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Educator MaterialsShort Film The Double Helix
IN-DEPTHFILMGUIDE
DESCRIPTIONThefilmTheDoubleHelixdescribesthetrailofevidenceJamesWatsonandFrancisCrickfollowedtodiscoverthedouble-helicalstructureofDNA.Theirmodel’sbeautifulandsimplestructureimmediatelyrevealedhowgeneticinformationisstoredandpassedfromonegenerationtothenext.
KEYCONCEPTSA. DNAisapolymerofnucleotidemonomers,eachconsistingofaphosphate,adeoxyribosesugar,andoneof
fournitrogenousbases:adenine(A),thymine(T),guanine(G),orcytosine(C).B. TherelativeamountsofA,T,G,andCbasesvaryfromonespeciestoanother;however,intheDNAofany
cellfromorganismswithinasinglespecies,theamountofAisequaltotheamountofTandtheamountofGisequaltotheamountofC.ThisfindingcanbeexplainedbythefactthatintheDNAdoublehelix,ApairswithTandGwithC.
C. EvenbeforethestructureofDNAwassolved,studiesindicatedthatthegeneticmaterialmustbeabletostoreinformation;befaithfullyreplicatedandbepassedonfromgenerationtogeneration;andallowforchanges,andthusevolution,tooccur.ThestructureofthedoubleheliximmediatelyshowedthatDNAhadtheseproperties.
D. Scientistsusedifferenttechniquestomeasurethingsthataretoolargeortoosmalltosee.ThestructureofDNAwasdeterminedbycombiningmathematicalinterpretationsofx-raycrystallographydataandchemicaldata.
E. Scientistsbuildmodelsbasedonwhattheyknowfrompreviousresearchtoderivetestablehypotheses.Datafromexperimentsareusedtorevisemodelsandaskadditionalresearchquestions.Theultimategoalistofindamodelthatisvalidinallormostoftheobservations.
F. Theprocessofscientificdiscoveryinvolvesbrainstormingandevaluatingideas,makingmistakes,andrethinkingthoseideasbasedonevidence.Failureisanimportantaspectofscientificdiscovery.
G. Communicationamongscientistsplaysacrucialroleinscientificdiscoveries.TounlockthestructureofDNA,WatsonandCrickalsoreliedonobservationsmadebyotherscientists.
CURRICULUMANDTEXTBOOKCONNECTIONSCurriculum StandardsNGSS(April2013) MS-LS3.A,MS-LS3.B,MS-ETS1.B,MS-PS1.B
HS-PS2.B,HS-PS4.C,HS-LS1.A,HS.LS3.A,HS.LS3.BAP(2012–13) 3.A.1,4.A.1IB(2016) 2.6,2.7,7.1
Textbook ChapterSectionsMillerandLevine,Biology(2010ed.) 12.1,12.2,12.3Reeseetal.,CampbellBiology(9thed.) 5.5,16.1,16.3
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Educator MaterialsShort Film The Double Helix
KEYTERMSchemicalbonds,chromosomes,DNA,genes,helix,hydrogenbonds,mutation,nucleotides,nucleus,proteins,structure,replication
PRIORKNOWLEDGEStudentsshould
• knowthatbiologicalmoleculesarecomposedofdifferenttypesofatoms,includingcarbon,oxygen,nitrogen,andhydrogenatoms;
• knowthattheshapesofbiologicalmoleculesdependonthearrangementofthecomponentatomsandtheirchemicalbonds,whichconstrainthedistancesbetweenatoms;
• knowthatgenesaremadeofDNA,thattheyareinheritedfromonegenerationtothenext,andthatmutationsarechangesintheDNAsequence;
• haveabasicunderstandingofDNAreplicationandthecentraldogmathatDNAistranscribedtoRNAandRNAistranslatedintoproteins;and
• befamiliarwiththescientificprocessoftestingideaswithevidence.
PAUSEPOINTSThefilmmaybeviewedinitsentiretyorpausedatspecificpointstoreviewcontentwithstudents.Thetablebelowlistssuggestedpausepoints,indicatingthebeginningandendtimesinminutesinthefilm.
Begin End ContentDescription ReviewQuestions Standards1
0:00
4:08
• Intheearly20thcenturyseveralscientistsweretryingtosolvethemysteryofthestructureofDNAinordertobetterunderstandinheritanceoftraits.
• JamesWatson&FrancesCrickmetinCambridgein1951.Bothwereinterestedinfindingthestructureofthegene.
• Inthe1920s,geneshadbeenlocatedinthenucleusandassociatedwithchromosomes.ScientistsneededtodetermineifgeneswereinproteinsorDNA.
• DNAisapolymerofnucleotidemonomers,eachconsistingofaphosphate,adeoxyribosesugar,andoneoffournitrogenousbases:adenine(A),thymine(T),guanine(G),orcytosine(C).
• OswoldAverydemonstratedthatDNAcouldcarrygeneticinformation.
• Whatarechromosomesmadeof?
• Wherearegenesfound?
• WhatisthestructureofDNA?
• WhywasOswoldAvery’sworksignificanttoWatsonandCrick?
NGSS(April2013)MS-LS3.A,MS-LS3.B,MS-ETS1.B,MS-PS1.A,HS-LS1.A,HS-LS3.A,HS-LS3.BAPBiology(2013)3.A.1,4.A.1IBBiology(2016)2.6,2.7,7.1
2 4:09 9:05 • X-raycrystallographyisatechniquefordeterminingmolecularstructure.Itcandeterminethelocationofatomswithina
• Whywasx-raycrystallographybeingusedto
NGSS(April2013)MS-LS3.A,
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Educator MaterialsShort Film The Double Helix
molecule.• MauriceWilkins,aphysicistatKingsCollege,
wasusingX-raycrystallographytodeterminethestructureofDNA.
• RosalindFranklinwasacolleagueofWilkins,butsheandWilkinsworkedseparately.
• LinusPaulingwasalsosearchingforDNA’sstructure.
• Pauling,WatsonandCrickbelievedDNAwasahelicalmolecule.WatsonandCrick’sfirstmodelofDNAwasincorrect.Theyreliedoninformationfromotherscientists.
determinethestructureofDNA?
MS-ETS1.B,MS-PS1.A,HS-PS4.C,HS-LS1.A,HS-LS3.A,APBiology(2013)3.A.1,4.A.1IBBiology(2016)2.6,7.1
3 9:06
13:50
• ThestructureofDNAwasdeterminedbycombiningmathematicalinterpretationsofx-raycrystallographydataandchemicaldata.
• WilkinsshowedWatsonanx-raycrystallographypicturetakenbyFranklin(Photo51).TheX-shapeddiffractionpatternischaracteristicofahelicalmolecule.
• Chargaffhadreportedthatbaseratioswerealwaysthesameinallorganisms.
• WatsonandCrickbuiltamodelofDNAasadoublehelix,withthebasesarrangedinside.
• WhatisthestructureofDNA?
• WhatwerethekeypiecesofevidencethatledWatsonandCricktodeterminethatstructure?
NGSS(April2013)MS-LS3.A,MS-ETS1.B,MS-PS1.A,HS-PS2.B,HS-PS4.C,HS-LS1.A,HS-LS3.AAPBiology(2013)3.A.1,4.A.1IBBiology(2016)2.6,7.1
4
13:51
16:53
• Solvingthestructurehadfar-reachingimplicationsforbiology.
• Thecomplementarynatureofthebases(A-TandG-C)providedamethodforreplicatinganewcomplementarycopy.
• DNA’sstructurerevealedhowgeneticinformationisstoredinthesequenceofthebasesandhowmutationscanhappen.
• HowdoesDNA’sstructureexplainthestabilityoflife?
• Howdoesitexplainthemutabilityoflife?
NGSS(April2013)MS-LS3.A,MS-LS3.B,MS-PS1.A,HS-PS2.B,HS-LS1.A,HS-LS3.A,HS-LS3.BAPBiology(2013)3.A.1,4.A.1IBBiology(2016)2.6,2.7,7.1
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Educator MaterialsShort Film The Double Helix
BACKGROUNDThediscoveryofthethree-dimensionalstructureofDNAwasmadepossiblebyearlierworkofmanyscientistswhohaduncoveredevidenceaboutheredity,genes,andDNA.ThefilmTheDoubleHelixmentionsmanyofthesefindings,whicharelistedinmoredetailbelow.
Thetrailofevidencebeginsinthe19thcentury,whenAustrianmonkGregorMendeldiscoveredpatternsinthewaycharacteristics,ortraits,areinheritedfromonegenerationtothenext.Doingexperimentsusinggardenpeaplants,Mendelfoundthattraitslikepeashapeandcolorarepassedfromparenttooffspringasdiscreteunitsinapredictableway.
Intheearly1900s,AmericangeneticistThomasHuntMorgandemonstratedthatthesediscreteunitsofheredity—orgenesastheywerebynowcalled—werelocatedonchromosomes.ChromosomeswereknowntobecomposedofDNAandprotein,butitwasunclearwhichofthetwotypesofmoleculeswasthesourceofgeneticinformation.
Mostresearchersfavoredproteinasthegeneticmaterial.Proteinsarebuiltfrom20distinctaminoacidcomponentsandshowgreatstructuraldiversityandspecificity.Incomparison,DNAseemedmonotonous.FrederichMiescher,aSwissphysician,hadfirstisolatedDNAfromwhitebloodcellsin1871.Shortlyafterthat,AmericanbiochemistPhoebusLeveneidentifiedthecomponentsofDNA:deoxyribosesugar,phosphate,andoneoffourdifferentnitrogenousbases.
In1938,BritishphysicistWilliamAstburytookthefirstx-raydiffractionimagesofDNA.HeusedtheseimagestobuildamodelofthestructureofDNAusingmetalplatesandrods.Whilehismodelwasverytentativeandcontainederrors,Astburycorrectlypositionedthebaseslyingflat,stackedlikeapileofpennies,0.34nmapart.
AseriesofexperimentssetthestageforestablishingthatgenesweremadeofDNAandnotproteins.FrederickGriffith’s1928experimentsshowedthatpneumococcalbacteriacouldtransfergeneticinformationbetweendifferentstrainsthroughaprocesshecalledtransformation.OswaldAvery,ColinMacLeod,andMaclynMcCartydeterminedthatthemoleculeresponsibleforthistransformationwasDNAandnotprotein.Averyandcolleagues’1944paperwasinitiallymetwithskepticism,asmanyscientistscontinuedtobelievethatproteinswerethegeneticmaterial.
Inthemeantime,moreinformationwasemergingaboutthestructureofDNA.AmericanbiochemistErwinChargaffreportedin1949thattheproportionsofthefournucleotidesinaDNAmoleculevariedbetween
ThenucleotidestructureofDNA.DNAconsistsofchainsofnucleotides,whicharemadeofasugar,phosphate,andoneoffourbases.DNAseemedlikeaboringsubstancecomparedtoproteins.
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species.However,withinaspecies,thepercentagesofadenine(A)andthymine(T)baseswerealwaysequal,aswerethepercentagesofguanine(G)andcytosine(C).ThesignificanceofthisfindingwasnotappreciateduntilWatsonhadtheinsighttouseittoinformthemodelofDNAhewasbuilding.
ThemostconvincingevidencethatDNAwasthemoleculeofhereditycamefromAlfredHersheyandMarthaChaseinapaperpublishedin1952.WorkingatColdSpringHarborLaboratoryonLongIsland,NY,theyusedradioactiveisotopesofsulfurandpotassiumtolabelproteinsandDNA,respectively,inbacteriophages—whicharevirusesthatinfectbacteria.TheexperimentshowedthatthebacteriophageDNA,andnottheproteins,enteredbacteriaforinfection.
AtthetimeoftheHershey-Chaseexperiment,anumberofgroupshadstartedworkingtodeterminethemolecularstructureofDNA.AmongthemwasLinusPaulingofCalTech,famousforhavingsolvedthestructureofseveralproteinsbybuildingmodelsbasedonchemicalbondingprinciplesandbiochemicalevidence.HewasaninspirationtoWatsonandCrickaswellasthepersonmostlikelytosolvethestructurebeforethem.In1951,Paulinghadproposedthatthepolypeptidechainsofproteinsfoldinα-helicalstructures.Today,theα-helixisknowntoformthebackboneoftensofthousandsofproteins.
Paulinghadturnedhismodel-buildingskillstothestructureofDNA.InEngland,MauriceWilkinsandRosalindFranklinatKing’sCollegeLondonwereusingx-raycrystallographytoanalyzeDNA’sstructure.Despiteafewconfusingblurryspots,theinitialimagestheyobtainedhintedthatDNAmightcomeintheformofatwistedspiral—orhelix.However,itwasnotclearhowthephosphates,sugars,andbaseswerearrayedwithinthathelix.
ShortlyafterWilkinsandFranklinbegantheirexperiments,WatsonandCrickdecidedtoworkonDNAaswell.InspiredbyPauling’swork,theystartedbuildingmodelsofDNAmolecules.UnlikeWilkinsandFranklin’s,theirapproachwastoformulateapossiblestructureofDNAandthendeterminewhetheritfitexperimentalobservations.Inoneoftheirfirstattempts,theycreatedahelixwiththreesugar-phosphatechainsheldtogetherbychemicalbondsfacilitatedbymagnesiumions,withthebasesprojectingoutwardfromthiscentralbackbone.
RelativeProportions(%)ofBasesinDNA
Organism A T G C
Human 30.9 29.4 19.9 19.8
Chicken 28.8 29.2 20.5 21.5
Grasshopper 29.3 29.3 20.5 20.7
SeaUrchin 32.8 32.1 17.7 17.3
E.coli 24.7 23.6 26.0 25.7
Chargaff’srule.ErwinChargaffdiscoveredthatinaDNAmolecule,theproportionofadenine(A)alwaysequalsthatofthymine(T)andtheproportionofguanine(G)alwaysequalsthatofcytosine(C).
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Franklinsawthatthemodeldidnotfitthex-rayevidence.Basedonhermeasurements,DNAfiberscontainedatleast10timesasmuchwaterasWatsonandCrick’smodelallowedfor,andtherewasnoevidencethatDNAwasassociatedwithmagnesiumions.Asconstructed,themodelcouldnotexplainhowthethreephosphatebackbonescouldbeheldtogetheratthecenterofthemolecule.(ShortlybeforeWatsonandCrickproducedtheirsuccessfuldouble-helixmodel,Paulingproducedhisownflawedtriple-helixmodel,againwiththephosphatesinthecenterofthemolecule,butwithoutmagnesiumions.Paulinghadhydrogenbondsholdingthephosphatechainstogether,butbasedonwhatwasknownaboutchemicalbondsthatmodelwasnotvalid.)
Franklin’smeasurementsofthewaterassociatedwithDNAsuggestedthatthephosphategroupswouldbelocatedinanaqueousenvironmentontheexterioroftheDNAmolecule.Ever-betterx-rayimages,includingFranklin’sfamousphotoB51,providedinformationaboutthedimensionsoftherepeatingsubunitsinaDNAmolecule.Inaddition,herimagesindicatedthatDNAmoleculeslookthesamewhentheyareturnedupsidedownandfronttoback.WhenCrickfoundoutaboutDNA’sdyadsymmetry,heinferredthatthephosphatechainsmustruninoppositeorientations,orantiparallel,tooneanother—abrilliantinsightthatFranklinandothershadmissed.
BuildingonthesecluesandWilkinsandFranklin’smeasurements,WatsonandCrickonceagainturnedtomodelstotesttheirhypothesesofDNAstructure.Thistimetheytriedbuildingmodelswithtwoantiparallelphosphatechainsontheoutsideofthemolecule.Inthisarrangement,thechainswouldhavetobeheldtogetherbythebasesontheinside,butWatsonandCrickwerenotsurehowthesebasesmightpairup.That’swhentheyrememberedChargaff’sratios.CrickreasonedthatAmustalwayspairwithTandGwithC.Butwherewerethebondsbetweenthesebases?TheyconsultedJ.N.Davidson’sTheBiochemistryofNucleicAcids,publishedin1950.However,aswithotherbooksofthattime,itcontainedincorrectforms,ortautomers,ofguanineandthymine(seefigure).Nomatterhowtheytried,thebasesdidnotformanicehydrogen-bondingpattern,asforexampleintheproteinbackboneofanα-helix.AvisitingAmericanchemist,JerryDonohue, lookedattheirbasestructuresandrealizedthatthesewerewrong.
CA B
DNAmoleculeshavedyadsymmetry.Onewaytoexplaindyadsymmetryistopretendthatthetwopencilsin(A)areaDNAcrystal.Whenflippedupsidedown(B)andfronttoback(C),itlooksthesameastheoriginal(A).
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Educator MaterialsShort Film The Double Helix
OnceWatsonincorporatedthenew,correctshapesofthebasesintohismodel,hesawwherethehydrogenbondswouldformandhismodelofDNAimmediatelyfellintoplace!
TheA-TandG-Cpairingswereconsistentwiththemeasurementsofthemoleculefromx-rayimages.Thehydrogenbondsbetweenthebasepairsmakethemoleculestructurallystable.TodayweknowthattherearetwohydrogenbondsinanA-TpairandthreehydrogenbondsinaG-Cpair.However,atthetimeWatsonandCrickbuilttheirmodeltheyhadinitiallyidentifiedtwohydrogenbondsbetweenbothA-TpairingsandG-Cpairings.
Thetextbookswerewrong.Guanineandthyminecanhavealternatemolecularstructuresbasedondifferentlocationsofaparticularhydrogenatom.Thesetwotypesofstructuresareknownastautomers.Thetautomericformsofeachbaseexistinequilibrium,butoneformismorestableandthereforepredominatesundertheconditionsfoundinsidemostcells.Intheearly1950s,chemistrytextbookshaddrawingsofthe“wrong”tautomersofguanineandthymine.JerryDonohuetoldJamesWatsonwhatthecorrectstructureswere.ThispieceofthepuzzleallowedWatsontobuildpairsofA’sandT’sandofG’sandC’swithaccuratehydrogenbonds.
Basepairingsusedtoconstructthedoublehelix.WatsonandCrickrejectedtheideaofathirdhydrogenbond(shownbythedottedlines)betweenguanineandcytosinebecausedatahintedthatsuchabondwouldbeweak.Laterevidenceshowedthatthereareinfactthreestronghydrogenbondsinaguanine-cytosinepair.(Thethirdbondisshowninblue.)
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Educator MaterialsShort Film The Double Helix
KeyEvidenceCamefromX-rayDiffraction
Developedearlyinthe20thcentury,x-raycrystallographyallowstheindirectobservationofmolecularstructurestoosmalltobeseenorphotographed.Thefather-sonteamofWilliamH.andWilliamL.BraggsharedtheNobelPrizein1915forusingx-raystorevealhowtherepeatingstructuresofcrystalsform.Tablesaltwasthefirstcrystalstructuresolvedbyx-raycrystallographyin1914,soonfollowedbytherepeatingcarbonstructureofdiamond.DorothyHodgkinandMaxPerutzwerepioneersinsolvingthestructuresoforganicmoleculescontainingmorecomplexatomicarrangements,includingcholesterol,penicillin,vitaminB12,insulin,andhemoglobin.
WilliamL.BraggwasdirectoroftheCavendishLaboratoryatthetimethatWatsonandCrickwerethere.Hehadbeenstrivingtodeterminethestructuresofproteincomponents,butLinusPaulinggottherefirstbydiscoveringthestructureoftheα-helix.
X-raycrystallographyinvolvesmountingamoleculeonastageandbombardingitwithabeamofx-rays.Thewavelengthsofx-raysaresoshortthattheybounceoffatomswithinthemolecule,scatteringatspecificanglesthatdependonthedistancesbetweenatomsofvarioussizes.Thescatteredx-raysproducepatternsthatcanbecapturedonphotographicfilmordigitally.Tointerpretthesepatterns,crystallographersmustdeterminewhenx-raysscatteredfromdifferentatomsoverlap.Thisoverlapchangestheintensityofspotsinthex-raypattern.
Two-dimensionalimagestakenatdifferentanglesareconvertedintoathree-dimensionalmodelofthemoleculeusingmathematicalcalculationstermedFouriertransformations,whichallowthepositionsofatomswithinthemoleculetobedetermined.Whenaveragedovermanyobservations,thesemeasurementscanbeaccuratetojustafractionofanångstrom(orone10-billionthofameter).
Thenumberofx-raysdiffractedbyasinglemoleculewouldsimplybetoosmalltobeobserved.Therefore,x-raycrystallographyexaminesmanyidenticalmoleculespackedintoahighlyregularthree-dimensionalarrayratherthansinglemolecules.Becausethex-raystravelthroughmanylayersofatoms,it’simportantthattheatomsalwaysoccurinthesamearrangementinallthemoleculesinasample.Iftheydon’t,thex-raysarebentintooverlappingpatterns,resultinginblurryorcompletelysmeareddiffractionpatternsthatcannotbeinterpretedorprovideverypoorresolution.Becausecrystalshavearepeatingarrangementofatomsinidenticalorientation,theyleaveapatternofsharp,clearspots.Forthisreason,biologicalmoleculesaretypicallyturnedintocrystals,orcrystallized,beforetheyareanalyzedwithx-rays.Infactmuchtimeandeffortmaybespentin“growing”crystalsofaparticularmoleculeofinterest,andthiscanbetherate-limitingstepinaresearchefforttosolveamolecularstructure.
Franklintookx-raysnotofcrystalsbutofDNAfibersthat,althoughthin,consistedofthousandsofindividualDNAmoleculestightlyarrayed,somewhatliketheindividualstrandsofthickrope,orhairgatheredintoaponytail.Whilenotincrystalform,theDNAhadasufficientlyregulararrangementtodiffractx-raysinawaythatproduceddecipherablepatterns.
ByexposingDNAtovaryinglevelsofatmospherichumidity,FranklinandhergraduatestudentRaymondGoslingdemonstratedthatDNAexistedintwoforms,whichtheycalledAandB.The“dry”Aformoccurswhentherelativehumidityislessthan75%andproducesascatteredx-raydiffractionpatternconsistingofmanydistinctspots.Watermoleculesclingtothe“wet”B-formDNA,causingthestrandstoelongate,producinganX-shapeddiffractionpattern.Becauseallmoleculesincellsareimmersedinliquid,theB-formDNAistheformthatexistsprimarilyinsidecells.Itwasnotuntil1980thatanactualcrystalstructureofmorethanacompleteturnofB-
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DNA,inwhichindividualatomsoftheDNAcouldbedistinguished,waspublished(Wing,R.etal.1980.Nature287:755-8).
FranklinhadinitiallyfocusedherattentiononA-formDNAbecauseshethoughtthoseimagescontainedmoreinformation.ItwasinfactoneoftheA-formphotosthathadrevealedthatthetwostrandsofDNAraninoppositedirections,althoughneitherFranklinnortheothershadbeenabletointerprettheevidencetomakethisconclusion—itwasFrancisCrickwhorealizeditssignificance.
ThefamousphotoB51,takenbyFranklinandGoslinginMay1952,wasofB-formDNA.(Asitwasthe51stphototaken,FranklinlabeledtheimageB51.)TheX-shapeddiffractionpatternischaracteristicofahelicalmolecule.
Independentlinesofevidencehaveconfirmedthatthediamondshapesformedbythearmsandlegsofthe“x”indicatetherepetitionofthehelicalpatternaswellastheplacementofthephosphatesugarbackboneontheexteriorofthemoleculeandthebasesintheinterior.Analysisoftheblurrysmearscomposingthe“x”ofthephotoallowsthecalculationofthedimensionsoftheDNAmolecule:aradiusof1.0nm,0.34nmbetweenbasepairs,and3.4nm(10basepairs)withinacompleteturnofthehelix.AlthoughthemodelbuiltbyWatsonandCrickwasbasedonfiberdiffraction,theirknowledgeofthechemicalnatureofthecomponentsofDNAallowedthemtobuildamodelthatincludedthepositionsofatomsandthechemicalbondsbetweenthem,whichwaslaterconfirmedbyhigh-resolutionx-raycrystallography.
TheAandBformsofDNA.The“dry”AformofDNAandthe“wet”BformofDNAhavedistinctstructuresasshownbytheirx-raydiffractionpatternsontheleft.
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TheDiscoveryoftheStructureofDNALedtoKeyInsights
OnthemorningofFebruary28,1953,WatsonconstructedthemodelofDNAthatunlockedthesecretoflife.Twomonthslater,WatsonandCrickpublishedtheirfindingsinNatureinapapertitled“Astructurefordeoxyribosenucleicacid.”Inthesameissue,immediatelyfollowingWatsonandCrick’spaper,wasapaperwrittenbyWilkinsandhiscolleaguesAlecStokesandHerbertWilson.ThethirdpaperintheserieswaswrittenbyFranklinandGosling.Wilkins’andFranklin’spaperspresentedtheevidencetheyhadobtained,corroboratingWatsonandCrick’sproposedstructureofDNA.
AnunderstandingofthestructureofDNAprovidedanexplanationofhowDNAfunctionedasthehereditarymaterial.WatsonandCricknotedthisintheirNaturepaper.Theywrote:“Ithasnotescapedournoticethatthespecificpairingwehavepostulatedimmediatelysuggestsapossiblecopyingmechanismforthegeneticmaterial.”BecauseAisalwayspairedwithTandGwithC,theorderofbasesononestranddeterminestheorderontheother.Thus,ifaDNAmoleculewereunwound,eachstrandcouldbecopiedintoacomplementarystrand,producinganexactreplicaoftheoriginalmolecule.Errorsinthecopyingmechanismcouldresultinmutations,orchangesintheDNAsequence,thatcouldbeinheritedbyfuturegenerations.
In1962,Watson,Crick,andWilkinswereawardedtheNobelPrizeinPhysiologyorMedicine“fortheirdiscoveriesconcerningthemolecularstructureofnucleicacidsanditssignificanceforinformationtransferinlivingmaterial.”Franklin’sdeathin1958fromovariancancerprecludedherfromreceivingmanyofthehonorsforthediscoveryofDNA’sstructure,includingthepossibilityofsharingintheNobelPrize,whichcannotbeawardedposthumously.
Ittookseveralyearsofsubsequentstudy,includingaclassic1958experimentbyAmericangeneticistsMatthewMeselsonandFranklinStahl,beforetheexactrelationshipbetweenDNAstructureandreplicationwasunderstood(Meselson,M.,andStahl,F.1958.ThereplicationofDNAinEscherichiacoli.ProceedingsoftheNationalAcademyofSciences44,671–682).
DISCUSSIONPOINTS
• Thewaythestoryunfoldsinthisshortfilmmaygivestudentstheimpressionthateachpieceofevidencefellintoplaceoneaftertheother,inasomewhatlinearpath.InrealitymostofthepieceswereinplayandmovingaroundinWatsonandCrick’sheadsandintheirdiscussionsastheyalmostcontinuouslythoughtabouttheproblemofDNAovermanymonths.Theprocesswasoneoftrialanderror,andcirclingback
PhotoB51.PhotoB51wastakenbyFranklinandGosling.ItrevealedthatB-formDNAwasadoublehelixwith10nucleotidebasepairswithinacompleteturnofthehelix.Thecrossindicatesahelix.Thedarkpatchesindicatethebases.
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repeatedly.AfterseeingphotoB51,WatsonsuspectedthatDNAmightbeadoublehelix,buthecontinuedtoconsiderbothadoubleortriplehelixuntilhebuiltadouble-helicalstructurethatfitalltheknownevidence.Similarly,Franklin’sx-raydatahadsuggestedthatthephosphatechainswereontheoutside,butthatpieceofevidencedidnotfituntilWatsonandCrickfiguredouthowthenitrogenousbasesmightpairtogetheratthecenter.
• Atthebeginningofthefilm,SeanCarrollsetsthestagefortheunfoldingofthestorybysaying,“Thethree-dimensionalarrangementofatomsinthosemoleculeshadtoexplainthestabilityoflife,sothattraitswerepassedfaithfullyfromgenerationtogeneration,andalsothemutabilityoflife.”Thenotionthathereditycouldbeexplainedbythe“arrangementofatoms”ofmoleculeswasfirstraisedbyworld-renownedphysicistErwinSchrödinger.In1944,SchrödingerpublishedthebookWhatIsLife?inwhichhearguedthatlivingthingsshouldbeconsideredintermsofmolecularandatomicstructure,astheyobeythesamelawsofchemistryandphysics.AccordingtoSchrödinger,genesarepassedfromgenerationtogenerationbecausethegeneticcodewasaresultofthearrangementofatomswithinamolecule.Thesethoughtsinspiredawholegenerationofresearchers,includingWatsonandCrick.Beforeyourstudentswatchthefilm,youmaywanttodiscusswiththemtheimportanceofunderstandingthree-dimensionalmolecularstructures.Askthem“Howdoesknowingthestructureofanyobjecttellyouaboutitsfunction?”
• Therearemanyopportunitiesthroughoutthefilmtodiscussthenatureofscientificinquiry.Forexample,sciencetriestoanswerquestionsaboutthenaturalworld.Intheearly1950s,theprinciplesofgeneticswereknown,butnooneknewwhatthegeneticmaterialwas,letalonehowanyphysicalorchemicalstructurerelatedtotheconsistencyofinheritanceandalsothecapacitytoevolvenewtraits.LeadstudentsinadiscussionofthequestionsscientistsweretryingtoanswerbydeterminingthestructureofDNA.
• Individualscientiststakedifferentapproachesintryingtounderstandaprocessorsolveaproblem,dueinparttodifferencesintraining,thetoolsavailablewheretheywork,andtheirpersonalities.FranklinandWilkinswantedtosolvethestructureofDNAbyobtainingx-raydiffractiondata;theyreliedonexperimentsandobservations.WatsonandCrickbuilttheoreticalmodelswhichallowedthemtoseewhetherthosemodelsagreedwithwhatwasknownaboutchemicalbondingandx-raydata.Manyscientistsuseacombinationofdata-gatheringandhypothesis-testingapproaches.
• Hypothesesmustbetestedandevaluatedagainstevidence.EarlierresearchersgatheredevidencethatDNAwasthegeneticmaterial.Evidencefromx-raydiffractionpatternsandChargaff’sbase-pairingratiossupportedWatsonandCrick’sball-and-stickmodelofthestructureofDNA.AskstudentstoidentifythekeypiecesofevidencethatWatsonandCrickusedtoconstructtheirmodelofDNA.Someofthekeyevidencepresentedinthefilmincludesthestructureofthenucleotide(aphosphatelinkedtoasugarlinkedtooneoffournitrogenousbases);Chargaff’sratios(A=TandG=C);andx-raydiffractionimages(showingthatDNAisahelixandthemolecule’sdimensions).
• Peopledoscience,andmakeintuitiveleapsofgenius,whilealsosometimesmissingtheobviousandmakingoutrightmistakes.AsDr.KarolinLuger,astructuralbiologistandHHMIinvestigatoratColoradoStateUniversity,pointsout,scientistscannotallowthemselvestobeparalyzedbyfearofmakingmistakes,andalthoughmostscientistswanttoprovethattheirhypothesesarecorrect,formallyspeaking,refutingahypothesisisalsouseful.PointouttostudentsthatbothPaulingandWatsoninitiallyhypothesizedatriplehelixwiththephosphatesontheinteriorandthebasespointingout,andwhythatwasbasedongoodlogicthatdidnothoweverfitthedata.Asnewevidenceemerges,modelsaremodifiedandsometimesrejected.
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Educator MaterialsShort Film The Double Helix
• ThefilmcontainsmanyillustrationsandanimationsofstructuresofDNAmoleculesandbasepairings.SomeoftheDNAanimationsarebasedonhistoricalmodelsthatarewrong;youmightusetheseanimationstotestyourstudents’knowledgeofmolecularstructures.IntheanimationinwhichJamesWatsonisbuildingtheDNAmoleculeandthebasesstartcomingtogether,onlytwohydrogenbondsareshownbetweenbases.TodayweknowthattherearetwohydrogenbondsinanA-TpairandthreehydrogenbondsinaG-Cpair.InanotheranimationshowingtheDNAmoleculereplicating,bothstrandsarebeingreplicatedinthesamedirection.TodayweknowthatthetwoDNAstrandsarereplicatedinantiparalleldirections,butthemechanismofDNAreplicationwasunknownatthetimeofthedouble-helixdiscovery.
• Studentsmightbeinterestedinknowingwhothepeoplefeaturedinthefilmare.JamesWatsoniscurrentlyatColdSpringHarborLaboratoryinNY;heistheonlyscientistwhowasinvolvedintheoriginalresearchwhowasinterviewedinthisfilm.TheinterviewswithFrancisCrickconsistofhistoricalfootageobtainedwhenCrickwasattheSalkInstituteinCalifornia.AfterleavingtheCavendishLaboratory,WatsonjoinedthedepartmentofbiologyatHarvardUniversity,wherehecontributedtoanunderstandingofRNA’sroleinthetransferofgeneticinformation.HewentontoleadtheColdSpringHarborLaboratoryinNY,wherehecontinuestoconductresearchfocusingonthegeneticbasisofcancerandtowriteaboutscienceinhisemeritusrole.DuringtheClintonadministration,WatsonspearheadedtheNationalInstitutesofHealth’sHumanGenomeProject,aneffortthatultimatelyinvolvednumerousscientificleadersandsequencingcenters,aswellashundredsofindividualresearchlabs.Watsonpublishedseveralleadingtextbooks,mostnotablyTheMolecularBiologyoftheGene,aswellasnumerouspopularbooksincludingtheclassicsciencediscoverystoryTheDoubleHelix.Thefilmnarrator,OliviaJudson,receivedherdoctoratefromOxfordandisanevolutionarybiologistbasedatImperialCollegeLondon.Sheiswellknownforher2002book,Dr.Tatiana’sSexAdvicetoAllCreation.Inaddition,shewasfeaturedintheNOVAdocumentary,WhatDarwinNeverKnew,andwroteaweeklyblogonevolutionarybiologyfortheNewYorkTimeswebsite.Thethreecommentatorsinthefilm,whoprovidedinsightsintothediscoveryanditssignificance,includeSeanB.Carroll,KarolinLuger,andRobertOlby.SeanB.CarrollisaHowardHughesMedicalInstituteinvestigatorattheUniversityofWisconsin-MadisonandHHMIvicepresidentforscienceeducation.Carrollisaninternationallyrecognizedevolutionarybiologist.Hisresearchfocusesonthewaynewanimalformshaveevolved.Hisstudiesofawidevarietyofanimalspeciesarerevealinghowchangesinthegenesthatcontrolanimaldevelopmentshapetheevolutionofbodypartsandbodypatterns.Inadditiontohisresearch,CarrolliswellknownforhisbooksMakingoftheFittest,EndlessFormsMostBeautiful,RemarkableCreatures,andIntotheJungle.Heisalsoaco-authorofthegeneticstextbookIntroductiontoGeneticAnalysis.Hismostrecentbook,publishedin2013,isentitledBraveGenius.KarolinLugerisanHHMIinvestigatoratColoradoStateUniversity.Luger’sresearchinterestsincludethestructuralbiologyofchromatin,thecomplexofDNAandproteinsthatformschromosomesinthenucleusofeukaryoticcells.In1997,shedeterminedthestructureofthebasicunitofDNApackaging,orthenucleosome,whichconsistsofasegmentofDNAwrappedaroundhistoneproteins.Usingthisstructureasastartingpoint,Luger’sworkhasshedlightonhowthenucleosomechangesshape,howchromatininteractswiththecell’stranscriptionmachinery,andhowsubtlechangesinhistonescanaffectoverallnucleosomestructure.RobertOlbyisasciencehistorianattheUniversityofPittsburgh.HeistheauthorofThePathtotheDoubleHelixandabiographyofFrancisCrickentitledFrancisCrick:HunterofLife’sSecrets.
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Educator MaterialsShort Film The Double Helix
RELATEDBIOINTERACTIVERESOURCESTeacherGuide:DNA(http://www.hhmi.org/biointeractive/teacher-guide-dna)
ThiscurriculumguideassistsinfilteringthroughthevastavailableresourcesfromBioInteractiveandHHMI,andorganizesthematerialaccordingtovarioustopicsrelatedtoDNA,includingDNAstructure.
BuildingBlocksofDNA(http://www.hhmi.org/biointeractive/building-blocks-dna)
Thisanimationshowsthefournitrogenousbases,adenine(A),thymine(T),guanine(G),andcytosine(C),thatmakeupDNA.
PairedDNAStrands(http://www.hhmi.org/biointeractive/paired-dna-strands)
ThisanimationshowsthatDNAhasadouble-helixstructure.Ifuntwisted,DNAwouldlookliketwoparallelstrands.EachstrandhasalinearsequenceofA,T,G,andCbases.Thepreciseorderoftheletterscarriesthecodedinstructions.Onestrandisacomplementaryimageoftheother:AalwayspairswithT,andGalwayspairswithC.
Chargaff’sRatio(http://www.hhmi.org/biointeractive/chargaffs-ratio)
ThisanimationfeaturesErwinChargaff’s1950paperstatingthatintheDNAofanygivenspecies,theratioofadeninetothymineisequal,asistheratioofguaninetocytosine.ThisbecameknownasChargaff'sratio,anditwasanimportantclueforsolvingthestructureofDNA.
Pauling’sTripleHelixModel(http://www.hhmi.org/biointeractive/pauling-triple-helix-model)
ThisanimationshowsLinusPauling'striple-helixmodel,oneofthefailedhypotheticalmodelsofDNA.Thisstructurewouldbeunstableundernormalcellularconditions.
DNAReplication(http://www.hhmi.org/biointeractive/dna-replication-schematic)
ThestructureofDNA,discoveredbyJamesWatsonandFrancisCrick,suggestsamechanismofreplication.Thisanimationshowsthatasthedoublehelixunwinds,eachstrandactsasatemplatefortheconstructionofthenewDNAmolecule.
TheChemicalStructureofDNA(http://www.hhmi.org/biointeractive/chemical-structure-dna)
ThisanimationshowshowDNA'schemicalpropertiescanbeharnessedforavarietyofbiotechnologyapplications.
WatsonConstructingBase-PairModels(http://www.hhmi.org/biointeractive/watson-constructing-base-pair-models)
ThisvideoshowsJimWatsonexplaininghow,duringtheprocessoftryingtoelucidatethestructureofDNA,hemadesomecardboardmodelstounderstandhowDNAnucleotidesarepaired.Themodelshelpedhimvisualizehowhydrogenatomsofpairednucleotidesinteractwitheachothertoformasymmetricalstructurethatfitsthedouble-helixmodel.
USINGTHEQUIZThequizcanhavetwolevels:useonlyquestions1-7forabasiclevelquizandincludequestions8-10formoredepth.Thequizisdesignedasasummativeassessmentthatprobesstudentunderstandingofthekeyconceptsaddressedinthefilm.However,someteachersusethequizbeforeandduringthefilmtoassessstudents’priorknowledgeandtoguidestudentsastheywatchthefilm.Teachersareencouragedtochoosetheusethatbest
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Educator MaterialsShort Film The Double Helix
fitstheirlearningobjectivesandtheirstudents’needs.Teachersareencouragedtomodifythequiz(e.g.onlyasksomeofthequestions,explaincomplicatedvocabularyforELLstudents)asneeded.
QUIZQUESTIONSANDANSWERS1. (KeyConceptF)Inthe1950swhenWatsonandCrickwereworkingontheirmodelofDNA,manyscientists
didnotthinkthatDNAcarriedthegeneticcode.
a. Whatwastheothertypeofmoleculethatsomescientiststhoughtmightcarrygeneticinformation?Proteins
b. Whydidthisothertypeofmoleculeseemlikealikelycandidate?
Proteinswerefavoredbymanyscientistsbecausetherearemanyproteins,theycomeindifferentshapesandhavemanydifferentfunctions.Also,proteinsaremadeof20differentaminoacidcomponents,whileDNAiscomposedofonlyfourbasicsubunits.2. (KeyConceptA)WhatarethechemicalcomponentsofaDNAnucleotide?
a. aphosphate,asugar,andanitrogenousbase
b. aphosphate,anitrogenousbase,andanaminoacid
c. anitrogenousbase,asugar,andanaminoacid
d. anitrogenousbase,ATP,andasugar3. (KeyConceptB)ThetwostrandsofaDNAmoleculeareheldtogetherbyhydrogenbondsbetweenthe
a. phosphategroupsoneachstrand
b. nitrogenousbasesoneachstrand
c. basesandthephosphate-sugarbackbone
d. carbonatomsinthesugars4. (KeyConceptB)Inthediagrambelow,strandsIandIIrepresentthetwocomplementarystrandsofa
portionofaDNAdoublehelix.ThesequenceofstrandIisindicatedbelow.WhatisthesequenceofstrandII?
StrandI-----------C-T-A-C-----------
StrandII-----------?-?-?-?-------------
a. AGCA b. CTAC c. TCGT d. GATG
5. (KeyConceptC)Theinstructionsforthetraitsofanorganismaredeterminedby
a. theproportionsofA,T,C,andGinDNAmolecules
b. theorderofnucleotidesinDNAmolecules
c. thelengthofDNAmolecules
d. thewaynucleotidesarepairedinthetwostrandsofaDNAmolecule
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Educator MaterialsShort Film The Double Helix
6. (KeyConceptsEandF)Scientistsbuildmodelsbasedonwhattheyknowfrompreviousresearchtoderivetestablehypotheses.Independently,bothWatsonandCrickandtheircompetitorLinusPaulingconstructedanincorrecttriple-helixmodelwiththenitrogenousbasesarrangedsotheywereontheexteriorofthemoleculeandthephosphategroupsontheinterior.
a. Althoughtheirmodelwaswrong,whatassumptionmadeitreasonabletobuildamodelwiththebasesprojecting totheoutside?
Thebasescontainthegeneticinformationastheyvaryinamountbetweenspeciesandintheirarrangementwithinthemolecule;basedonthisinformationitwasreasonabletoassumethatthebaseswouldbeontheoutsideoftheDNAmolecule.
b. WhatevidencecausedWatsonandCricktorevisetheirmodel?
Basedonthefilm,studentsshouldrealizethatinformationaboutthedimensionsofthedoublehelixfromFranklin’simages,aswellasChargaff’spairingrulesforthebases,ledthemtoarevisedmodel.ThefilmsaysthatWatsonhadmisrememberedsomekeymeasurementsfromFranklin’swork.HerimagesshowedtherewasmuchmorewaterinDNAthanthetriple-helixmodelallowed.Further,thetriple-helixmodelcouldnotexplainhowthethreephosphatebackbonescouldbeheldtogether.(Studentsmaynotknowtheselasttwopointsastheyarenotexplicitlystatedinthefilm.)
7. (KeyConceptC)EvenbeforethestructureofDNAwassolved,studiesindicatedthatthegeneticmaterialmusthavethefollowingproperties:
• beabletostoreinformation;
• befaithfullyreplicatedandbepassedonfromgenerationtogeneration;and
• allowforchanges,andthusevolution,tooccur.
ExplainhowthestructureofthedoublehelixshowedthatDNAhadtheseproperties.Writeoneortwosentencesperpoint.
Theorderofthebases,A,T,G,andC,containedinformation.BecauseAisalwayspairedwithTandGwithC,theorderofbasesononestranddeterminestheorderontheotherstrand.Thus,ifaDNAmoleculewereunwound,eachstrandcouldbecopiedintoacomplementarystrand,producinganexactreplicaoftheoriginalmolecule.Errorsinthecopyingmechanismcouldresultinmutations,orchangesintheDNAsequence,thatcouldbeinheritedbyfuturegenerations.
8. (KeyConceptF)In1928,FrederickGriffithconductedanexperimentinwhichheinjectedmicewithdifferentkindsofbacteria.Whenbacteriathatcausedisease(pathogenic)wereinjectedinhealthymice,thesemicegotsickanddied.Othertypesofbacteria(nonpathogenic)didnotcausethemicetodie.GriffithtooktheDNAfromdeadpathogenicbacteriaandtransferreditintolivingnonpathogenicbacteria.Thesealteredbacteriaweretheninjectedintohealthymice.Themicediedofthesamediseasecausedbythepathogenicbacteria.Basedonthisinformation,whichstatementwouldbeavalidconclusion?
a. Whenanorganismdies,theDNAchanges;itnolongerprovidesthesamegeneticinformation.
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Educator MaterialsShort Film The Double Helix
b. WhenDNAfromoneorganismistransferredtoanotherorganism,theDNAnolongerfunctions.
c. DNAindifferenttypesofbacteriacarriesexactlythesametypeofinformation.
d. WhenDNAfromoneorganismistransferredtoanotherorganism,itcangivenewtraitstothesecondorganism.
9. (KeyConceptF)ThefollowingtableisasampleofthedataErwinChargaffpublishedin1952.
Proportions*ofNitrogenousBasesintheDNAofDifferentOrganismsOrganism Tissue %Adenine %Guanine %Cytosine %ThymineYeast 31.3 18.7 17.1 32.9Seaurchin Sperm 32.8 17.7 18.4 32.1Rat Bone
marrow28.6 21.4 21.5 28.4
Human Thymus 30.9 19.9 19.8 29.4Human Sperm 30.3 19.5 19.9 30.3*Definedasmolesofnitrogenousconstituentsper100gofatomsofphosphate.Source:E.ChargaffandJ.Davidson,Eds.TheNucleicAcids.AcademicPress,1955.
a. Whichofthefollowingobservationscanbesupportedbythedatainthetable?(Placeacheckmarkintheboxnexttothecorrectstatements.)
√Ineachanimal,thepercentageofadenineisthesameasthepercentageofthymineintheDNA.
☐Theproportionsofadenine+thymineandguanine+cytosinearethesameinallorganisms.
☐Largerorganismshavegreateramountsofeachnitrogenousbasethansmallerorganismshave.
☐ThetotallengthofDNAmoleculesinallorganismsisaboutthesame.
b. Inoneortwosentences,explainhowtheseobservationshelpedWatsonandCrickdeveloptheirmodelofDNA.
Itsuggeststhatinthedoublehelix,adeninealwayspairswiththymineandguaninealwayspairswithcytosine.
c. Inoneortwosentences,explainwhytheproportionsofnitrogenousbasesintheDNAoftwodifferenthumantissues(thymusandsperm)areaboutthesame.
AllbodycellsandtissuesinaparticularorganismcontainthesameDNA.
10. TheimageontherightshowsthefamousphotoB51takeninMay1952byRosalindFranklinandherstudentRaymondGosling.Thisx-raydiffractionpatternprovidedinformationaboutthepositionsofatomsinaDNAmolecule.
a. (KeyConceptG)IdentifytheclueinthisphotothatrevealedthatDNAisahelix.
TheclueistheXshapeintheimage.
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Educator MaterialsShort Film The Double Helix
b. (KeyConceptD)Measurementsrevealedthatthedistancebetweenthetwostrandswasalwaysequal.ExplainhowthisinformationhelpedWatsonandCrickbuildasuccessfulmodelofDNA.
WatsonandCrickconcludedthatalargepurinemolecule(adenineorguanine)alwaysbondswithasmallerpyrimidinemolecule(cytosineorthymine)—thatway,thedistancebetweenthetwostrandsofDNAisalwaysthesame.
c. (KeyConceptE)WasthisinformationconsistentwiththedataobtainedbyChargaff(question9)?Explainyouranswer.
Yes.ChargaffdiscoveredthatintheDNAofanorganism,theproportionofadenine(apurine)isthesameastheproportionofthymine(apyrimidine).Thesameistrueforguanine(apurine)andcytosine(apyrimidine).Thesedataareconsistentwithamodelinwhichapurineononestrandalwaysbondswithapyrimidineontheotherstrand.
KEYREFERENCESWatson,J.D.,andCrick,F.H.C.1953.Astructurefordeoxyribosenucleicacid.Nature171:737-738.Wilkins,M.H.F.,Stokes,A.R.,andWilson,H.R.1953.Molecularstructureofdeoxypentosenucleicacids.Nature171:738-740.Franklin,R.,andGosling,R.G.1953.Molecularconfigurationinsodiumthymonucleate.Nature171:740-741.Griffith,F.1928.Thesignificanceofpneumococcaltypes.JournalofHygiene27:113-159.Avery,O.T.,MacLeod,C.M.,andMcCarty,M.1944.Studiesonthechemicalnatureofthesubstanceinducingtransformationofpneumococcaltypes.TheJournalofExperimentalMedicine79:137-158.Hershey,A.D.,andChase,M.1952.Independentfunctionsofviralproteinandnucleicacidingrowthofabacteriophage.JournalofGeneralPhysiology36:39-56.Chargaff,E.1950.Chemicalspecificityofnucleicacidsandmechanismoftheirenzymaticdegradation.Experientia6(6):201-209.Judson,H.F.TheEighthDayofCreation:MakersoftheRevolutioninBiology.(1979)Watson,J.TheDoubleHelix:APersonalAccountoftheDiscoveryoftheStructureofDNA.(1968)Olby,R.ThePathtotheDoubleHelix:TheDiscoveryofDNA(UniversityofWashingtonPress,Seattle:1974&revised1994).Olby,R.FrancisCrick:HunterofLife'sSecrets(ColdSpringHarborLaboratoryPress:August25,2009)
AUTHORSWrittenbyCindyGay,SteamboatSpringsHighSchool,CO,LauraBonetta,PhD,HHMI,andMaryColvard,Cobleskill-RichmondvilleHighSchool(retired),Deposit,NewYorkEditedbyLauraBonetta,PhD,DennisLiu,PhD,ErikoClements,PhD,HHMIandSusanDodge,consultant;copyeditedbyLindaFelaco;illustrationsbyHeatherMcDonald,PhDReviewedbyKarolinLuger,PhD,ColoradoStateUniversity.
FIELDTESTERSAyseAydemir,LindaCiota,DonnaBalado,JenniferKaltenbach,LauraJulien,LilleenFerraro,EllenPerry,StaceyStrandberg