北大生物化學完整chapter7

NucleotidesDeoxyribonucleicAcids(DNA)RibonucleicAcids(RNA)DaochunKongTheCollegeofLifeSciencesPekingUniversityDeoxyribonucleicAcids(DNA)In1868,FriedrichMiescher,aSwissbiologist,isolatedaphosphorus-containingsubstancefromthenucleiofpuscells(leukocytes),whichhecalled“nuclein”.Hefurtherfoundthatthe“nuclein”consistedoftwoportions.Oneportionisacidic,whichwenowknowasDNA.Theotherportionisbasic,whicharemostlyhistoneproteins.Thereofre,the“nuclein”Miescherisolatedwasactuallychromatin.Later,hefoundthatasimilarsubstanceintheheadsofsalmonspermcells.1.TheDiscovery:2.DNA,theGeneticSubstance:1).DNAwassuspectedtobegeneticmaterial,but,therewasmorereasontoassignthisroletoproteins.Proteinswerethoughttobelarger,morecomplex(composedoftwentydifferentaminoacids),andthereforeweremoresuitableforageneticrole.DNAwasthoughttobejustarepeatingpolymerofonekindoftetranucleotideunit.Itwasjustaboutseventyyearsago,in1930s,DNAwaswidelybelievedtoexistonlyinanimalcells,andRNAwasinplantcells.Plantandanimalcellsweresometimesdistinguishedbasedonthischemicalfeature.

2).In1928,FrederickGriffithdiscoveredthephenomenonoftransformationinbacteria

Streptococcuspneumoniaehastwoforms,

Slimycolonies(virulentSstrain)formsmucouscapsules,itkillsmice.But,heat-killedstraindoesnotkillmice.Roughcolonies(non-virulentRstrain)lackscapsules,itdoesnotkillmice.Mixedheat-killedS-strainwithliveR-strain,resultingorganismskilledmice.

Conclusion:somechemicalincellsofSstrainisstableinheattreatment,retainsgeneticinformation,isabletotransmitthatinformationtoR-strainbacteria,convertthemtoS.3).

In1944,Avery,MacLeod,andMcCartyfoundthatDNApreparedfromavirulentstrainofpneumococcuscouldtransformanonvirulentstrainofthisorganismintoavirulentone.VirulentbacteriaInjectionMousediesNonvirulentbacteriaMouselivesAveryOT,MacLeodCM,McCartyM(1944)J.Exp.Med.79:137.

NonvirulentbacteriaDNAfromvirulentbacteria

DNAcarriedageneticmessagefromonetypeofcellscouldbeassimilatedandexpressedbyanothertypeofcells.Conclusion:Argument:

Atthattime,noteveryoneacceptedAveryetal’sconclusion.Thecriticsattackedthat,

a.TheDNAwaspossiblycontaminatedbyslightamountofproteinsthatwasresponsiblefortheobservedtransformation.b.DNAmaybegeneticsubstancejustinbacteria.

Whatareyourexperimentstoeaseoreliminatethesecritics’doubts?4).

In1952,AlfredD.HersheyandMarthaChasediscoveredthatitwasbacteriophageT2DNAthatenteredhostcell,E.coli,andcarriedgeneticinformationforphagereplication.

Atthattime,itwasalreadyknown,a.BacteriophagecontainsonlyDNAandproteins.b.PhageproteinsdonothavephosphorusP,butdocontainsulfurSinaminoacids.DNAdoesnothaveS,butlotsofP.HersheyandChase’sexperiment5).

In1953,WatsonandCrick,basedonexperimentalevidencesprovidedbytheotherpeople,suggestedamodelforstructureofDNA---acomplementary,doublestranded,andhelicalstructureofDNA.Thismodelwasprovedcorrectseveralyearslater.Basedontheaboveandotherstudies,itisfinallydemonstratedthatDNAisindeedgeneticsubstance.Ithastwodiscretefunctions---Oneistocarrygeneticinformationthatbringsaboutthespecificphenotypeofthecell;Anotherisitsownreplication.Howmanykindsofnucleicacids?Chemicalconstituentsofnucleicacids3.Structureofnucleicacids?4.Chemicalpropertiesofnucleicacids?3.ChemistryofNucleicAcidsTwoKindsofNucleicAcidsinAllOrganismsDNA---DeoxyribonucleicacidsRNA---Ribonucleicacids1.DNAisalonglinearpolymercomposedoffourkindsofmonomerunitscalled

deoxyribonucleotides(A,G,T,C)linkedtoeachotherinachain.DNA5’---AGTCCCTAGGACTATTTAAAACCTGGAGAGT---3’2.Single-strandedDNA(ssDNA)anddouble-strandedDNA(dsDNA).Innature,mostofDNAisintheformofdsDNA.RNARNAisalsolinearpolymerscomposedoffourtypesofribonucleotides(A,G,C,U).

5’…AGCUAAGUCCUUUGGGAACCCAAGGUUAACC…3’RibosomalRNAs(rRNA)MessengerRNAs(mRNAs)TransferRNAs(tRNAs)andOtherRNAmolecules

ChemicalStructureofNucleotidesAlexanderR.Todd(1954)chemicalstructureofthenucleicacids.PNAS.40:748-755DifferencesofNucleotidesbetweenDNAandRNADeoxyadenylateDeoxyguanylateDeoxythymidylateDeoxycytidylate

DeoxyribonucleotidesAdenylateGuanylateUridylateCytidylate

RibonucleotidesSomeRareBasesinDNAandRNA

5-MethylcytidineN6-MethyladenosineN2-Methyladenosine5-Hydromethylcytidine

InosinePseudouridine7-Methylguanosine4-ThiouridineRarebasesinDNARarebasesinRNADNARNADNA&RNAPolymer

Individualnucleosidesarejoinedtogetherbyphospho-diesterbonds,notetherorpyrophosphatelinkages.Levene&Simms(1925,1927)JBC.65:519;70:327.4.StructureofDNAErwin

Chargaff1).ErwinChargaffmeasuredamountsoffournucleotidesinDNAfromdifferentorganismsCompositionsofdeoxyribonucleotidesinSea-UrchinDNARatiosofdeoxyribonucleotidesinSea-UrchinDNACompositionsofdeoxyribonucleotidesinSalmonSpermDNARatiosofdeoxyribonucleotidesinSalmonSpermDNADr.Chargaffdiscoveredthat,

a.TheamountsofdeoxynucleotidesA,G,C,TinDNAvariedindifferentorganisms.ThisresultrejectedthetetranucleotidehypothesisanddemonstratedthepotentialfortheexistenceofalargenumberofspeciesofDNA.b.TheratiosofAtoTandGtoCareoneorveryclosetoone.ThisratioiscalledChargaff’sratioorrule.Thesebaseregularitiessuggestedtheexistenceofsomekindofbasepairing.2).Dr.Donohue,atheoreticalchemist,suggestedthatthatatphysiologicalcondition,pH7.0,fourbasesareatketoform,notenolform.

JerryDonohue:KetoformKeto,enolenolformKetoorenolformofbasesDeoxyadenylateDeoxyguanylateDeoxythymidylateDeoxycytidylate

DeoxyribonucleotidesAdenylateGuanylateUridylateCytidylate

RibonucleotidesG,C,T,andUallcanhaveketoorenolformifpHchanges.But,theketoformofallbasespredominatesatpH7.0.Potentialhydrogen-bondingdonorandacceptorsitesPossiblebasepairsDonohuej.(1956)hydrogen-bondedhelicalconfigurationsofpolynucleotides.PNAS.42:60-65Mostpossiblebase-pairingsindeoxyribonucleicacidsRosalindFranklin&MauriceWilkinsX-rayDiffractionofDNA3).X-raydiffractionpatternofBformofDNABasedoninformationfromx-raydiffractionpatternofDNA,Dr.Franklindeducedthat,X-raydiagramofDNAshowsthefeaturescharacteristicofhelicalstructures.DNAfibershavetwoperiodicities.Amajoroneof3.4Aasecondaryoneof34A.ThediameterofthesuggestedhelicalstructureofDNAis20A.(A:Angstrom)Dr.Franklinalsosuggestedthat,Thesugar-phosphategroupswereoutside.ThebackboneoftheAformofDNAisantiparallel.JamesD.Watson&FrancisH.C.CrickWatsonandCrickproposedamodelofdoublehelixstructureforDNAWatson&Crick(1953)Molecularstructureofnucleicacids.171:737-738.TwohelicalDNAchainswoundaroundthesameaxistoformaright-handeddoublehelix.Sugarandphosphateformthebackboneofthehelixandareontheoutsideofthehelix.Thepurineorpyrimidinebasesfrombothstrandsarestackedinside.Theringsofbasesareperpendiculartothelongaxis.Duetohelicalstructure,therearetwogroovesonthesurfaceoftheduplex.Thedistancebetweentwoneighbourringbasesis3.4A.Perturnhas10.5bases,36A.Twostrandsareantiparallel.ApairswithT,andGpairswithC.Theyarelinkedbyhydrogenbonds.WatsonandCrick’smodelforstructureofDNAstates:ASchemeforspatialpositionsofchemicalgroupsinDNAPositionsofhydrogenbondsinATandGCbasepairDNAreplicationsuggestedbyWatsonandCrickOldNewMeselson-Stahlexperiment15N-DNAisabout1%greaterthan14N-DNA.TheycanbeSeparatedbycentrifugationtoEquilibriuminacesiumchlorideDensitygradient.OtherformsofhelicalDNAstructuresNoWaterwaterGCGCStructureofRNANucleotidecompositionofRNA

single-strandedRNAchain.RNA,unlikeDNA,hasnoregularsecondarystructure.But,ittendstohavearight-handedhelicalconformationdominatedbybase-stackinginteractions.RNA<>InternaldoublehelicalstructureAtypicalRNAmoleculeHairpin