short film the double helix educator...

RevisedMarch2017www.BioInteractive.org Page1of17

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



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,



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



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.



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).



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).



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.)



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-



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.



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.



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.



• 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.



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



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



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.



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.



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

short film the double helix educator...

Documents