武汉大学 分子生物学全部课程 3

WelcomeEachofYoutoMyMolecularBiologyClassMolecularBiologyoftheGene,5/E---Watsonetal.(2004)PartI:ChemistryandGeneticsPartII:MaintenanceoftheGenomePartIII:ExpressionoftheGenomePartIV:RegulationPartV:Methods3/08/05PartII:MaintenanceoftheGenomeDedicatedtothestructureofDNAandtheprocessesthatpropagate,maintainandalteritfromonecellgenerationtothenextMaintenanceoftheGenomeCh6:ThestructuresofDNAandRNACh7:Chromosomes,chromatinsandthenucleosomeCh8:ThereplicationofDNACh9:ThemutabilityandrepairofDNACh10:HomologousrecombinationatthemolecularlevelCh11:Site-specificrecombinationandtranspositionofDNA3/08/05CHAPTER6TheStructuresofDNAandRNAHowdothestructuresofDNAandRNAaccountfortheirfunctions?OUTLINE1.DNAStructure2.DNATopology3.RNAStructureDNASTRUCTUREDNASTRUCTURE(1)DNAiscomposedofpolynucleotidechainsStructure:twistingaroundeachotherintheformofadoublehelix.SchematicmodelSpace-fillingmodelNucleoside&Nucleotide,thefundamentalbuildingblockofDNAglycosidicbondphosphoesterbondDNApolarity:isdefinedbytheasymmetryofthenucleotidesandthewaytheyarejoined.Phosphodiesterlinkages:repeating,sugar-phosphatebackboneofthepolynucleotidechainBasesinDNApurinespyrimidinesadenineguaninecytosinethymineN9N1DNASTRUCTURE(2)Eachbaseshasitspreferredtautomericform(RelatedtoCh9)Thetwostrandsofthedoublehelixareheldtogetherbybasepairinginanantiparallelorientation,Whichisastereochemicalconsequenceofthewaythatadenineandthymine,andguanineandcytosine,pairwitheachother.(Relatedtoreplicationandtranscription)DNASTRUCTURE(3)TheTwoChainsoftheDoubleHelixHaveComplementarySequencesExample:Ifsequence5’-ATGTC-3’ononechain,theoppositechainmusthavethecomplementarysequence3’-TACAG-5’DNASTRUCTURE(4)Watson-CrickBasePairing(Relatedtoreplicationandtranscription)Thestrictnessoftherulesfor“Waston-Crick”pairingderivesfromthecomplementaritybothofshapeandofhydrogenbondingpropertiesbetweenadenineandthymineandbetweenguanineandcytosine.A:CincompatibilityHydrogenBondingIsImportantfortheSpecificityofBasePairingDNASTRUCTURE(5)Thehydrogenbondsbetweencomplementarybasescontributetothethermodynamicstabilityofthehelix(why?)andthespecificityofbasepairingStackinginteractionsbetweenbasessignificantlycontributetothestabilityofDNAdoublehelixThedoublehelixhasMinorandMajorgrooves(What&Why)DNASTRUCTURE(5)(SeetheStructuralTutorialofthischapterfordetails)Itisasimpleconsequenceofthegeometryofthebasepair.TheMajorgrooveisrichinchemicalinformation(Whatarethebiologicalrelevance?)DNASTRUCTURE(6)Theedgesofeachbasepairareexposedinthemajorandminorgrooves,creatingapatternofhydrogenbonddonorsandacceptorsandofvanderWaalssurfacesthatidentifiesthebasepair.A:H-bondacceptorsD:H-bonddonorsH:non-polarhydrogensM:methylgroupsThedoublehelixexistsinmultipleconformations.DNASTRUCTURE(7)TheBform(10bp/turn),whichisobservedathighhumidity,mostcloselycorrespondstotheaveragestructureofDNAunderphysiologicalconditionsAform(11bp/turn),whichobservedundertheconditionoflowhumidity,presentsincertainDNA/proteincomplexes.RNAdoublehelixadoptsasimilarconformation.DNAstrandscanseparate(denature)andreassociate(anneal)DNASTRUCTURE(8)Keytermstounderstand1.Denaturation2.Hybridization3.Annealing/renature4.Absorbance5.Hyperchromicity6.Tm(meltingpoint)DNATOPOLOGYDNATOPOLOGY(1)Structure(1):Linkingnumberisaninvarianttopologicalpropertyofcovalentlyclosed,circularDNA(cccDNA)Linkingnumberisthenumberoftimesonestrandhavetobepassedthroughtheotherstrandinorderforthetwostrandstobeentirelyseparatedfromeachother.SpeciesofcccDNA1.Plasmidandcircularbacterialchromosomes2.LinearDNAmoleculesofeukaryoticchromosomesduetotheirextremelength,entrainmentinchromatinandinteractionwithothercellularcomponents(Ch7)Structure(2):LinkingnumberiscomposedofTwistandWritheThelinkingnumberisthesumofthetwistandthewrithe.Twististhenumberoftimesonestrandcompletelywrapsaroundtheotherstrand.WritheisthenumberoftimesthatthelongaxisofthedoublehelicalDNAcrossesoveritselfin3-Dspace.DNATOPOLOGY(2)LocaldisruptionofbasepairsFunction(1):DNAincellsisnegativelysupercoiled;nucleosomesintroducesnegativesupercoilingineukaryotesNegativesupercoilsserveasastoreoffreeenergythataidsinprocessesthatrequirestrandseparation,suchasDNAreplicationandtranscription.StrandseparationcanbeaccomplishedmoreeasilyinnegativelysupercoiledDNAthaninrelaxedDNADNATOPOLOGY(3)Function(2):Topoisomerases(P115-119)1.Thebiologicalimportanceoftopoisomerase?2.Thefunctionaldifferenceofthetwotypesoftopoisomerases?3.Theworkingmechanismoftopoisomerase(Seetheanimationfordetail)DNATOPOLOGY(4)RNASTRUCTURERNASTRUCTURE(1)RNAcontainsriboseanduracilandisusuallysingle-strandedBiologicalrolesofRNA1.RNAisthegeneticmaterialofsomeviruses2.RNAfunctionsastheintermediate(mRNA)betweenthegeneandtheprotein-synthesizingmachinery.3.RNAfunctionsasanadaptor(tRNA)betweenthecodonsinthemRNAandaminoacids.4.RNAservesasaregulatorymolecule,whichthroughsequencecomplementaritybindsto,andinterfereswiththetranslationofcertainmRNAs.5.SomeRNAsareenzymesthatcatalyzeessentialreactionsinthecell(RNasePribozyme,largerRNA,self-splicingintrons,etc).Structure(1):RNAchainsfoldbackon