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OPERONS * Indicated slides borrowed from: Kim Foglia
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Slide by Kim Foglia modified
PROKARYOTES Slide by Kim Foglia modified Genes with related functions grouped together = Operon example: all enzymes in a metabolic pathway promoter = RNA polymerase binding site single promoter controls transcription of all genes in operon transcribed as one unit & a single mRNA is made operator = DNA binding site of repressor protein regulatory genes = code for regulatory proteins (EX: repressors)
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Slide by Kim Foglia modified
When gene is turned ON: Polymerase binds promoter mRNA is made (transcription) mRNA is turned into proteins by ribosomes RNA polymerase DNA TATA operator 1 2 3 4 promoter mRNA enzyme1 enzyme2 enzyme3 enzyme4 gene1 gene2 gene3 gene4 RNA polymerase gene1 gene2 gene3 gene4 repressor TATA gene1 gene2 gene3 gene4 DNA operator promoter When gene is turned OFF Repressor binds operator RNA polymerase can’t transcribe gene Slide by Kim Foglia modified
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INDUCIBLE OPERONS Usually OFF/repressor INACTIVE
REPRESSABLE OPERONS Usually ON/repressor usually ACTIVE Can be turned off (repressed) Genes for enzymes that make product always needed EX: trp operon makes enzymes used in essential amino acid synthesis INDUCIBLE OPERONS Usually OFF/repressor INACTIVE Can be turned on (induced) Genes for enzymes that are only needed sometimes EX: lac operon makes enzymes used in lactose digestion
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DIFFERENCE BETWEEN THE TWO IS WHAT MAKES REPRESSOR ACTIVE OR INACTIVE!
REPRESSOR is usually INACTIVE Cells need to make tryptophan If tryptophan is available, don’t need to make it Tryptophan ACTIVATES repressor TURNS GENE OFF REPRESSOR is usually ACTIVE Cells don’t need to make lactose digesting enzymes if no lactose present If lactose is available, need to digest it Lactose INACTIVATES repressor TURNS GENE ON
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Repressible trp operon: Codes for enzymes that synthesize tryptophan
Synthesis pathway model When excess tryptophan is present, it binds to trp repressor protein & triggers repressor to bind to DNA blocks (represses) transcription RNA polymerase repressor trp TATA gene1 gene2 gene3 gene4 DNA promoter operator trp trp repressor repressor protein = INACTIVE trp trp trp trp trp trp conformational change in repressor protein makes it ACTIVE! trp tryptophan trp repressor tryptophan – repressor protein Complex = ACTIVE trp SLIDE FROM: Kim Foglia
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Inducible lac operon codes for enzymes for lactose digestion
SLIDE FROM: Kim Foglia Inducible lac operon codes for enzymes for lactose digestion lac Digestive pathway model When lactose is present, binds to lac repressor protein & triggers repressor to release DNA induces transcription RNA polymerase repressor TATA lac gene1 gene2 gene3 gene4 DNA promoter operator 1 2 3 4 mRNA enzyme1 enzyme2 enzyme3 enzyme4 repressor repressor protein =ACTIVE lactose lac conformational change in repressor protein makes it INACTIVE! repressor lactose – repressor protein Complex = INACTIVE lac
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Using REPRESSORS to TURN OFF genes =
NEGATIVE CONTROL REPRESSIBLE and INDUCIBLE operons ARE BOTH TYPES OF NEGATIVE CONTROL ! ! ! !
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Using ENHANCER REGIONS and ACTIVATORS to TURN ON genes =
POSITIVE CONTROL
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POSTIVE CONTROL of Lactose operon
What happens when concentration of glucose is LOW? LOW GLUCOSE → HIGH cAMP cAMP makes CAP active; starts transcription GLUCOSE IS FOOD OF CHOICE Image from:
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POSTIVE CONTROL of Lactose operon
What happens if BOTH GLUCOSE AND LACTOSE are present? GLUCOSE IS FOOD OF CHOICE Repressor is inactive but CAP activator is not activated Gene is unable to turn on at significant rate Image from:
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WHAT ABOUT EUKARYOTES Genes for proteins that work together in a pathway are spread out on different chromosomes (NO OPERONS) Separate control sequences for each gene BOTH POSTIVE (enhancers) and NEGATIVE (repressors) control
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ACTIVATORS BIND TO ENHANCER region
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WHERE DO ACTIVATOR proteins come from?
MAKE A CONNECTION WHERE DO ACTIVATOR proteins come from? Images from
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ACTIVATED ENHANCERS attach to TRANSCRIPTION FACTORS and FOLD DNA back onto itself
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RNA POLYMERASE attaches to TRANSCRIPTION INTIATION COMPLEX to start transcription (GENE is turned ON!) Image from:
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Different genes have different enhancer regions and are controlled by different activators
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DIFFERENT ENHANCER sequences can TURN ON a gene in DIFFERENT KINDS of cells at DIFFERENT TIMES.
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