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Page 1: OPERONS-REPRESSIBLE AND INDUCEIBLEstecklescience.weebly.com/uploads/2/3/2/2/23227308/operon_notes.pdfAn operon is a region in prokaryotic DNA that codes for several genes at the same

OPERONS-REPRESSIBLEANDINDUCEIBLE

AnoperonisaregioninprokaryoticDNAthatcodesforseveralgenesatthesametime.Theoperoniscomposedofapromoterregionthatcontainstheoperatorandseveralgeneslinedupnexttoeachother.Eachgenehasastartandstopcodon,butsincetheyaresoclose,theenzymesformoneafteranother.ThemRNAwillbetranscribediftheRNAPolymerasecangetpasttheoperator.Ifthereisarepressorproteinintheoperatorthantheenzymesarenotsynthesized.IftherepressorproteinisNOTintheoperator,thenthemRNAwillbetranscribed.Therepressorproteiniscodedupstreamfromtheoperon,soitisthereifitneedstobe.

Intheaboveexample,willtheRNAPolymerasetranscribethegenes?Explain.Intheaboveexample,willRNAPolymerasetranscribethegenes?Explain.

GeneforRNAOperatorGenesRepressorPolymeraseProtein(Upstream)

AnoperonisaregioninprokaryoticDNAthatcodesforseveralgenesatthesametime.Theoperoniscomposedofapromoterregionthatcontainstheoperatorandseveralgeneslinedupnexttoeachother.Eachgenehasastartandstopcodon,butsincetheyaresoclose,theenzymesformoneafteranother.ThemRNAwillbetranscribediftheRNAPolymerasecangetpasttheoperator.Ifthereisarepressorproteinintheoperatorthantheenzymesarenotsynthesized.Therepressorproteinistranscribedupstreamfromtheoperon,soitisthereifitneedstobe.

Wewilllearnabout2operonsbutthereareseveralotherswewillnottalkabout.

AnoperonisaregioninprokaryoticDNAthatcodesforseveralgenesatthesametime.Theoperoniscomposedofapromoterregionthatcontainstheoperatorandseveralgeneslinedupnexttoeachother.Eachgenehasastartandstopcodon,butsincetheyaresoclose,theenzymesformoneafteranother.ThemRNAwillbetranscribediftheRNAPolymerasecangetpasttheoperator.Ifthereisarepressorproteinintheoperatorthantheenzymesarenotsynthesized.Therepressorproteinistranscribedupstreamfromtheoperon,soitisthereifitneedstobe.

Wewilllearnabout2operonsbutthereareseveralotherswewillnottalkabout.

RepressorProtein

Page 2: OPERONS-REPRESSIBLE AND INDUCEIBLEstecklescience.weebly.com/uploads/2/3/2/2/23227308/operon_notes.pdfAn operon is a region in prokaryotic DNA that codes for several genes at the same

Wewilllearnabout2operonsbutthereareseveralotherswewillnottalkabout.ThefirstiscalledtheTRPOperon-REPRESSIBLE.Prokaryotesneedtheaminoacidtryptophan(Trp)inordertobuildproteinsandenzymes.IftheTrpisobtainable,theprokaryoteswilluptakeitandbuildallofthenecessaryproteinsandenzymes.IfTrpisnotpresent,theprokaryotesneedtosynthesizeitthemselves.Theycanachievethisthroughaseriesofenzymaticreactions.RNAPolymerasewillbindtothepromoterregionandtranscribethemRNAneededforthenecessaryenzymes.TheywillcontinuetosynthesizetheTrpuntiltheyhaveenough.Oncethatamountisreached,allofthegenesthatcodefortheenzymesareturnedofftogether.ThishappenswhenTrpbindsallostericallytotherepressorproteinandaltertheactivesite.TherepressorproteinwillnowfitintheoperatorblockingtheRNAPolymerasefromtranscribingmRNA.Thisiscalled“REPRESSINGTHEGENES”IfthereisnoTrpinthecell,therepressorproteinisproducedbutWILLNOTFITintheoperatorsite.TheRNAPolymerasewillmovedownandtranscribemRNAthatwillcodeforthe5enzymesneededtosynthesizemoreTrp.IfTrpispresent,itwillallostericallybindtotherepressorproteinandchangeitsshape.TherepressorproteinwillNOWFITINTOtheoperatorandblocktheRNAPolymerase.

Page 3: OPERONS-REPRESSIBLE AND INDUCEIBLEstecklescience.weebly.com/uploads/2/3/2/2/23227308/operon_notes.pdfAn operon is a region in prokaryotic DNA that codes for several genes at the same

ThesecondiscalledtheLacOperon-INDUCEIBLELactoseisadisaccharidecomposedofglucoseandgalactose.ProkayoteslikeEcoliliveinourgutandareexposedtolactosewhenweeatthingslikecheese.Thesebacterianeedglucose.Inordertogettheglucosefromthelactose,theyneedaseriesofenzymesthatcanbreakdownthelactose.Interestingly,theEcoliwillonlyproducetheseenzymeswhenlactose,oritsisomerallolactosearepresent.Veryeconomical.IfthereisNOTlactoseorallolactosearound,arepressorproteinisintheoperatorsiteblockingtheRNAPolymerasefrommakingenzymesneededtobreakdownlactose.Ifallolactoseisaround,itwillallostericallybindtotherepressorproteinandchangetheactivesite.ItwillNOLONGERFITintotheoperatorregion.Iftherepressorproteinisnotthere,theRNAPolymerasecantranscribetheenzymesneededtobreakdowntheallolactoseintotheglucoseitneeds.Thisiscalled“INDUCINGTHEGENES”

AnoperonisaregioninprokaryoticDNAthatcodesforseveralgenesatthesametime.Theoperoniscomposedofapromoterregionthatcontainstheoperatorandseveralgeneslinedupnexttoeachother.Eachgenehasastartandstopcodon,butsincetheyaresoclose,theenzymesformoneafteranother.ThemRNAwillbetranscribediftheRNAPolymerasecangetpasttheoperator.Ifthereisarepressorproteinintheoperatorthantheenzymesarenotsynthesized.Therepressorproteinistranscribedupstreamfromtheoperon,soitisthereifitneedstobe.

Wewilllearnabout2operonsbutthereareseveralotherswewillnottalkabout.

RepressorProtein

Allolactosebindsandalterstherepressorprotein.Itcannolongerbondtotheoperatorsite.

RNAPolymerasecantranscribethemRNAthatwillgoontocreatetheenzymesneededtodigestthelactose.

Page 4: OPERONS-REPRESSIBLE AND INDUCEIBLEstecklescience.weebly.com/uploads/2/3/2/2/23227308/operon_notes.pdfAn operon is a region in prokaryotic DNA that codes for several genes at the same

TryptophanSynthesisRepressible

LactoseDigestionInducible


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