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Ch 18

Ch 18. Gene Regulation. Consider: A multicellular organism (Pliny) Do each of his cells have the same genes?. Yes, with an exception: germ cells are haploid. Do each of his cells express the same genes?. Examples to support your claim.

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Ch 18

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  1. Ch 18 Gene Regulation

  2. Consider: A multicellular organism (Pliny) Do each of his cells have the same genes? Yes, with an exception: germ cells are haploid Do each of his cells express the same genes? Examples to support your claim Do cells express the same genes at the same time and at the same level? Examples

  3. How are Genes Regulated?

  4. How might genes be regulated? Regulation of enzyme production Regulation of enzyme activity Precursor Feedback inhibition LE 18-20 Enzyme 1 Gene 1 Gene 2 Enzyme 2 Regulation of gene expression Gene 3 Enzyme 3 Enzyme 4 Gene 4 Gene 5 Enzyme 5 Tryptophan Give an example of how proteins are regulated.

  5. Bacterial Operons: Gene Regulation Model Genes grouped into operons - Promoter to help initiate transcription - Operator: DNA sequence acts as on-off switch - Genes encode metabolic enzymes Operon regulated by repressors and/or activators in response to environment.

  6. DNA trpE trpB trpA trpC trpD 5’ mRNA proteins E D C B A Trp Operon Group of genes that encode enzymes for tryptophan synthesis (an amino acid) RNA polymerase

  7. Trp Operon ON most of the time TrpR gene also ON: makes inactive repressor protein LE 18-21a trp operon Promoter Promoter Genes of operon trpE DNA trpB trpA trpC trpD trpR Operator Stop codon RNA polymerase Regulatory gene Start codon 3¢ mRNA 5¢ mRNA 5¢ D B E C A Inactive repressor Protein Polypeptides that make up enzymes for tryptophan synthesis Tryptophan absent, repressor inactive, operon on If the cell is not synthesizing much protein (e.g. low nutrients), will it need to continue to make trp? How to shut off the trp operon?

  8. DNA LE 18-21b_1 mRNA Active repressor Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off

  9. operator TrpR DNA LE 18-21b_2 No RNA made mRNA conformational Active repressor Protein change Tryptophan (corepressor) Tryptophan present, repressor active, operon off

  10. Is the trp operon repressible or inducible? ON unless excess trp binds and activates repressor protein-> Active TrpR binds operator Blocks transcription Trp operon OFF

  11. Do inducible operons exist? (usually OFF; need signal to turn ON Lac operon: group of genes involved in catabolism of lactose

  12. LE 18-22b Lac operon DNA lacl lacY lacA lacZ RNA polymerase 3¢ mRNA mRNA 5¢ 5¢ Transacetylase Permease -Galactosidase Protein Inactive repressor Allolactose (inducer) Lactose present, repressor inactive, operon on Enzymes facilitate lactose import and breakdown for cellular energy

  13. Let’s assume bacteria prefer glucose to lactose as a carbon source. If glucose is available in the surroundings, does it make sense for the lac operon to be ON? If it’s a waste of energy then how do bacteria repress (turn OFF) the Lac operon?

  14. Promoter Regulatory gene LE 18-22a Operator lacl lacZ DNA No RNA made 3¢ mRNA RNA polymerase 5¢ Active repressor Protein Lactose absent, repressor active, operon off

  15. Inducible gene products • usually function in catabolic pathways (lactose metabolism) • Repressible gene products • -products usually function in anabolic pathways • (trp synthesis) Trp and lac operons (similarities): - Negatively controlled - Blocked by a repressor

  16. Positive Gene Regulation • Activator protein turns on Lac operon • catabolite activator protein (CAP) Glucose high cAMP low Glucose low cAMP high CAP-cAMP binds Lac promoter and induces transcription When would this occur, when glucose is high or low? Low

  17. Promoter LE 18-23a DNA lacl lacZ RNA polymerase can bind and transcribe Operator CAP-binding site Active CAP cAMP Inactive lac repressor Inactive CAP Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized

  18. Promoter LE 18-23b DNA lacl lacZ CAP-binding site Operator RNA polymerase can’t bind Inactive CAP Inactive lac repressor Lactose present, glucose present (cAMP level low): little lac mRNA synthesized

  19. Oh gee, am I supposed to induce or repress? Do you have questions too?

  20. DNA LE 18-21b_1 mRNA Active repressor Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off

  21. Promoter LE 18-23a DNA lacl lacZ RNA polymerase can bind and transcribe Operator CAP-binding site Active CAP cAMP Inactive lac repressor Inactive CAP Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized

  22. A mutation arises in the TrpR gene that inactivates the bindingsite for the co-repressor. How will the mutant phenotype differ from wildtype? If the operator is deleted from the lac operon predict how gene expression will be altered relative to wildtype under the following conditions a. lactose present, glucose absent b. lactose absent, glucose present c. lactose present, glucose present d. lactose absent, glucose absent

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