Chapter19Chapter19Nucleicacidsareimportantmacromolecules,andfoundinallkindsoforganisms.DNADNAstoresgeneticDNAstoresgeneticHershey&DeoxyDeoxyribonucleicacid,Ribonucleicacid,RNADNAIsDNAIsfoundineukaryotenuclearprokaryotenucleoidmitochondriaandchloroplastprokaryoteplasmidsomevirusandHumanDNADNAfromNucleoidofBacteriophageBacteriophageDNAand個(gè)基因，一個(gè)單倍體細(xì)胞中所有不同的染色體上的基因RNAisRNAisfoundmessengerRNAtransferRNAribosomalRNAothereukaryoteTranscription proteinsynthesisgeneexpression19.119.1Nucleotidesarethebuildingblocksofnucleicacids.RiboseRiboseandRNAcontainsDNAcontainsPurinesPurinesandAdenineA腺嘌呤,GuanineG（inDNA、ThymineT胸腺嘧啶,CytosineC（inCytosineC胞嘧啶,UracilU（inNitrogenousbaseHeterocyclicbase NucleosideNucleosideNitrogenousbaseconnectsriboseordeoxyribosebyaβ-D-N-glycosidebond.glycosidearedenotedas1,2,Positionsinribosearedenotedas1’,2’,3’…… Purinenucleosideshave2conformations:Purinenucleosideshave2conformations:synandanti.alwaysanti.otherotherderivativesofnucleoside.Innaturallyoccurringnucleotides,thephosphorylgroupsareusuallyattachedtotheoxygenatomofthe5’-hydroxylgroup.NucleosideNMPcanNMPcanbefurtherphosphorylatedtoformnucleosidediphosphates(NDP)andnucleosidetriphosphatesNTPhas2high-energyOtherOther19.2DNAisdouble-BasesBases++Nucleicacidsisalinearorcyclicpolymerof(deoxy-)ribonucleotideresidueslinkedby3’-5’phosphodiesterbonds.ChargaffAChargaffA=T,C=A+C=G+T,A+G=dissymmetryratioPrimarystructureofTheprimarystructureofanucleicacidisthesequenceofitsnucleotidesjoinedby3’-5’phosphodiesterlinkage.PhosphatePhosphateR1 5’endofthenucleicacidhasafreephosphorylgroup.3’endofthefreehydroxyl4′ ThedirectiontheDNAisDNA’sprimarystructurecanberepresentedschematically.DNAisDNAisdoubleDNAmoleculehas2antiparallelpolynucleotidechains.StructureStructureofSkeletalSpace-fillingBall-and-stickstrandsformdoublehelix.ThebasesareontheinsideofthehelixwhereasThebasesareontheinsideofthehelixwhereasthephosphate–sugarbackboneareontheoutside.areperpendiculartothehelixaxis.Theplaneofthesugarsarenearlyhorizontaltotheaxis. DNAdoublehelixDNAdoublehelixdiameteris2.37nmangleofrotationbetweeneachbasepairis34.6°pitchis3.4nm,thedistancebetween2adjacentbasepairisTheThedistancebetween2adjacentbasepairis0.33nm.Twistbetweeneachbasepairisbasepair(bp)3Basespairs, 652441.170.850.750.57DNAsequencemaybeAngleofrotationbetweeneachbasepairmayvaryfrom28°to42°.BasespairmayformpropellerWeakforcesstabilizethedoublehelixStackinginteractions堿基堆積力HydrogenbondsCharge-charge StackingStackingAbsoptionAbsoptionofandnucleicacidhaveanabsorbancemaximumnear DisruptionoftheH-bondsbetweenbasepairsandofbasestackingcausesunwindingthedoublehelixtoform2singleButnocovalentbondsareDenaturationDenaturationofDNAtakesplaceoveranarrowrangeoftemperature.MeltingtemperatureMeltingMeltingMeltingcurveofDNAissigmoidshape,indicatingthatdenaturationisacooperativeFactorsrelatedFactorsrelatedwithGCIonicstrengthofHelixHelixIfIfasolutionofdenaturedDNAisrapidlycooledtowellbelowitsTm,theresultingDNAwillbeonlypartiallybasepaired.Ifsolutioniscooledslowly,denaturedDNAeventuallycompletelyrenatures,thisprocessisalsocalledanneal（退火）.2complementarystrandsofRNAandDNA(orstrandsarefrom2differentDNA)formRNA-DNAhybriddoublehelix（orDNA-DNAhybrid）thatareonlyslightlylessstablethanthecorrespondingDNAdoublehelix.AvarietyAvarietyofDNAinrelativehumidityofInrelativehumidityoftakesonazigzag AABZ 3strandsand4strands Hoogsteenbasepair19.319.3DNAcanbeDNADNAcanbeMostcircularDNAmoleculessupercoiledinPositiv