Experimental evidence to Prove RNA as genetic material

Evidence to Prove RNA as genetic material

1. K. Narayanapura, Kothanur (PO), Bengaluru 560077
Tel[phone] Fax. [phone]
e-mail:[email protected], www.kristujayanti.edu.in
Experimental Evidence to
prove RNA as genetic material
Dr. Manikandan Kathirvel
Assistant Professor,
Department of Life Sciences,
Kristu Jayanti College (Autonomous),
Bengaluru

2. WhileDNAisthegeneticmaterialforthevastmajorityoforganisms,therearesomevirusesthatuseRNAastheir
geneticmaterial.
ThesevirusescanbeeithersingleordoublestrandedandincludeSARS,influenza,hepatitisCandpolio,aswellasthe
retroviruseslikeHIV-AIDS.

3. RNAASGENETICMATERIAL
Experimentswithtobaccomosaicvirus(TMV):
Withthehelpoftransformationexperimentandbacteriophageinfection,itwasshownthatDNAisthegeneticmaterial.
RNAisthegeneticmaterialinviruseswasdemonstratedin1956withtheexperimentsconductedontobaccoplantbyA.Gierer
andG.Schramm.
1.Allvirusesarenotlimitedtobacterialhosts.
2.Virusesinfectandparasiteplantcells,andevensomeanimalcellscontainRNAonly.Intheseviruses,RNAactasgenetic
material.
3.Oneplantvirus,Tobaccomosaicvirus(TMV),thatcontainsRNA,notDNAwasanimportanttoolforgeneticExperiments.
4.TMVinfectstobacco,causingtheinfectedregionsonleavestobecomediscoloredandbristled.DifferentstrainsofTMV
produceclearlydifferentinheritedlesionsontheinfectedleaves.
5.ThecommonTMVvirusproducesagreenmosaicdisease,butavariantHolmesribgrass(TMV-HR),producesring
spotlesions.Moreover,theaminoacidcompositionsoftheproteinsofthesetwostrainsdiffer.

4. H.Fraenkel-ConratandB.SingerExperiment:
ThefirstevidencethatRNAalsohasthecapacitytocarrygeneticinformationcame
fromexperimentsconductedwithtobaccomosaicvirus(TMV).Thisvirusdoes
notcontainanyDNA,andiscomposedofRNAsurroundedbyahollowcylinderof
proteinsubunits.
H.Fraenkel-ConratandB.Singerfirstdevelopedthetechniquesforseparating
TMVparticlesintoRNAandprotein.
•TheyusedRNAandproteinsseparatelyintestsforinfectivity,itcouldbeshown
thatRNAalonewasabletocauseinfection.Suchapropertywasnotfoundin
theproteinfraction.
•Theyfoundthatviruscouldbebrokenintocomponentpartsandtheycould
againbereassembledorreconstitutedtoformfunctionalvirus.
•ItwaspossibletosynthesizechimericvirusparticlesusingRNAfromonestrain
andproteinfromanotherstrain.
Whenthesereassembledviruseswereusedtoinfecttobaccoleaves,theprogeny
virusesproducedwerealwaysfoundtobephenotypicallyandgenotypically
identicaltotheparentstrainfromwhichtheRNAhadbeenobtained.
Suchchimerashadserologicalpropertiesofthosestrainsfromwhichproteinwas
derived,butinotherproperties,itresembledtheotherstrainfromwhichRNAwas
used.

5. In one experiment, two viruses used
weretobaccomosaicvirus(TMV)and
Holmesrib-grassvirus(HRV)(isolatedfromPlantago
lanceolataandtherefore,isalsoknownasPlantagostrain).
Proteinsofthesetwovirusesdifferinhavingdifferent
frequenciesandsequencesofaminoacids.
Alsothesevirusesgivedifferentsymptoms.
Onleavesofaparticularvarietyoftobacco,
•whileTMVproducesmottlingofleaves,
•HRVproducesdistinctringpatterns.
ReciprocalchimerasusingRNAofonestrainandproteinofthe
otherstraincouldbeobtained.
Itwasfoundthatwhenthesechimeras(hybrid)wereusedfor
infection,theprogenyhadproteinswhichcorrespondedtothe
virusfromwhichRNAoftheinfectingvirusparticleswas
derived.
Thisprovedthatspecificityofvirusproteinswasdeterminedby
RNAaloneandthatproteinscarriednoinformation.

6. ThereassembledviruseswiththeTMV-common
RNAandTMV-HRVproteinproducedagreen
mosaicdiseasecharacteristicofTMV-common.
RecoveredvirushadproteincharacteristicofTMV
common.
Thisprovedthatspecificityofvirusproteinswas
determinedbyRNAaloneandthatproteinscarried
nogeneticinformation.HenceRNAcarriesgenetic
informationnotproteins.
ThegeneticRNAisusuallyfoundtobesingle
strandedbutinsomeitisdoublestrandedasin
reovirus,woundtumorvirus.

7. Thank you