分子生物学的中心法则

第三节分子生物学的中心法则TheCentralDogmaofMolecularBiology1)中心法则总述2)DNA自我复制3)从DNA到RNA(转录)4)遗传密码5)从RNA到蛋白质(翻译)6)基因一、中心法则简介TheCentralDogmaofMolecularBiology二、DNA复制DNAReplicatingItselfReplication，复制：在亲代DNA双链的每一条链上按碱基配对而准确地形成一条新的互补链，结果生成两个与亲代链相同的DNA双链。DNA复制示意图DNA的半保留复制在DNA复制时，亲代DNA的双螺旋先解旋，然后以每条链为模板，按照碱基配对原则，在这两条链上各形成一条互补链。在每一个新形成的双螺旋中，一条链是从亲代DNA来的，另一条是新形成的。3’5’5’3’3’5’5’3’3’5’5’3’leadingstrand(synthesizedcontinuously)laggingstrand(synthesizeddiscontinuously)Eachreplicationforkhasaleadingandalaggingstrand•TheleadingandlaggingstrandarrowsshowthedirectionofDNAchainelongationina5’to3’direction•ThesmallDNApiecesonthelaggingstrandarecalledOkazakifragments(100-1000basesinlength)replicationforkreplicationforkDNA的半不连续复制RNAprimer5’3’3’5’3’5’directionofleadingstrandsynthesisdirectionoflaggingstrandsynthesisreplicationfork5’3’5’3’MovementofthereplicationforkMovementofthereplicationforkRNAprimerOkazakifragmentRNAprimer5’复制体(replicon)在DNA复制过程中，由众多的酶和蛋白质参与DNA的复制作用。复制体的基本活动包括：1）双链的解开；2）RNA引物的合成；3）DNA链的延长；4）切除RNA引物，填补缺口，连接DNA片段；5）切除和修复错配碱基。三、转录Transcription:makinganRNAcopyofaDNAsequenceTranscription，转录：以DNA为模板，按碱基配对将其中所含的遗传信息传给RNA，形成一条与DNA链互补的mRNA链的过程。编码链，有义链模板链，无义链TranscriptionofasegmentofDNAtoformamoleculeofRNA.1）RNA聚合酶与DNA模板的结合2）起始3）延长4）终止TranscriptionRNApolymerase,RNA聚合酶closedpromotercomplex,封闭的启动子复合物openpromotercomplex,开放的启动子复合物initiationelongationterminationRNAproductRNAchainsaresynthesizedina5’to3’directionProkaryoticRNApolymerasestructureRNApolymeraseofE.coliisamultisubunitproteinSubunitNumberRolea2uncertainb1formsphosphodiesterbondsb1bindsDNAtemplates1recognizespromoterandfacilitatesinitiationa2bb’sa2bb’+sholoenzymecorepolymerasesigmafactorRNApolymeraseholoenzyme(+sfactor)•closedpromotercomplex(moderatelystable)•thesigmasubunitbindstothe-10region•onceinitiationtakesplace,RNApolymerasedoesnotneedveryhighaffinityforthepromoter•sigmafactordissociatesfromthecorepolymeraseafterafewelongationreactions•elongationtakesplacewiththecoreRNApolymerase•openpromotercomplex(highlystable)•theholoenzymehasveryhighaffinityforpromoterregionsbecauseofsigmafactors•sigmacanre-bindothercoreenzymesThesigmacycless四、遗传密码TheGeneticCode:TranslationofRNAcodeintoprotein三联体密码子的特点：1）方向性2）无标点性3）简并性4）通用性5）摆动性起始密码子：ATG。终止（无义）密码子：UAAUGAUAG。同义密码子：编码同一个氨基酸的几个密码子。五、翻译TranslationistheprocessofconvertingthemRNAcodonsequencesintoanaminoacidsequence.Translation，翻译：在RNA控制下，根据核酸链上每三个核苷酸决定一个氨基酸的三联体密码规则，合成出具有特定氨基酸顺序的蛋白质肽链的过程。ProteinSynthesisandProteinProcessinga).RNAstructureb).Proteinsynthesisi).Initiationofproteinsynthesisii).Peptidebondformation;peptidyltransferaseiii).ElongationandterminationRNA类型结构作用mRNA信使RNA单链；原核生物的mRNA为多顺反子，真核生物的为单顺反子。DNA原始遗传信息的直接接受者；合成蛋白质直接模板。tRNA转运RNA二级结构是三叶草型结构；三级结构为倒L型结构。转运氨基酸到核糖体上；通过反密码子识别mRNA上的密码子。rRNA核糖体RNA16S(smallribosomalsubunit)23S(largeribosomalsubunit)5S(largeribosomalsubunit)含量大，维持核糖体的空间结构；使mRNA和tRNA结合在适当位置；识别mRNA上的启动和终止信号。核糖体是蛋白质生物合成的场所；mRNA是合成的模板；tRNA是模板与“合成原料”氨基酸之间的接合体。mRNArRNA原核生物的核糖体为70S，由50S和30S的大小亚基组成；真核生物的核糖体为80S，由60S和40S的大小亚基组成。rRNA是构成核糖体的骨架。16S(smallribosomalsubunit)23S(largeribosomalsubunit)5S(largeribosomalsubunit)RibosomestructureAPPPPPPPPP-sitepeptidyltRNAsiteA-siteaminoacyltRNAsitemRNA5’Smallsubunit(40S)Largesubunit(60S)RibosomewithboundtRNAsandmRNAThemodeloftRNA.a.a.+tRNA+ATP氨酰-tRNA+AMP+PPi氨酰-tRNA合成酶氨酰-tRNA合成酶能识别tRNA，并催化反应。对L-氨基酸有极高的专一性，每种氨基酸都有一个专一的酶。氨酰-tRNA合成酶的高度专一性，防止在蛋白质合成时错误的氨基酸掺入多肽。TertiarystructureSecondarystructuretRNAProteinsynthesis起始延长终止一个新生肽链的诞生，除了必须有mRNA模板，tRNA及核糖体外，还需要许多蛋白因子。如，起始因子(IF)，延长因子(EF)和终止因子(RF)等的参与。肽链合成的方向是N-C,mRNA信号被转译的方向是5`-3`。ProteinSynthesismRNA5’cap40SsubunitMeIF2AUGInitiatortRNAboundtothesmallribosomalsubunitwiththeeukaryoticinitiationfactor-2(eIF2)Initiationofproteinsynthesis:mRNAbindingThesmallsubunitfindsthe5’capandscansdownthemRNAtothefirstAUGcodonmRNA5’40SsubunitMeIF2AUG•theinitiationcodonisrecognized•eIF2dissociatesfromthecomplex•thelargeribosomalsubunitbinds60SsubunitmRNA5’MAUG•aminoacyltRNAbindstheA-site•firstpeptidebondisformed•initiationiscompleteGCCAmRNA5’MAUGGCCA氨酰-tRNANH2CH3-S-CH2-CH2-CHO=CPeptidebondformation•peptidebondformationiscatalyzedbypeptidyltransferase(肽基转移酶)•peptidebondformationresultsinashiftofthenascentpeptidefromtheP-sitetotheA-siteNH2CH3-S-CH2-CH2-CHO=COtRNANH2CH3-CHO=COtRNANCP-siteA-siteOHtRNANHCH3-CHO=COtRNAP-siteA-sitePUCAPPPPPUCAGCAGGGUAGAPPPPElongationGCAGGGUAG•followingpeptidebondformationtheunchargedtRNAdissociatesfromtheP-site•theribosomeshiftsonecodonalongthemRNA,movingpeptidyltRNAfromtheA-sitetotheP-site;thistranslocationrequirestheelongationfactorEF2•thenextaminoacyltRNAthenbindswithintheA-site;thistRNAbindingrequirestheelongationfactorEF1EF1EF2•energyforelongationisprovidedbythehydrolysisoftwoGTPs:•onefortranslocation•oneforaminoacyltRNAbindingPUCAGCAGGGUAGPPPPTermination•whentranslationreachesthestopcodon,areleasefactor(RF)bindswithintheA-site,recognizingthestopcodon•releasefactorcatalyzesthehydrolysisofthecompletedpolypeptidefromthepeptidyltRNA,andtheentirecomplexdissociatesRFPUCAGCAGGGUAGPPPPPPP六基因GeneGene基因，遗传功能的单位。它是一种DNA序列，在有些病毒中则是一种RNA序列，它编码