DNA的生物合成和损伤修复DNA Biosynthesis and DNA Damage Repa

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1、Chapter15DNABiosynthesisandDNADamageRepairSectionOneTheGeneralFeaturesofReplicationofChromosomalDNADNAreplicatessemiconservatively.⑴.WatsonandCrickpredictedthatDNAmightsemiconservativelyreplicate.thehypothesis___________p___________d___________p___________p_____

2、_______d____________p⑵.In1958MeselsonandStahldemons-tratedthesemiconservativenatureofDNAreplicationinEcoli.Theexperiment:CsClequilibriumgradientdensityultracentri-fugationof15NlabeledEcoliDNA.ThedensityofDNAwasincreasedbylabelingitwith15N,aheavyisotopeofnitrogen

3、.ThiswasdonebygrowingEcoli15ge-nerationsinamediumthatcontained15NH4Clasitsonlynitrogensource.15NDNAwasextractedandsubjectedtoCsClequilibriumgradientdensityultracen-trifugation.TheDNAbandpositionwasrecorded.Therewasonebandof15NDNA.Thebacteriaweretransferredtoan14

4、NH4Clmediumandgrownforonegeneration.TheDNAdensitywasdeterminedagain.ThepositionoftheDNAbandwascomparedwiththatofthe15NDNA.Whatwastheresult?TherewasoneDNAband.Ithadalowerdensitythan15NDNAbecauseitspositionwasaboveonthatof15NDNA.Itwas15N/14NhybridDNA.Afteranotherg

5、enerationgrowinginthe14NH4ClmediumthebacterialDNAdensitywasdetermined.ThereweretwoDNAbands.OnehalfoftheDNAwas14NDNA,andanotherhalfwashybridDNA.Insucceedinggenerationstheratioof14NDNAtohybridDNAincreasedgradually.ThehybridDNAbecamelessandless..summaryDNAreplicate

6、sinasemiconservativeman-ner.Whenthetwoparentalstrandssepa-rate,eachservesasthetemplateformakinganew,complementarystrand.2.ThepointatwhichseparationofthestrandsandsynthesisofnewDNAtakesplaceisknownasthereplicationfork.ThereplicationforkisY-shaped.Twoarms(V)aresep

7、aratedstrandswhichactasthetemplateandDNAsynthesisisactivelytakingplace.Thebody(I)istheparentalDNA.3.DNAreplicationisusuallybidirectional.⑴.Replicon:Anypiecewhichreplicatesasasingleunitiscalledareplicon.AllbacterialchromosomesandmanyphageandvirusDNAmoleculesareci

8、rcularandcomprisesinglereplicons.Incontrasteukaryoticchromosomesconsistofmultiplereplicons.⑵.Origin:Theinitiationwithinarepliconalwaysoccursatafixedpointknownastheori