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1、NucleotidesDeoxyribonucleicAcids(DNA)RibonucleicAcids(RNA)DaochunKongTheCollegeofLifeSciencesPekingUniversityDeoxyribonucleicAcids(DNA)In1868,FriedrichMiescher,aSwissbiologist,isolatedaphosphorus-containingsubstancefromthenucleiofpuscells(leukocytes),whichhecalled“nuclein”
2、.Hefurtherfoundthatthe“nuclein”consistedoftwoportions.Oneportionisacidic,whichwenowknowasDNA.Theotherportionisbasic,whicharemostlyhistoneproteins.Thereofre,the“nuclein”Miescherisolatedwasactuallychromatin.Later,hefoundthatasimilarsubstanceintheheadsofsalmonspermcells.1.The
3、Discovery:2.DNA,theGeneticSubstance:1).DNAwassuspectedtobegeneticmaterial,but,therewasmorereasontoassignthisroletoproteins.Proteinswerethoughttobelarger,morecomplex(composedoftwentydifferentaminoacids),andthereforeweremoresuitableforageneticrole.DNAwasthoughttobejustarepea
4、tingpolymerofonekindoftetranucleotideunit.Itwasjustaboutseventyyearsago,in1930s,DNAwaswidelybelievedtoexistonlyinanimalcells,andRNAwasinplantcells.Plantandanimalcellsweresometimesdistinguishedbasedonthischemicalfeature.2).In1928,FrederickGriffithdiscoveredthephenomenonoftr
5、ansformationinbacteriaStreptococcuspneumoniaehastwoforms,Slimycolonies(virulentSstrain)formsmucouscapsules,itkillsmice.But,heat-killedstraindoesnotkillmice.Roughcolonies(non-virulentRstrain)lackscapsules,itdoesnotkillmice.Mixedheat-killedS-strainwithliveR-strain,resultingo
6、rganismskilledmice.Conclusion:somechemicalincellsofSstrainisstableinheattreatment,retainsgeneticinformation,isabletotransmitthatinformationtoR-strainbacteria,convertthemtoS.3).In1944,Avery,MacLeod,andMcCartyfoundthatDNApreparedfromavirulentstrainofpneumococcuscouldtransfor
7、manonvirulentstrainofthisorganismintoavirulentone.VirulentbacteriaInjectionMousediesNonvirulentbacteriaMouselivesAveryOT,MacLeodCM,McCartyM(1944)J.Exp.Med.79:137.NonvirulentbacteriaDNAfromvirulentbacteriaDNAcarriedageneticmessagefromonetypeofcellscouldbeassimilatedandexpre
8、ssedbyanothertypeofcells.Conclusion:Argument:Atthattime,noteveryoneacceptedAveryetal’scon
