutationGene mutationGene mutation is a double-edged sword.Gene mutations may causes various diseases.However, gene mutation is a source of geneticvariation and provides the raw material for natural selection.Genetic variations introduced by

3、genemutation allow species to adjust to different environmental conditions.染色体畸变 碱基错配 复制错误 突变DNA 复制跳格自发突变脱嘌呤脱氨基基因突变化学错误 氧化损伤放射线产生嘌呤二聚体诱发突变碱基类似物:5BU,2AP碱基修饰物:NA,HA,烷化剂DNA 插入剂:吖啶类化学诱变剂14.1 Classification of MutationsConcept of a mutation in the protein-coding region of a gene. (Note that not all mutat

4、ions lead to altered proteins and that not all mutations are in protein-coding regions.)Spontaneous mutations vs. inducedmutationsSpontaneous mutations: mutations that are not associated with any specific agents, therefore are generally assumed to be accidental. (Low mutation rate!)Spontaneous mutat

5、ions usually arise as a resultof normal biological and chemical processes. (Especially DNA replication!)Spontaneous mutations vs. inducedmutationsInduced mutations: mutations that result from the influence of extraneous factors.Mutagen: agents that increase mutation rate.RadiationChemicals: preserva

6、tives, pesticides, herbicides, even natural substances in food.smoke,Mutations DefinedAny base-pair change in any part of a DNA molecule can be considered a mutation.Somatic mutations体细胞突变occur in somatic cells; the mutant phenotype will occur only in the descendants of that cell and will not be tra

7、nsmitted to the progeny or passed to the next generationGerminal mutations生殖细胞突变occur in germ-line cells and will be transmitted through the gametes to the progeny.Autosomal / X-linked mutationThe size of the patch will depend on the timing of the mutationThe earlier the mutation, the larger the pat

8、chAn individual who has somatic regions that are genotypically different from each other is calleda genetic mosaicTherefore, the mutation can be passed on to future generationsTherefore, the mutation cannot be passed on to future generationschimera mosaic嵌合体Types of Mutations Changes in chromosome n

9、umber and structure Point mutation-changes at specific nucleotidein a gene (A,T,C,G) Insertion mutations-insert fragment of DNA Deletion mutations-delete fragment of DNA 表 21-1 突变的各种类型 转换 Py 与 Py，Pu 与 Pu 之间变换，多见 点突变，碱基替换 颠换 Py 与 Pu 之间变换，少见 插入 1-2 个碱基 移码突变 发生移码 丢失 1-2 个碱基 缺失突变 缺失大片段 DNA（十几到几千个碱基） 点突变

10、（point mutation）是DNA多核苷酸链中单个碱基或碱基对的改变。CategorizingGene pointMutations1. Silent substitutions同义: the mutation changes one codon for an amino acid into another codon for that same amino acid.2. Missense错义mutations: the codon for one amino acid is replaced by a codon for another amino acid.3. Nonsense无

11、义mutations: the codon for one amino acid is replaced by a translation termination (stop) codon. nucleotide substitutions. If a purine replaces a purine or a pyrimidine replaces a pyrirnidine, a transition(转换） has occurred. If a purine and a pyrimidine are interchanged, a transversion（颠换） has occurre

12、d.1碱基替换（base substitution）DNA链中碱基或碱基对被置换、替代的突变形式。 同类碱基之间的替换，又被称之为转换（transition）被另外一种碱基所置换，则称之为颠换（transversion）（1）同义突变silent mutation遗传子所编码的氨基酸种类保持不变（碱基替换发生在第三位）（2）无义突变nonsense mutation遗传子变成为不编码任何氨UAA、 UAG 或基酸的终止UGA的突变。（3）移码突变frameshift改变了自发突变位点到开放阅读框的终止子之间的全部序列通常会导致蛋白产物丧失功能。（4）错义突变（missense mutation

13、)遗 传 改变。子所编码的氨基酸种类Describe the differencestransition mutation 转换purine/pyrimidine)(purine/pyrimidine totransversion mutation颠换 (purine/pyrimidine topyrimidine/purine)missense mutation错义突变 (different amino acid)nonsense mutation无义突变(stop)silent mutation 同义突变(same amino acid)frameshift mutation 移码突变(c

14、hanges everything after it)Which of the above point mutations would never affect the protein that the gene encodes?Why do frameshift mutations usually result in a nonfunctional protein?A nonsense mutation and its effect on translationDisruption of FGF (Fibroblast Growth Factor)Receptor FunctionFibro

15、blast Growth Factor Signaling is Essential forMesoderm(中胚层)Production in Frog EmbryosDominant mutations in human FGFR-3 (TM domain)临床表现为特殊类型的侏儒-短肢型侏儒.躯干与四肢不成比例， 头颅大而四肢短小 .成纤维细胞生长因子受体的基因发生了点突变，位置在第4对染色体的短臂上。Achondroplasia (软骨发育不全）Mutation: Spontaneous or InducedAll mutations are described as either s

16、pontaneous or induced. Although these two categories overlap to some degree.Spontaneous mutations occur without a known cause due to unknown agents in the environment.Induced mutations result from exposure or organisms to mutagens, physical and chemical agents that cause changes in DNA, such as ioni

17、zing irradiation, ultraviolet light, or certain chemicals.spontaneous mutations are those that just happen in nature. No specific agents are associated with their occurrence, and they are generally assumed to be random changes in the nucleotide sequences of genes.Most spontaneous mutations are thoug

18、ht to occur during the enzymatic process of DNA replication.Once an error is present in the genetic code, it may be reflected in the amino acid composition of the specified protein. If the changed amino acid is present in a part of the molecule critical to the structure or biochemical activity, a fu

19、nctional alteration can result.Fragile- Xsyndrome脆性X综合征Triplet repeat expansion主要表现为中度到重度的智力低下，其它常见的特征尚有身长和体重超过正常儿，发育快，前额突出，面中部发育不全，下 颌大而前突，大耳，高腭弓，唇厚，下唇突出，The role of such repeated sequences in normal and mutant genes remains a mystery.Their locations within the gene vary in each case.In Huntington(

20、神经性舞蹈病) and Kennedy肯尼迪diseases(一种罕见的呈X连锁隐性遗传的运动神经元病,好发于成年男性, 表现为缓慢进行性的肢体和延髓肌肉) , the repeat lies within the coding portion of the gene. This is not the case in theother two disorders, however.In the gene responsible for fragile-X syndrome, the repeat is upstream (5) in an area that is most often inv

21、olved in regulating gene expression.In the case of myotonic dystrophy强直性肌营养不良 , the repeat is downstream (the 3-end).肌强直表现受累的骨骼肌收缩后松弛显著延迟，导致明显的肌肉僵硬，肌电图出现特征性连续高频电位放电现象。Induced mutations; In contrast to such spontaneous events, those that result from the influence of any artificial factor are consider

22、ed to be.It is generally agreed that any natural phenomenon that heightens chemical reactivity in cells will induce mutations.For example, radiation from cosmic and mineral sources and ultraviolet radiation from the sun are energy sources that most organisms are exposed to and, as such, may be facto

23、rs that cause spontaneous mutations.Most chemical mutagens are carcinogens and thus cause cancer Chemical mutagens（化学诱变剂)1. Base analogs（碱基类似物）: mispair，direct mutagenesis，2. Nitrous acid（亚硝酸）:deaminates C to produceU（脱氨基作用）3. Alkylating agents（烷化剂）indirect4. Arylating agents（芳香化剂）mutagenesischemica

24、ls, e.g.associated with our lifepesticides herbicides smokenatural and additive substances in food. preservativesmechanisms generate / study mutationRadiation Physical mutagens（物理诱变剂) 1. High-energy ionizing radiation(电离辐射）:X-rays and -rays strand breaks（断链）， base/sugar destruction（碱基/核糖损伤）2. Nonion

25、izing radiation（非电离辐射） :UV light pyrimidine dimers（嘧啶二聚体）loss-of-function mutation vs. gain-of-function mutationA loss-of-function mutation: reduces or eliminates (null mutation) the function of the gene.A gain-of function mutation: enhances the function of a gene or results in new function of a gen

26、e.loss-of-function 功能丧失突变造成蛋白质活性减弱或者消除，通常是隐性的，在杂合体中不表现，单也有例外，例如单倍体机能不全，机体中不能承受杂合子50%蛋白质活性的下降，杂合子单个野生型等位基因不能提供足够数量的基因产物来行成正常表型。完全丧失基因功能的突变无效突变null mutationgain-of-function获得功能的突变赋予了蛋白质异常的活性，并可能产生新的表型，很多发生在基因的调节序列loss-of-functionleaky mutation渗漏突变 :基因功能的失活不完全，但在杂合子状态下不能产生足够多的基因产物，但较之无效突变其表型得而变化不十分明显。它

27、是处于野生型和营养缺陷型间的中间型.gain-of-functionOther Categories of MutationVarious types of mutations are classified on the basis of their effect on the organism. Note that a single mutation may well fall into more than one category.1. morphological TraitThe most easily observed mutations are those affecting a m

28、orphological Trait. For example, all of MendelspeacharactersandmanygeneticvariationsencounteredinthestudyofDrosophilafitthisdesignation. morphology.Theycauseobviouschangesinmorphological mutations visible phenotype不同眼色的果蝇而患白化病的患者则是由于机体中缺少一种酶酪氨酸酶，患者体内的黑色素细胞不能将酪氨酸酶的最终变成黑色素。机体中控制酪氨酸酶的基因位于第11号常染色体上。因此在遗

29、传的方式上白化病是属于常染色体上的隐性遗传。苹果biochemical mutations & auxotrophsIn bacteria andfungi, the inability to synthesize a particular amino acid or vitamin is an example of a typical nutritional mutation.2. Nutritional or biochemicalvariations : A second broad category of mutations includes those that exhibit nu

30、tritional or biochemical variations in phenotype. .In humans, sickle-cell anemia and hemophiliaare examples of biochemical mutations3. Behavior mutations :A third category consists of mutations that affectbehaviorpatternsofanorganism.Forexample,mating behavior or circadian rhythms of animalsmaybealt

31、ered.Theprimaryeffectofbehavior mutations is often difficult to discern.Sincethismutationmaybein(1)theflightmuscles, (2) the nerves leading to them, or (3) thebrainThestudyofitbehaviorandthegeneticfactorsinfluencinghasbenefitedimmenselyfrominvestigations of behavior mutations.Still another group con

32、sists of lethal mutations. Nutritional and biochemical mutations can also fall into this category. A mutant bacterium that cannot synthesize a specific amino acid it needs will be unable to grow and divide if plated on a medium lacking that amino acid. Various humanbiochemicaldisorders,suchasTay-Sac

33、hsdiseaseGM2 神 经 节 苷 脂 贮 积 症 变 异 型andHuntingtondisease神经性舞蹈病,arelethal different points in the life cycle of humans.atregulatoryregulatorymutations:candisruptnormalactivateprocessesandpermanentlyor inactivate a gene. Our knowledge of geneticregulationdependsonthestudyofmutationsthat disrupt this pro

34、cess.14.3The Molecular Basis ofMutation and factors causingmutationBases are Normally In the keto（ 酮 类 ） Formif in The enol（烯醇） form; Base pair incorrectlyProduction of a mutation as a result of a mismatch caused by wobble base pairing（摇摆碱基对） .Spontaneous Chemical Changes(a) Deamination of cytosine胞

35、嘧啶to uracil尿嘧啶 .(b) Deamination of 5-methylcytosine ( 5-甲基胞嘧啶5mC) to thymine胸腺嘧啶 .Based onMolecular Change Substitutions In and delsMolecular EffectCauses ofSpontaneous Mutationsn Spontaneous mutations can arise by three types of chemical changesn 1.Depurination a purine嘌呤 siteThe most commonn 2.Dea

36、minationNH groupn 3.Tautomeric shift 互变异构 keto酮类 -enol烯醇and base modifiedn Due to DNA replicaton errorMutagens Alter DNA Structure inDifferent Waysn Chemical mutagens come into three main typesn 1.n 2.n 3.Base modifiersIntercalating agents嵌入剂Base analogues相似物亚硝酸羟胺芥氮乙酰磺酸原黄素5-溴尿嘧啶2-氨基嘌呤16-53Copyright

37、The McGraw-Hill Companies, Inc. Permission required for reproduction or display1.Tautomeric Shifts碱基的互变异构体In 1953, Watson and Crickpairingnormal baseThey recognized that the purines andpyrirnidines found in DNA could exist in tautomeric forms; that is, each can exist in alternative chemical forms, d

38、iffering by only a single proton shift in the molecule.Watson and Crick suggested that tautomeric shifts could result in base-pair changes or mutations酮式氨基烯醇亚氨基compares the normal base-pairing relationships with the rare unorthodox (非正规的）pairings.The biologically unstable tautomers involve keto酮类-en

39、ol 烯醇类pairs for thymine and guanine, and amino氨基-imino亚氨基的 pairs for cytosine and adenine.The effect leading to mutation occurs during DNA replication when a rare tautomer in the template strand matches with a noncompletmentary base.Normal base pairingIn the next round of replication, the mismatched

40、 members of the base pair are separated, and each specifies its normal complementary base. The end result is a transition mutation( Figure 14-6).2. Base AnalogsBase analogs, which are mutagenic chemicals, are molecules that can substitute for purines or pyrimidines during nucleic acid biosynthesis.(

41、1)5-bromouracilThe halogenated卤化derivative of uracil in the number-5 position of the pyrimidine ring 5- bromouracil (5-BU 5- 溴尿嘧啶 )* is a good example.TThymineCCytosineFigure 14-7Base analogues: 5-bromouracil (5-BU 5-溴尿嘧啶 )5-BUT (keto) = A T=A CG transitions5-BUC (ionzd) = G CG T=A Transitions3.Base

42、 modifier.(1)AlkyIating Agents烷化剂The sulfur-containing mustard gases含硫芥子wereoneofthefirstgroupsofchemical气mutagens discovered.This discovery was made in studies involving chemical warfare during World War I.Mustard gases are alkylating agents; that is, they donate an alkyl group such as CH3 or CH3CH

43、2 to amino or keto groups in nucleotides.Alkylating agents: EMSGC A=TT=A CGAs with base analogs, base-pairing affinities are altered and transition mutations result. In the case of 6-ethylguanine, this molecule acts like a base analog of adenine, causing it to pairwith thyminetransitionsDeaminationd

44、eamination of C U or 5-methyl-C T CG T=Atransitions* U repairable T not. hot spotsDeamination(nitrous acid （HNO2）可以使碱基发生氧化脱氨作用 H CACUAHNO2AHGTCCGAACUT HNO2depurination脱嘌呤作用Gbreak sugar basemammalian cells loose 1 104purines / cell / gen.= lossof A orglycosidic bond(3) hydroxylamine (羟胺HA)是一种还原剂，可将胞嘧

45、啶（C）上的氨基变为醇基，使C与A配对，从而是GC对转换为AT对 A CA=TGC4. Intercalates(插入剂） -Acridine吖啶Dyes and Frameshift MutationsA group of aromatic芳香剂 moleculesknown as acridine dyes. Proflavin原黄素, themost widely studied acridine mutagen, and acridine orange are examples.Acridine dyes are of about the same dimension as a nit

46、rogenous含氮的 base pair and are known to intercalate or wedge楔入between purines and pyrimidines of intact DNA.acridine orange2- a-acridineproflavin14.4Ultraviolet and High Energy RadiationThe dimers distortIonizing RadiationAs shown in Figure 14-9, X-rays, gamma rays, andcosmicrayshaveevenshorterwavele

47、ngthsthandoes UV radiation and are therefore more energetic.Asaresult,theyarestrongenoughtopenetratedeeplyintotissues,causingionizationofthemolecules encountered along the way. Hermann J.MullerandLewisJ.Stadlerestablishedinthe1920sthatthesesourcesofionizingradiation电离辐射 are mutagenic. Since that tim

48、e, the effectsofionizingradiation,particularlyX-rays,havebeen studied intensely.DNA Repair Mechanisms in E. coliLight-dependent repair (photo-reactivation).Excision repair. Mismatch repair.Post-replication repair.Error-prone repair system (SOS response). John Wiley & Sons, Inc.Photoreactivation Repa

49、ir: Reversal of UV Damage in ProkaryotesAs shown in Figure 14-10, UV light is mutagenic as a result of the creation of pyrimidine dimers.UV Light-DependentRepair: Photolyase Cleaves Thymine Dimers.-No endonuclease-No Poly-No ligase John Wiley & Sons, Inc.DNA Pol 35外切酶活性活光裂合酶一般切除修复UvrABC 系统AP 内切酶直接修复

50、切除修复修复特殊切除修复糖基酶GO 系统-MutM,MutY,MutT错配修复-Dam, MutL,MutH,UvrD重组修复-RecASOS 修复-RecA,LexA,UvrAB,UmuC,HimA复制后修复Excision Repair in Prokaryotes and Eukaryotesin contrast , 暗修复Investigations in the early 1960s suggested that,inadditiontoPRE,alight-independentrepairsystem exists in prokaryotes such as E. coli

51、 thatrepairsdamagetoDNAcausedagents.byThebothbasicexogenousandendogenousmechanism of this type of repair, referred to asexcision repair, has been conserved throughoutevolution,eukaryoticoccurringinallprokaryoticandorganisms.Theprocessconsistsofthree basic steps.Excision Repair(steps)A DNA repair end

52、onuclease核酸内切酶orendonuclease-containing complex recognizes, binds to, and excised the damaged base or bases.A DNA Polymerase fills in the gap, using the undamaged complementary strand of DNA as a template.DNA ligase seals the break left by DNA polymerase.Types of Excision RepairBase excision repair

53、pathways removeabnormal or chemically modified bases.Nucleotide excision repair pathwaysremove larger defects, such as thymine diners.1.The distortion or error is recognized and enzymatically clipped out by a nuclease. This excision may affect only a single base, a single nucleotide, or include several nucleotides adjacent to the error as well, leaving agap in the helix.2. DNA polymerase I fills this gap b