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  • 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
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OPERONS-REPRESSIBLE AND INDUCEIBLE An operon is a region in prokaryotic DNA that codes for several genes at the same time. The operon is composed of a promoter region that contains the operator and several genes lined up next to each other. Each gene has a start and stop codon, but since they are so close, the enzymes form one after another. The mRNA will be transcribed if the RNA Polymerase can get past the operator. If there is a repressor protein in the operator than the enzymes are not synthesized. If the repressor protein is NOT in the operator, then the mRNA will be transcribed. The repressor protein is coded upstream from the operon, so it is there if it needs to be. In the above example, will the RNA Polymerase transcribe the genes? Explain. In the above example, will RNA Polymerase transcribe the genes? Explain. Gene for RNA Operator Genes Repressor Polymerase Protein (Upstream) Repressor Protein

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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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