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OPERONS * Indicated slides borrowed from: Kim Foglia Image from: http://www.eldontaylor.com/blog/wp-content/uploads/bigstock-On-Off-Switch-Shows-Energy-Sup-519917201-300x300.jpg Slide by Kim Foglia modified PROKARYOTES 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) When gene is turned ON: Polymerase binds promoter mRNA is made (transcription) mRNA is turned into proteins by ribosomes DNA RNA TATA polymerase 1 2 3 4 mRNA promoter operator enzyme1 gene1 enzyme2 gene2 enzyme3 gene3 enzyme4 gene4 gene1 gene2 gene3 gene4 gene1 gene2 gene3 gene4 RNA polymerase TATA promoter repressor operator Slide by Kim Foglia modified DNA When gene is turned OFF Repressor binds operator RNA polymerase can’t transcribe gene • 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 DIFFERENCE BETWEEN THE TWO IS WHAT MAKES REPRESSOR ACTIVE OR INACTIVE! REPRESSOR is usually INACTIVE Cells need to make tryptophan REPRESSOR is usually ACTIVE Cells don’t need to make lactose digesting enzymes if no lactose present If tryptophan is available, don’t need to make it If lactose is available, need to digest it Tryptophan ACTIVATES repressor TURNS GENE OFF Lactose INACTIVATES repressor TURNS GENE ON 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 trp repressor TATA gene1 gene2 gene3 gene4 DNA trp repressor promoter operator trp repressor protein = INACTIVE trp trp trp trp trp trp conformational change in repressor protein makes it ACTIVE! SLIDE FROM: Kim Foglia trp trp repressor tryptophan trp tryptophan – repressor protein Complex = ACTIVE SLIDE FROM: Kim Foglia Inducible lac operon codes for enzymes for lactose digestion lac lac lac Digestive pathway model lac lac lac When lactose is present, binds to lac repressor protein & triggers repressor to release DNA lac RNA lac repressor TATA polymerase gene1 gene2 gene3 gene4 1 2 3 4 enzyme1 enzyme2 enzyme3 enzyme4 mRNA promoter – induces transcription operator repressor lac conformational change in repressor protein makes it INACTIVE! lac repressor DNA repressor protein =ACTIVE lactose lactose – repressor protein Complex = INACTIVE Using REPRESSORS to TURN OFF genes = NEGATIVE CONTROL REPRESSIBLE and INDUCIBLE operons ARE BOTH TYPES OF NEGATIVE CONTROL ! ! ! ! http://3.bp.blogspot.com/-tArOpdcSKlQ/VDyOFD4hJ8I/AAAAAAAABw8/rEaxBPVNf-g/s1600/Electical%2BLight%2Bswitch...cartoon.jpeg Using ENHANCER REGIONS and ACTIVATORS to TURN ON genes = POSITIVE CONTROL http://cdn.xl.thumbs.canstockphoto.com/canstock8046278.jpg POSTIVE CONTROL of Lactose operon What happens when concentration of glucose is LOW? LOW GLUCOSE → HIGH cAMP GLUCOSE IS FOOD OF CHOICE cAMP makes CAP active; starts transcription Image from:http://image.slidesharecdn.com/18regulationofgeneexpression-130613012903-phpapp02/95/18-regulation-of-gene-expression-15-638.jpg?cb=1371087103 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:http://image.slidesharecdn.com/18regulationofgeneexpression-130613012903-phpapp02/95/18-regulation-of-gene-expression-15-638.jpg?cb=1371087103 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 ACTIVATORS BIND TO ENHANCER region Image from: https://cellularphysiology.wikispaces.com/file/view/18_09ActivatorAction_3-L.jpg/465203668/18_09ActivatorAction_3-L.jpg MAKE A CONNECTION WHERE DO ACTIVATOR proteins come from? Images from http://images.slideplayer.com/1/273672/slides/slide_49.jpg http://image.slidesharecdn.com/45lecturepresentation-101204060035-phpapp01/95/chapter-45-textbook-presentation-33-728.jpg?cb=1291442560 ACTIVATED ENHANCERS attach to TRANSCRIPTION FACTORS and FOLD DNA back onto itself Image from: https://cellularphysiology.wikispaces.com/file/view/18_09ActivatorAction_3-L.jpg/465203668/18_09ActivatorAction_3-L.jpg RNA POLYMERASE attaches to TRANSCRIPTION INTIATION COMPLEX to start transcription (GENE is turned ON!) Image from: https://cellularphysiology.wikispaces.com/file/view/18_09ActivatorAction_3-L.jpg/465203668/18_09ActivatorAction_3-L.jpg Different genes have different enhancer regions and are controlled by different activators Image from: http://image.slidesharecdn.com/18regulationofgeneexpression-130613012903-phpapp02/95/18-regulation-of-gene-expression-35-638.jpg?cb=1371087103 DIFFERENT ENHANCER sequences can TURN ON a gene in DIFFERENT KINDS of cells at DIFFERENT TIMES. Image from: https://cellularphysiology.wikispaces.com/file/view/18_09ActivatorAction_3-L.jpg/465203668/18_09ActivatorAction_3-L.jpg