Gene-X-ppt-(Gene10-基因十)--Chapter01

Chapter1GenesAreDNA1.1Introduction•chromosome–Adiscreteunitofthegenomecarryingmanygenes.–EachchromosomeconsistsofaverylongmoleculeofduplexDNAandanapproximatelyequalmassofproteins,andisvisibleasamorphologicalentityonlyduringcelldivision.•structuralgene–AgenethatcodesforanyRNAorpolypeptideproductotherthanaregulator.1.2DNAIstheGeneticMaterialofBacteriaandViruses•BacterialtransformationprovidedthefirstsupportthatDNAisthegeneticmaterialofbacteria.•Duringtransformation,geneticpropertiescanbetransferredfromonebacterialstraintoanotherbyextractingDNAfromthefirststrainandaddingittothesecondstrain.FIGURE04:TheDNAofS-typebacteriacantransformR-typebacteriaintothesameS-type1.2DNAIstheGeneticMaterialofBacteriaandViruses•transformingprinciple–DNAthatistakenupbyabacteriumandwhoseexpressionthenchangesthepropertiesoftherecipientcell.•PhageinfectionshowedthatDNAisthegeneticmaterialofviruses.WhentheDNAandproteincomponentsofbacteriophagesarelabeledwithdifferentradioactiveisotopes,onlytheDNAistransmittedtotheprogenyphagesproducedbyinfectingbacteria.FIGURE06:ThegeneticmaterialofphageT2isDNA1.3DNAIstheGeneticMaterialofEukaryoticCells•DNAcanbeusedtointroducenewgenetictraitsintoanimalcellsorwholeanimals.•transfection–Ineukaryoticcells,theacquisitionofnewgeneticmarkersbyincorporationofaddedDNA.•Insomeviruses,thegeneticmaterialisRNA.FIGURE07:Eukaryoticcellscanacquireanewphenotypeastheresultoftransfection1.4PolynucleotideChainsHaveNitrogenousBasesLinkedtoaSugar–PhosphateBackbone•Anucleosideconsistsofapurineorpyrimidinebaselinkedtothe1′carbonofapentosesugar.•ThedifferencebetweenDNAandRNAisinthegroupatthe2′positionofthesugar.–DNAhasadeoxyribosesugar(2′–H);RNAhasaribosesugar(2′–OH).1.4PolynucleotideChainsHaveNitrogenousBasesLinkedtoaSugar–PhosphateBackbone•Anucleotideconsistsofanucleosidelinkedtoaphosphategrouponeitherthe5′or3′carbonofthe(deoxy)ribose.•DNAcontainsthefourbasesadenine,guanine,cytosine,andthymine;RNAhasuracilinsteadofthymine.1.4PolynucleotideChainsHaveNitrogenousBasesLinkedtoaSugar–PhosphateBackbone•Successive(deoxy)riboseresiduesofapolynucleotidechainarejoinedbyaphosphategroupbetweenthe3′carbonofonesugarandthe5′carbonofthenextsugar.•Oneendofthechain(conventionallywrittenontheleft)hasafree5′endandtheotherendofthechainhasafree3′end.FIGURE08:Apolynucleotidechain1.5SupercoilingAffectstheStructureofDNA•Supercoilingoccursonlyin“closed”DNAwithnofreeends.•supercoiling–ThecoilingofaclosedduplexDNAinspacesothatitcrossesoveritsownaxis.PhotocourtesyofNirupamRoyChoudhury,InternationalCentreforGeneticEngineeringandBiotechnology(ICGEB)FIGURE09_BOT:AsupercoiledDNAhasatwistedandcondensedform1.5SupercoilingAffectstheStructureofDNA•ClosedDNAiseithercircularDNAorlinearDNAinwhichtheendsareanchoredsothattheyarenotfreetorotate.•AclosedDNAmoleculehasalinkingnumber(L),whichisthesumofthetwistingnumber(T)andthewrithingnumber(W).•ThelinkingnumbercanbechangedonlybybreakingandreformingbondsintheDNAbackbone.1.6DNAIsaDoubleHelix•TheB-formofDNAisadoublehelixconsistingoftwopolynucleotidechainsthatrunantiparallel.FIGURE11:Apolynucleotidechain1.6DNAIsaDoubleHelix•ThenitrogenousbasesofeachchainareflatpurineorpyrimidineringsthatfaceinwardandpairwithoneanotherbyhydrogenbondingtoformonlyA-TorG-Cpairs.•complementary–Basepairsthatmatchupinthepairingreactionsindoublehelicalnucleicacids(AwithTinDNAorwithUinRNA,andCwithG).FIGURE12:Flatbasepairslieperpendiculartothesugar-phosphatebackbone1.6DNAIsaDoubleHelix•Thediameterofthedoublehelixis20Å,andthereisacompleteturnevery34Å,withtenbasepairsperturn(~10.4basepairsperturninsolution).•Thedoublehelixhasamajor(wide)grooveandaminor(narrow)groove.1.6DNAIsaDoubleHelix•overwound–B-formDNAthathasmorethan10.5basepairsperturnofthehelix.•underwound–B-formDNAthathasfewerthan10.5basepairsperturnofthehelix.1.7DNAReplicationIsSemiconservative•semiconservativereplication–DNAreplicationaccomplishedbyseparationofthestrandsofaparentalduplex,eachstrandthenactingasatemplateforsynthesisofacomplementarystrand.•Thesequencesofthedaughterstrandsaredeterminedbycomplementarybasepairingwiththeseparatedparentalstrands.FIGURE14:BasepairingprovidesthemechanismforreplicatingDNA1.7DNAReplicationIsSemiconservative•TheMeselson–Stahlexperimentused“heavy”isotopelabelingtoshowthatthesinglepolynucleotidestrandistheunitofDNAthatisconservedduringreplication.FIGURE15:ReplicationofDNAissemiconservative1.8PolymerasesActonSeparatedDNAStrandsattheReplicationFork•ReplicationofDNAisundertakenbyacomplexofenzymesthatseparatetheparentalstrandsandsynthesizethedaughterstrands.•denaturation–InDNA,thisinvolvestheseparationofthetwostrandsduetobreakingofhydrogenbondsbetweenbases.•renaturation–ThereassociationofdenaturedcomplementarysinglestrandsofaDNAdoublehelix.1.8PolymerasesActonSeparatedDNAStrandsattheReplicationFork•Thereplicationforkisthepointatwhichtheparentalstrandsareseparated.•TheenzymesthatsynthesizeDNAarecalledDNApolymerases.FIGURE16:Thereplicationfork1.8PolymerasesActonSeparatedDNAStrandsattheReplicationFork•Nucleasesareenzymesthatdegradenucleicacids;theyincludeDNasesan