Ch. 16 The Molecular Basis of...
Transcript of Ch. 16 The Molecular Basis of...
TheMolecularBasisTheMolecularBasisofInheritanceofInheritanceBIOL222
Ch.16
Overview:LifeOverview:Life’’sOpera:ngInstruc:onssOpera:ngInstruc:ons
• JamesWatsonandFrancisCrick
• 1953‐produceddouble‐helicalmodelforthestructureofDNA
TheSearchfortheGene:cMaterialTheSearchfortheGene:cMaterial
• T.H.Morgan
• Showedthatgenesarelocatedonchromosomes
Therefore…
• Twocomponentsofchromosomes—DNAand
protein—becamecandidatesforthegeneOcmaterial
• TheroleofDNAinheredity
• Firststudiedinbacteria/bacteriophages
EvidenceThatDNACanTransformBacteriaEvidenceThatDNACanTransformBacteria• FrederickGriffith‐1928
• twostrainsofabacterium
• onepathogenicandoneharmless
• heat‐killedpathogenicremains
• mixedwithlivingharmlessstrain
• somelivingcellsbecamepathogenic
• Calledtransforma:on
• Changeingenotypeandphenotype
• duetoassimilaOonofforeignDNA
Fig.Fig. 16‐216‐2
Living S cells(control)
Living R cells(control)
Heat-killedS cells (control)
Mixture ofheat-killedS cells andliving R cells
Mouse diesMouse dies Mouse healthy Mouse healthy
Living S cells
RESULTS
EXPERIMENT
• OswaldAvery,MaclynMcCarty,andColinMacLeod‐1944
• transformingsubstancewasDNA
• onlyDNAtransformedharmlessbacteriaintopathogens
• SequenOallydeacOvatedproteins,RNA,orDNA
EvidenceThatDNACanTransformBacteriaEvidenceThatDNACanTransformBacteria
EvidenceThatViralDNACanProgramCellsEvidenceThatViralDNACanProgramCells
• MoreevidenceforDNAasthegeneOcmaterialcamefromstudiesofvirusesthatinfectbacteria
• bacteriophages
• widelyusedinmoleculargeneOcsresearch
Bacterialcell
Phage head
Tail sheathTail fiberDNA
100
nm
• AlfredHersheyandMarthaChase‐1952
• showedDNAisgeneOcmaterial
• phageknownasT2
• LookedtoseeifviralDNAorprotein
• entersthebacterium
• concludedinjectedDNA
• providesgeneOcinformaOon
EvidenceThatViralDNACanProgramCellsEvidenceThatViralDNACanProgramCells
Fig.Fig. 16‐4‐116‐4‐1
EXPERIMENT
Phage
DNA
Bacterial cell
Radioactiveprotein
RadioactiveDNA
Batch 1:radioactivesulfur (35S)
Batch 2:radioactivephosphorus (32P)
Fig.Fig. 16‐4‐216‐4‐2
EXPERIMENT
Phage
DNA
Bacterial cell
Radioactiveprotein
RadioactiveDNA
Batch 1:radioactivesulfur (35S)
Batch 2:radioactivephosphorus (32P)
Emptyproteinshell
PhageDNA
Fig.Fig. 16‐4‐316‐4‐3
EXPERIMENT
Phage
DNA
Bacterial cell
Radioactiveprotein
RadioactiveDNA
Batch 1:radioactivesulfur (35S)
Batch 2:radioactivephosphorus (32P)
Emptyproteinshell
PhageDNA
Centrifuge
Centrifuge
Pellet
Pellet (bacterialcells and contents)
Radioactivity(phageprotein)in liquid
Radioactivity(phage DNA)in pellet
EvidenceThatDNAIstheGene:cMaterialEvidenceThatDNAIstheGene:cMaterial
• DNAisapolymerofnucleoOdes
• base,asugar,andaphosphategroup
• ErwinChargaff‐1950
• DNAcomposiOonvaries
• fromonespeciestothenext
• evidenceofdiversitypointstoDNA
• Chargaff’srules
• equalnumberofAandTbases
• andequalGandCbases
StructuralModelofDNAStructuralModelofDNA
• RosalindFranklinandMauriceWilkins
• X‐raycrystallography
• FranklinproducedapictureoftheDNA
(a) Rosalind Franklin (b) Franklin’s X-ray diffraction photograph of DNA
• WatsonandCrick
• basespairedlikewithlike
• resultedinanon‐uniformwidth
• purinewithapyrimidine
• uniformwidthconsistentwithX‐ray
StructuralModelofDNAStructuralModelofDNA
Purine + purine: toowide
Pyrimidine + pyrimidine: toonarrow
Purine + pyrimidine:width consistentwith X-ray data
• ThereforeWatsonandCrick…
• pairingmorespecific
• dictatedbythebasestructures
• Determinedadenine(A)withthymine(T)
• guanine(G)withcytosine(C)
• explainsChargaff’srule:
• inanyorganismtheamountofA=T,andthe
amountofG=C
StructuralModelofDNAStructuralModelofDNA
Fig.Fig. 16‐816‐8
Cytosine (C)
Adenine (A) Thymine (T)
Guanine (G)
Fig.Fig. 16‐716‐7
(c) Space-filling model
Hydrogen bond 3′ end
5′ end
3.4 nm
0.34 nm3′ end
5′ end
(b) Partial chemical structure(a) Key features of DNA structure
1 nm
• WatsonandCrick
• specificbasepairing
• suggestedapossiblecopyingmechanism
• twostrandsofDNAarecomplementary
• eachstrandactsasatemplate
DNAReplica:onDNAReplica:on
Fig.Fig. 16‐9‐116‐9‐1
A T
GC
T A
TA
G C
(a) Parent molecule
Fig.Fig. 16‐9‐216‐9‐2
A T
GC
T A
TA
G C
A T
GC
T A
TAG C
(a) Parent molecule (b) Separation ofstrands
Fig.Fig. 16‐9‐316‐9‐3
A T
GC
T A
TA
G C
(a) Parent molecule
A T
GC
T A
TAG C
(c) “Daughter” DNA molecules,each consisting of oneparental strand and onenew strand
(b) Separation ofstrands
A T
GC
T A
TA
G C
A T
GC
T A
TAG C
• Semiconserva:vemodel
• Eachdaughtermolecule
• oneold,onenewstrand
• CompeOngmodels
• conservaOvemodel
• thetwoparentstrandsrejoin
• dispersivemodel
• eachstrandisamixofoldandnew
DNAReplica:onDNAReplica:on
Fig.Fig. 16‐1016‐10
Parent cellFirstreplication
Secondreplication
(a) Conservativemodel
(b) Semiconserva-tive model
(c) Dispersivemodel
DNAReplica:onDNAReplica:on
• DNAreplicaOon
• remarkablespeedandaccuracy
• MorethanadozenenzymesandotherproteinsparOcipate
DNA Replication
• Originsofreplica:on
• SiteswheretwoDNAstrandsseparate
• opensreplicaOon“bubble”
• eukaryoOcchromosome
• hundreds‐thousandsoforiginsofreplicaOon
• ReplicaOonproceedsinbothdirecOonsfromeachorigin
• unOltheenOremoleculeiscopied
DNAReplica:onDNAReplica:on
Fig.Fig. 16‐1216‐12
Origin ofreplication Parental (template) strand
Daughter (new) strand
Replication forkReplicationbubble
TwodaughterDNAmolecules
(a) Origins of replication in E. coli
Origin of replication Double-stranded DNA molecule
Parental (template) strandDaughter (new) strand
Bubble Replication fork
Two daughter DNA molecules
(b) Origins of replication in eukaryotes
0.5 µm
0.25 µm
Double-strandedDNA molecule
• Replica:onfork
• Y‐shapedregion
• endofeachreplicaOonbubble
• wherenewDNAstrandsareelongaOng
• Helicases
• untwistDNAatreplicaOonforks
• Single‐strandbindingprotein
• Binds/stabilizessingle‐strandedDNA
• unOlitcanbeusedasatemplate
• Topoisomerase
• corrects“overwinding”aheadofreplicaOonforks
• bybreaking,swiveling,andrejoiningDNAstrands
DNAReplica:onDNAReplica:on
Fig.Fig. 16‐1316‐13
Topoisomerase
Helicase
PrimaseSingle-strand bindingproteins
RNAprimer
5′5′
5′ 3′
3′
3′
• Primer
• ShortRNAstrandrequiredforiniOaOon
• Primase
• EnzymethatstartsanRNAchainfromscratch
• addsRNAnucleoOdes
• UsesDNAasatemplate
• 5–10nucleoOdeslong
• 3′endstarOngpointfornewDNA
DNAReplica:onDNAReplica:on
• DNApolymerases
• catalyzeelongaOonofnewDNA
• atareplicaOonfork
• 500nucleoOdespersecondinbacteria
• 50persecondinhumancells
DNAReplica:onDNAReplica:on