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Download Co-dominance • WT protein will make WT phenotype. Mutant gene
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Transcript
Co-dominance • WTproteinwillmakeWTphenotype.Mutantgeneproductwillgenerateamutantgenotype. • Heterozygotewillhaveahalf-wayphenotypebetweenWTandmutant(co-dominance).Bothgeneproductsexerttheir effectsonthecell&theheterozygoteexhibitsphenotypesofbothhomozygotes. • Forexample:Bloodgroups:Antigens(geneproduct)presentonRBCs;Threealleles. Incompletedominance Wherethemutantalleleexertssomeeffectbutnotequallybalancedwiththewildtypeallele.Thismutantgeneproductmight havesomemutantphenotypecreatingactivitybutitisnotasequallystrongasthewildtype. PenetranceandExpressivity • Variablepenetrance:AllorganismshavethesamegenotypeyetonlySOME(i.e.half)showthephenotype. • Variableexpressivity:AllorganismshavethesamegenotypebutEXPRESSphenotypetodifferentEXTENTS. • Itisallaboutthegenesbeingexpressedandwhattheallelesdoandwhatthegeneproductsendupdoing. • Canhavebothvariablepenetranceandvariableexpressivitycomingoutbytheonealleleormutant. LifeCyclesofBacteriophageI(L2) BacteriophageGenetics • Bacteriophage(phage)arevirusesthatinfectbacteria. • PhagethatinfectEscherichiacolicanbeeithervirulentortemperate. o Virulentphageincludethe'T'seriesphage(T1,T2,T3,T4etc.) o Temperatephageinclude:phageλ&φ80 • Differentphagemaycarrydifferentformsofgeneticmaterial:e.g.dsDNAinT-seriesandλ,ssDNA,RNA.Canbecircular (covalentlyclosedcircles)orlinear. LyticCycle:Phageλ • Adsorptionofphagetohostcell.λphagerecognises&attachestothehost(E.coli). • Insertionofphagenucleicacid(i.e.DNA)intohost.Fromhead--->tail&intoE.coli.Emptyphageshelloutside. • CircularisationofphageDNA(linearDNAintheheadofphagecircularisesinsideE.coli). • Recruitmentofhostmachinery.PhageDNAreplicates&isexpressed(someuponentryintoE.colitotakeoverE.coli machinerytotranslate&transcribegenes,e.g.RNApolymerase,anddirectittophageDNAreplication). • Productionofphagecomponents.Allgenesthatmakeproteinstomakenewλareexpressed(E.coligenesaren't). o PhageproducesenzymestobreakdownE.coli(host)&itsDNAthatdisintegrates&weakens. • Assemblyofphagefromcomponents. • PackagingofDNAintophage(heads). • Lysis:Releaseofprogenyphage(>100).EventuallyE.coliisweakenoughthatitbursts&phageprogenycomeout. Bacteriophage • Plaque:Theareaona"lawn"ofbacteria(onagarplate)whereallthecellshavebeenlysed(brokenopen). o Clearplaques:AlmostallE.colihavebeenlysed(phagearelytic:killallbacteriaincontactwith). o Turbidplaques:SomeE.colihavebeenlysed(living&growingE.coli).Somephagearetemperatethatundergo lifecyclesthatcanco-existwithorkillbacteria.Therearelysogens(E.coliwithintegratedλDNA).Thephageisnot expressedinitsindependentform. • Eachplaquederivesfromaninitialsinglephageinfection. 6 • Phagewithineachplaque(>10 )areallclones(geneticallyidentical).Allphageareclones(exceptspont.mutation). • Mosttemperatephageinfectionsfollowthelyticcycletoproduceclearplaquesonabacteriallawn. • ThelysogenicstatecanbereversedwheretheintegratedphageDNAcomesout&triggerlyticcycle. LysogenicCyclee.g.λ • Adsorptionofphagetohostcell.λ infectsthecell,detectsproteinsonthesurfaceofE.coli,attachestocell. • Insertionofphagenucleiacid(DNA)intohost.Atthemolecularlevel'decide'whethertogodownlyticpathway,make progeny&killtheE.coli,ortogodownlysogenicpathway&co-existwiththeE.coli. • CircularisationofphageDNA. • Expressionofc1repressor.The'decision'isbasedonhowmuchexpressionofc1geneonλgenome.c1geneencodes transcriptionfactor(proteinsthatrecogniseaspecificDNAsequence,bind&eitheractiveorrepressgeneexpression). Thec1genemakesc1proteinwhichhasaspecificbindingsiteontheλ chromosome. • Repressionofgenesinvolvedinthelyticcycle.Boundc1repressorblocksexpressionofgeneseithersideofitontheλ chromosome,thusblockingthelyticcycle(stopsexpressionofgenestomakemorephage&lysecell).Itisnow undergoingthelysogenicpathway. • Productionofintegrasefromλ intgene.Blockageoflyticgenesalsoleadstoexpressionoflysogenicgenes:int. (integrase),arecombinationenzymethataidscrossoveroftwospecificsequencesofDNA(notgenes)&operateson thirdgeneticelementonλ chromosome:attP(attachmentphage);onE.colichromosomethereisasimilarsequence: attB(attachmentbacterial).Thesethreesequencesarehomologous. • Recombinationbetweenbacterial&phageDNA.int.canrecruitsomeofE.colimachinery&thecomplexcanform:int. canbind,formasinglecrossoverbetweenthetwoattsites,joiningλ toE.colichromosome. • (Stably)integratedphageDNAknownasaprophage.ThisE.coli(lysogen)cancontinuetodivide(thisisseeninturbid plaques).Integrationoftemperatephageintothebacterialchromosomeoccursatspecificsites.
