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M O L EC U LAR B A S IS OF I N H E RI T ANCE

M O L EC U LAR B A S IS OF I N H E RI T ANCE. T H E D N A :. DNA is a long polymer of deoxyribonucleotides. The length of the DNA depends on, number of nucleotide pair present in it. Characteristics of the organism depend on the length of the DNA. Bacteriophage ø174 has 5386 nucleotides.

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M O L EC U LAR B A S IS OF I N H E RI T ANCE

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  1. MOLECULARBASISOFINHERITANCE

  2. THEDNA: • DNAisalongpolymerofdeoxyribonucleotides. • ThelengthoftheDNAdependson,numberofnucleotidepairpresentinit. • CharacteristicsoftheorganismdependonthelengthoftheDNA. • Bacteriophageø174has5386nucleotides. • Bacteriophagelambdahas48502basepairs. • Escherichiacolihave4.6X106basepairs. • Humangenome(haploid)is3.3X109bp.

  3. Structureofpolynucleotidechain: • Anucleotidehasthreecomponent:- • Anitrogenbase • Apentosesugar(riboseinRNAanddeoxyriboseinDNA) • Aphosphoricacid. • Therearetwotypesofnitrogenbases: • Purines(AdenineandGuanine) • Pyrimidines(Cytosine,UracilandThymine) • Adenine,GuanineandCytosineiscommoninRNAandDNA. • UracilispresentinRNAandThymineispresentinDNAinplaceofUracil. • PentosesugarisriboseinRNAandDeoxyriboseinDNA. • AnitrogenbaseattachedtothepentosesugaratC1ofpentose • sugarby

  4. Phosphoricacidattachedtothe5’ OH ofanucleosidebyPhosphodiesterlinkageacorrespondingnucleotideisformed.(Ribonucleotideordeoxyribonucleotidesdependingonthesugarunit). • Two nucleotides are joined by 3’-5’ Phosphodiesterlinkagetoformdinucleotide. • Morethantwonucleotidesjoinedtoformpolynucleotidechain. • Polynucleotide chain has a free phosphate moiety at 5’ end of sugar, is referred to as • 5’ end • In the other end of the polymer with 3’-OH group called 3’ end. • Thebackboneofthepolynucleotidechainissugarandphosphate. • Nitrogenbaseslinkedtothesugarmoietyprojectfromthebackbone. • InRNAeverynucleotidehasanadditional–OH group at 2’ of ribose. • InRNAUracilisfoundinplaceofthymine. • 5-methyluracilistheothernameofthymine.

  5. HistoryofDNA: • DNAisanacidicsubstanceinthenucleuswasfirstidentifiedbyFriedrichMeischerin 1869. He named it as ‘Nuclein” • 1953doublehelixstructureofDNAwasgivenbyJamesWatsonandFrancisCrick,basedonX-raydefractiondataproducedMauriceWilkinsandRosalindFranklin. • Hallmarkoftheirpropositionwasbasepairingbetweentwostrandsofpolynucleotidechains.ThiswasbasedonobservationofErwinChargaff. • Chargaff ’s observation was that for a double stranded DNA,theratiobetweenAdenineandThymine,andGuanineandCytosineareconstantandequalone.

  6. SalientfeaturesofDoublehelix structureofDNA: • Madeoftwopolynucleotidechains. • Sugarandphosphateformsthebackboneandbasesprojectedtoinside. • Twochainshaveanti-parallelpolarity. • Twostrandsareheldtogetherbyhydrogenbondpresentinbetweenbases. • AdenineofonestrandpairswithThymineofanotherstrandbytwohydrogenbondsandviceversa. • GuanineofonestrandpairswithCytosineofanotherstrandbythreehydrogenbondsand • viceversa. • Apurinecomesoppositetoapyrimidine.Thisgeneratesapproximatelyuniformdistancebetweenthetwostrandsofthehelix. • Thetwochainsarecoiledinaright– handedfashion. • Thepitchofthehelixis3.4nmor34A0 • Thereareroughly10bpinturn. • Thedistancebetweenthebpinahelixis0.34nmor3.4A0. • Theplaneofonebasepairstacksovertheotherindoublehelix. • H-bondconfersstabilityofthehelicalstructureoftheDNA. • Centraldogmaofflowofgeneticinformation:DNA→ RNA→ Protein.

  7. PackagingofDNAHelix: • Distancebetweentwoconjugativebasepairsis0.34nm,thelengthoftheDNAinatypicalmammaliancellwillbe6.6X109bpX • 0.34X10-9/bp,itcomesabout2.2meters. • ThelengthofDNAismorethanthedimensionofatypicalnucleus(10-6m),howissuchalongpolymerpackagedinacell? • Packaginginprokaryotes: • Theydonothavedefinitenucleus. • TheDNAisnotscatteredthroughoutthecell. • DNAisheldtogetherwithsomeproteinsinaregioniscalled • ‘nucleoid’. • TheDNAinnucleoidisorganizedinlargeloopsheldbeproteins.

  8. PackaginginEukaryotes: • Ineukaryotesthepackagingismorecomplex. • Thereisasetofpositivelycharged,basicproteincalledHistones. • HistonesarepositivelychargedduetorichinbasicaminoacidslikeLysinesandarginines. • Histonesareorganizedtoformaunitofeightmoleculescalledhistoneoctamere. • NegativelychargedDNAwrappedaroundpositivelychargedhistoneoctameretoformastructurecallednucleosome. • Atypicalnucleosomecontains200bpofDNAhelix. • Nucleosomeconstitutestherepeatingunitofastructureinnucleuscalledchromatin,threadlikestainedbodiesseeninthenucleus. • Thenucleosomesareseenas‘beads-on-string’ structurewhenviewedunderelectronmicroscope. • Thechromatinispackagedtoformchromatinfibersthatarefurthercoiledandcondensedatmetaphasestagetoformchromosome. • PackagingathigherlevelrequiredadditionalsetofproteinscalledNon-histoneChromosomal(NHC)proteins. • Inatypicalnucleussomelooselycoiledregionsofchromatin(lightstained)iscalledeuchromatin. • ThechromatinthatmoredenselypackedandstainsdarkarecalledHeterochromatin. • Euchromatinistranscriptionallyactivechromatinandheterochromatinisinactive.

  9. THESEARCHOF GENETICMATERIAL: Transformingprinciple: • GivenbyFrederickGriffithin1928. • HisexperimentbasedonStreptococcuspneumoniae(causedpneumonia). • Thereischangeinphysicalformofbacteria. • Therearetwocoloniesofbacteria: • SmoothshinycoloniescalledSstrain. • RoughcoloniescalledRstrain. • S-strainbacteriahaveamucous(polysaccharide)coat. • R-straindoesnothavemucouscoat. • S-strainisvirulentandcausedpneumoniainmiceanddiedwheninfected. • R-strainisnon-virulentanddoesnotcausedpneumoniainmicewheninfected. • HeatkilledS-Strainisnon-virulentanddoesnotcausespneumonia. • TheheatkilledS-StrainmixedwithliveR-Straininjectedintomice;themicedevelopedpneumoniaanddied. • HerecoveredliveS-Strainbacteriaformthedeadmice.

  10. Biochemicalcharacterizationoftransformingprinciple: • BiochemicalnatureoftransformingprinciplewasdiscoveredbyOswaldAvery,ColinMacleodandMaclynMcCarty.(1933-44) • Priortotheirworkgeneticmaterialwasthoughttobeprotein. • Theyworkedtodeterminethebiochemicalnatureofthe • ‘transforming principle’ of Griffith’s experiment. • Theypurifiedbiomolecules(proteins,DNAandRNA)fromtheheatkilledScellstoseewhichonecouldtransformliveRcellstoScells. • HeatkilledS-Strain+protease+LiveR-Strain → transformation. • HeatkilledS-Strain+RNase+LiveR-Strain → transformation. • HeatkilledS-Strain+DNase+LiveR-Strain → Notransformation.

  11. Conclusionoftheexperiments: • ProteinofheatkilledS-Strainisnotthegeneticmaterial • RNAofheatkilledS-Strainisnotthegeneticmaterial. • DNAofheatkilledS-Strainisthegeneticmaterial,becauseDNAdigestedwithDNasemixedwithR-strainunabletotransformR-StraintoS-Strain.

  12. Conclusionofexperiment: • R– StrainbacteriahadsomehowbeentransformedbytheheatkilledS-Strainbacteria. • Some‘transforming principle’,transferredfromheatkilledS-Strainbacteria,hadenabledtheR-Straintosynthesizesmoothpolysaccharidecoatandbecomevirulent(SStrain). • ThetransformationofR-StraintoS-StrainisduetotransferofGeneticmaterial. • Howeverthebiochemicalnatureofgeneticmaterialwasnot • definedfromhisexperiment

  13. TheGeneticMaterialisDNA: • ‘DNA is the genetic material’ is proved by AlfredHersheyandMarthaChase(1952). • Theyworkedonthevirusthatinfectsbacteria • calledbacteriophage. • Duringnormalinfectionthebacteriophagefirstattachesthebacteriacellwallandtheninsertsitsgeneticmaterialintothebacterialcell. • Theviralgeneticmaterialbecameintegralpartofthebacterialgenomeandsubsequentlymanufacturesmorevirusparticleusinghostmachinery. • HersheyandChaseworkedtodiscoverwhetheritwasproteinorDNAfromthevirusesthatenteredthebacteria.

  14. Experiment:(blendersexperiment) • Theygrewsomevirusesonamediumhavingradioactivephosphorusandsomeothersonmediumhavingradioactivesulfur. • VirusesgrowninradioactivePhosphorushaveradioactiveDNAbutnotradioactiveproteinbecausePhosphoruspresentinDNAnotinprotein. • VirusesgrowninradioactivesulfurhaveradioactiveproteinnotradioactiveDNAbecausesulfurpresentinproteinbutnotinDNA. • Infection:radioactivephageswereallowedtoattachtoE.colibacteria; thephagestransferthegeneticmaterialtothebacteria. • Blending:theviralcoatswereseparatedfromthebacteriasurfacebyagitatingtheminablender. • Centrifugation:Thevirusparticleswereseparatedfromthebacteriabyspinningtheminacentrifugemachine.

  15. Observation: • BacteriainfectedwithvirusesthathadradioactiveDNAwereradioactiveandnoradioactivityinthesupernatant. • Bacteriainfectedwithvirusesthathadradioactiveproteinwerenotradioactive,butradioactivityfoundinthesupernatant. • ConclusionofExperiment: • DNAistheinfectingagentthatmadethebacteriaradioactivehenceDNAisthegeneticmaterialnottheprotein.

  16. PROPOERTIESOFGENETICMATERIAL(DNAVERSUSRNA): • Criteriaforgeneticmaterial: • Itshouldbeabletogenerateitsreplica(replication) • Itshouldbechemicallyandstructurallystable. • Itshouldprovidethescopeforslowchanges(mutation)thatrequiredforevolution. • It should be able to express itself in the form of ‘Mendelian • Character’. • Proteindosenotfulfillthecriteriahenceitisnotthegeneticmaterial. • RNAandDNAfulfillthecriteria.

  17. RNAisunstable: • 2’-OHgrouppresentateverynucleotide(ribosesugar)inRNAisareactivegroupandmakesRNAliableandeasilydegradable. • RNAisalsonowknownascatalyst,hencereactive. • RNAisunstableandmutatesfaster.ConsequentlytheviruseshavingRNA • genomeandhavingshorterlifespanmutateandevolvefaster. • DNAismorestable: • StabilityasoneofthepropertiesofgeneticmaterialwasveryevidentinGriffith’s ‘transforming principle’ itself that heat, which killed the bacteria at leastdidnotdestroysomeofthepropertiesofgeneticmaterial. • Twostrandsbeingcomplementaryifseparatedbyheatingcometogether,whenappropriateconditionsareprovided. • PresenceofThymineinplaceofuracilconfersadditionalstabilitytoDNA • DNAischemicallylessreactiveandstructurallymorestablewhencomparedtoRNA. • ThereforeamongthetwonucleicacidstheDNAisabettergeneticmaterial.

  18. Bettergeneticmaterial(DNAorRNA) • PresenceofthymineattheplaceofuracilconfersmorestabilitytoDNA. • BothDNAandRNAareabletomutate. • InfactRNAbeingunstablemutateatafasterrate. • RNAcandirectlycodeforthesynthesisofproteins,henceeasilyexpress. • DNAhoweverdependsonRNAforproteinsynthesis. • TheproteinsynthesismachineryhasevolvedaroundRNA. • BothRNAandDNAcanfunctionsasgeneticmaterial,butDNAbeingmorestableispreferredforstorageofgeneticinformation. • ForthetransmissionofgeneticinformationRNAisbetter.

  19. RNAWORLD: • RNAisthefirstgeneticmaterial. • EssentiallifeprocessesevolvedaroundRNA. • RNAusedtoactasageneticmaterialaswellascatalyst. • ButRNAbeingcatalystwasreactiveandhenceunstable. • HenceDNAhasevolvedfromRNAwithchemical • modificationsthatmakeitmorestable. • DNAbeingdoublestrandedandhavingcomplementarystrandfurtherresistschangesbyevolvingaprocessofrepair.

  20. TypesofRNA: • InprokaryotestherearethreemajortypesofRNAs:mRNA(messenger),tRNA(transfer),andrRNA(ribosomal). • AllthreeRNAsarerequiredtosynthesizeproteininacell. • ThemRNAprovidesthetemplateandhavinggeneticinformationintheformofgeneticcode. • ThetRNAbringstheaminoacidsandreadthegeneticcode • ofmRNA. • TherRNAisthestructuralpartoftheribosomeandalsoascatalyticroleduringprocessoftranslation

  21. REPLICATION:THEPROCESS: • WatsonandCrickproposedaschemeforreplicationofDNA. • The Original statement that “It has not escaped our notice that the specificpairingwehavepostulatedimmediatelysuggestsapossiblecopyingmechanismforthegeneticmaterial(WatsonandCrick,1953) • Theschemesuggestedthatthetwostrandswouldseparateandactastemplateforthesynthesisofnewcomplementarystrands. • NewDNAmoleculemusthaveoneparentalstrandandonenewstrand. • ThisschemeofreplicationiscalledSemiconservativetypeofreplication.

  22. ExperimentalProofofsemiconservativenatureofreplication: • Itisnowprovedexperimentallythatreplicationissemiconservativetype. • ItwasfirstshowninEscherichiacoliandsubsequentlyinhigherorganism. • MathewMesselsonandFranklinStahlperformedthefollowingexperimentin1958.

  23. STEPSOFTHEEXPERIMENTS: • TheygrewE.coliin15NH4Clmediumformanygenerations.(15Nisheavynitrogennotradioactiveelement) • Theresultwasthat15NwasincorporatedintonewlysynthesizedDNAandothernitrogencontainingcompoundas • well. • ThisheavyDNAmoleculecouldbedistinguishedfromnormalDNAbycentrifugationinacesiumchloride(CsCl)densitygradient. • ThentheytransferredtheE.coliintoamediumwithnormal14NH4Clandletthemgrow.(E.colidividesin20minutes) • Theytooksamplesatdefinitetimeintervalsasthecellsmultiplied,and • extractedtheDNAthatremainedasdouble-strandedhelices. • VarioussampleswereseparatedindependentlyonCsClgradientstomeasurethedensitiesofDNA. • TheDNAthatwasextractedfromthecultureonegenerationafterthetransferfrom15Nto14Nmediumhadahybridorintermediatedensity. • DNAextractedfromthecultureafteranothergeneration(after40min.) • was composed of equal amount of this hybrid DNA and of ‘light ‘DNA.

  24. ExperimentbyTaylorandcolleagues: • UsedradioactivethymidinetodetectdistributionofnewlysynthesizedDNAinthechromosomes. • TheyperformedtheexperimentonViciafaba(fababeans)in1958. • TheyprovedthesemiconservativenatureofDNAreplication ineukaryotes

  25. ReplicationMachineryandEnzymes: • InalllivingcellssuchasE.colireplicationrequiresasetofenzymes. • E.colicompletesthereplicationofitsDNAinwithin38min. • Theaveragerateofpolymerizationhastobeapprox.2000bppersec. • Thepolymerizationprocessmustbeaccurate;anymistakeduringreplicationwouldresultintomutation. • Deoxyribonucleosidetriphosphates(dATP,dGTP,dCTP,dTTP)servedualpurposes: • Provideenergyforpolymerization. • Actsassubstratesforpolymerization. • ThereplicationprocessoccurswithinasmallopeningoftheDNAhelixcalledreplicationfork. • Theregionwhere,replicationforkformediscalledoriginofreplication. • Thereplicationforkisformedbyanenzymecalledhelicase.

  26. Twoseparatedstrandiscalledtemplatestrands. • MainenzymeisDNA-dependentDNApolymerase,sinceitusesaDNAtemplatetocatalyzethepolymerizationofdeoxyribonucleotides. • DNApolymerasecatalysespolymerizationonlyinonedirection • i.e.5’→3’. • On one strand (template with 3’→5’ polarity) the replication is • continuoushencecalledleadingstrand. • In another strand (template with 5’→3’ polarity) the polymerization • takesplaceintheformofshortfragmentcalledOkazakifragment. • TheshortfragmentsarejoinedbyDNAligase,hencecalledlaggingstrand. • IneukaryotesreplicationtakesplaceinS-phaseofcellcycle. • Afailureofcytokinesisafterreplicationresultsintopolyploidy.

  27. TRANSCRIPTION: • ‘The process of copying genetic information from one strand of the DNA into RNA is termed as transcription’. • Transcriptionvs.Replication: • PrincipleofcomplementaritygovernstheprocessoftranscriptionexceptAdenosineofDNAformsbasepairwiththeUracilinsteadofthymine.DuringreplicationAdeninepairswiththymineinsteadofuracil. • DuringreplicationoncestartedthewholeDNAisduplicated, • whereastranscriptiontakesplaceonlyasegmentofDNA. • Inreplicationbothstrandactsastemplate,whereasintranscriptiononlyonestrandisactsastemplatetosynthesizeRNA. • InreplicationDNAcopiedfromaDNA,whereasin • transcriptionRNAcopiedfromtheDNA.

  28. WhybothstrandsofDNAnotcopied duringtranscription: • IfbothstrandofDNAactsastemplate,theywouldtranslatedintotwoRNAofdifferentsequencesandinturniftheycodeforproteins,thesequenceofaminoacidsintheproteinwouldbedifferent.HenceonesegmentofDNAwouldbecodingfortwodifferentproteins. • ThetwoRNAmoleculesifproducedfromsimultaneouslywouldbecomplementarytoeachother,hencewillformdoublestrandedRNA.ThiswouldpreventRNAtranslationintoprotein.

  29. Transcriptionunit: • AtranscriptionunitinDNAconsistsofthreeregions: • Apromoter • Thestructuralgene • Aterminator. • DNAdependentRNApolymerasecatalysesthepolymerizationin • onlyone directionthat is5’→3’.

  30. Structuralgene: • TheDNAstrandhavingpolarity3’→5’ iscalledtemplatestrandfortranscription. • TheotherstrandofDNAhavingpolarity5’→3’ iscalledcodingstrand. • ThesequencesofnitrogenbaseintheRNAtranscribedfromthetemplatestrandaresameasthecodingstrandofDNAexcepthavingThymineinplaceofUracil. • Allthereferencepointdefiningatranscriptionunitismadewiththecodingstrandonly,notthetemplatestrand.

  31. PromoterandTerminatorpresentoneithersideofstructuralgene. • The promoter located towards 5’ end (upstream)ofthestructuralgene. • ItisashortsequenceofDNAthatprovidesbindingsiteforRNA • polymerase.(mostlyTATA,CommonlycalledTATAbox) • Presenceofthepromoterdefinesthetemplateandcodingstrands. • Ifthepositionofpromoterischangedwithterminatorthedefinitionofcodingandtemplatestrandwillbereversed. • Terminator: • Theterminatorlocated towards 3’ end (downstream)ofcodingstrand. • Itterminatestheprocessoftranscription. • ItisalsoashortsegmentofDNAwhichrecognizestheterminationfactor.(ρ-factor)

  32. Transcriptionunitandthegene: • Geneisdefinedasthefunctionalunitofinheritance. • GenesarelocatedontheDNA. • TheDNAsequencecodingfortRNAandrRNAmoleculealsodefineagene. • Cistron:asegmentofDNA(structuralgene)codingforapolypeptide. • Monocistronic:mostofeukaryoticstructuralgenecodesforsinglepolypeptide. • Polycistronic:Mostprokaryoticstructuralgenecodeformorethanonepolypeptides. • Ineukaryotesthemonocistronicstructuralgenshaveinterruptedcoding • sequences,thegenesaresaidtobesplitgene: • ThecodingsequencesorexpressedsequencesarecalledExons. • ExonsareinterruptedbyIntrons. • Exonsaresaidtobethosesequencesthatappearinmatureorprocessed • mRNA. • IntronsneverappearinmatureofprocessedmRNA.Theyaresplicedout

  33. Processoftranscription:prokaryotes. • ThereisasingleDNAdependentRNApolymerasethatcatalysestranscriptionorsynthesisofallthreetypesofRNAsinprokaryotes. • Theprocessoftranscriptioncompletedinthreesteps: • Initiation: • RNApolymerasebindstothespecificsiteofDNAcalledpromoter. • PromoteroftheDNAisrecognizedbyinitiationfactororsigma(σ). • RNApolymerasealongwithinitiationfactorbindstothepromoter. • Elongation: • RNApolymeraseunzippedtheDNAdoublehelixandformsanopenloop. • ItusesribonucleosidetriphosphatesassubstrateandpolymerizesinaDNAtemplatefollowingtheruleofcomplementarity. • OnlyashortstretchofpolymerizedRNAremainsbindswiththeenzyme. • Theprocessofpolymerizationcontinuedtilltheenzymereachestheterminatorgene. • Termination: • RNApolymeraserecognizestheterminatorgenebyatermination-factorcalledrho(ρ) • factor. • TheRNApolymeraseseparatedfromtheDNAandalsothetranscribedRNA.

  34. Additionalcomplexitiesineukaryotes: • TherearethreedifferenttypesofRNApolymerasesinthenucleus: • RNApolymeraseItranscribesrRNA(28S,18S,and5.8S) • RNApolymeraseIItranscribesheterogeneousnuclearRNA(hnRNA). • RNApolymeraseIIItranscribestRNA,5srRNAandsnRNA. • Posttranscriptionalprocessing:(occursinsidethenucleus) • (a)Splicing: • Theprimarytranscript(hnRNA)containbothexonsandintronsandrequiredtobeprocessedbeforetranslationallyactive(mRNA). • Theintronsareremovedandexonsarejoinedinadefinedorder. • ThisprocessiscatalyzedbySnRNP,intronsremovedasspliceosome. • (b)Capping:anunusualnucleotidecalledmethylguanosinetriphosphateis added to the 5’ end of hnRNA. • (c)Tailing:Adenylateresidues(200-300) are added at 3’ end of hnRNAinatemplateindependentmanner. • TheprocessedhnRNAisnowcalledmRNAandtransportedoutofthe • nucleusfortranslation.

  35. GENETICCODE: • Contributiontodiscovery: • Theprocessofreplicationandtranscriptionbasedoncomplementarity. • Theprocessoftranslationisthetransferofgeneticinformationformapolymerofnucleotidestoapolymerofaminoacids.Thereisnocomplementarityexistbetweennucleotidesandaminoacids. • Ifthereischangeinthenucleicacid(geneticmaterial)thereischangeinaminoacidsinproteins. • Theremustbeageneticcodethatcoulddirectthesequenceofaminoacidsinproteinsduringtranslation. • GeorgeGamowproposedthecodeshouldbecombinationofbases,hesuggestedthatin • ordertocodeforallthe20aminoacids,thecodeshouldbemadeupofthreenucleotides. • HarGovindKhoranaenablesinstrumentalsynthesizingRNAmoleculeswithdesiredcombinationsofbases(homopolymerandcopolymers). • Marshall Nirenberg’s cell– freesystemforproteinsynthesisfinallyhelpedthediscoveryofgeneticcode. • SeveroOchoaenzyme(polynucleotidephosphorylase)wasalsohelpfulinpolymerizingRNAwithdesiredsequencesinatemplateindependentmanner(enzymaticsynthesisofRNA)

  36. Salientfeaturesofgeneticcode: • Thecodonistriplet.Threenitrogenbasesequencesconstituteonecodon. • Thereare64codon,61codesforaminoacidsand3codonsarestopcodon. • Onecodoncodesforonlyoneaminoacid,henceitisunambiguous. • Degeneracy:someaminoacidsarecodedbymorethanonecodon. • Commaless:thecodonisreadinmRNAinacontinuousfashion.Thereisnopunctuation. • Universal:FrombacteriatohumanUUUcodesforphenylalanine. • Initiationcodon:AUGisthefirstcodonofallmRNA.Andalsoitcodesformethionine(met),hencehasdualfunction. • Non-overlapping:Thegeneticcodereadslinearly • Direction:thecodeonlyreadin5’ → 3’ direction. • Anticodon:EachcodonhasacomplementaryanticodonontRNA. • Non-sensecodon:UAA,GUA,andUAGdonotcodeforaminoacidandhas • noanticodononthetRNA.

  37. MutationandGeneticcode: • RelationshipbetweenDNAandgenesarebestunderstoodbymutation. • Pointmutation: • Itoccursduetoreplacementnitrogenbasewithinthegene. • Itonlyaffectsthechangeofparticularaminoacid. • Bestunderstoodthecauseofsicklecellanemia. • Frameshiftmutation: • Itoccursduetoinsertionordeletionofoneormorenitrogenbasesinthegene. • Thereischangeinwholesequenceofaminoacidfromthepointofinsertionordeletion. • Bestunderstoodinβ-thalasemia.

  38. tRNA-theAdaptormolecule: • ThetRNAiscalledsRNA(solubleRNA) • Itactsasanadaptermolecule. • tRNAhasananticodonloopthatbase • complementarytothecodon. • Ithasanaminoacidaccepterendtowhichitbindswithaminoacid. • EachtRNAbindwithspecificaminoacidi.e61typesoftRNAfound. • OnespecifictRNAwithanticodonUACcalledinitiatortRNA. • ThereisnotRNAforstopcodons.(UAA,UGA,UAG) • Thesecondarystructureislikeclover-leaf. • TheactualstructureoftRNAiscompact,looks • like inverted ‘L’.

  39. TRANSLATION: • Itreferstopolymerizationofaminoacidstoformapolypeptide. • Thenumberandsequenceofaminoacidsaredefinedbythe • sequenceofbasesinthemRNA. • Theaminoacidsarejoinedbypeptidebond. • AminoacidsareactivatedinthepresenceofATPandlinkedtotheirspecifictRNAiscalledchargingof • tRNAoraminoacylationoftRNA. • Ribosomeisthecellularfactoryforproteinsynthesis. • RibosomeconsistsofstructuralrRNAand80differentproteins. • Ininactivestateribosome(70S)presentintwosubunits:- • Alargesubunit50S. • Asmallsubunit30S.

  40. Initiation: • TheprocessoftranslationorproteinsynthesisbeginswithattachmentofmRNAwithsmallsubunitofribosome. • TheribosomebindstothemRNAatthestartcodon(AUG). • AUGisrecognizedbytheinitiatortRNA. • Elongation: • Largersubunitattachedwiththeinitiationcomplex. • Larger subunit has two site ‘A’ site and ‘P’ site. • InitiatortRNAaccommodated in ‘P’ site of large subunit, the subsequent • amino-acyl-tRNAenters into the ‘A’ site. • ThesubsubsequenttRNAselectedaccordingtothecodonofthemRNA. • CodonofmRNAandanticodonoftRNAarecomplementarytoeachother. • Formation of peptide bond between two amino acids of ‘P’ and‘A’ site, • catalyzedbyribozyme,(23SrRNAinbacteria) • ThemovesfromcodontocodonalongthemRNAcalledtranslocation.

  41. Termination: • Elongationcontinuesuntilastopcodonarrives at ‘P’ site. • ThereisnotRNAforstopcodon. • Areleasefactorbindstothestopcodon. • Furthershiftingofribosomeleadstoseparationofpolypeptide. • AnmRNAalsohassomeadditionalsequencesthatarenot • translatedcalleduntranslatedregions(UTR).

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