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Questions and Answers
What type of enzymes are expressed continuously at a fixed rate?
In the presence of lactose, what happens to the Lac operon?
What molecule does CAP (catabolite activator protein) bind to in order to induce changes?
What effect does the repressor have on gene expression in the Lac operon?
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Which operon is an example of positive regulation?
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In which scenario is the Tryptophan operon transcribed?
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What is the role of the negative regulator in the Tryptophan operon?
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Which factor is required for full expression of the Lac operon?
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What distinguishes an inducible operon like the Lac operon from a repressible operon like the Tryptophan operon?
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What is the overall effect of glucose presence on the Lac operon functioning?
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Study Notes
Bacterial Gene Expression
- Constitutive enzymes are expressed at a fixed rate.
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Other enzymes are expressed as required
- Repressible Enzymes: Production is repressed by a molecule
- Inducible Enzymes: Production is induced by a molecule
Operon Examples
- Lactose Operon (Lac Operon): Inducible operon involved in lactose metabolism.
- Tryptophan Operon (Trp Operon): Repressible operon involved in tryptophan biosynthesis.
Operon Structure
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Regulator Protein: A DNA binding protein that binds to the operator site.
- Blocks RNA polymerase from transcribing genes in the operon.
Lac Operon
- Inducible System: The lac operon is only transcribed when lactose is present.
- Repressor Protein: Binds to the operator site in the absence of lactose, preventing transcription.
- Inducer: Lactose acts as the inducer, binding to the repressor and causing it to detach from the operator. This allows RNA polymerase to transcribe the genes.
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Positive Regulation: The Lac operon also features positive regulation by cAMP Receptor Protein (CRP).
- CAP/CRP: Activated by cAMP, which builds up in the absence of glucose.
- Function: Binds to the lac operon promoter and enhances RNA polymerase binding, further increasing expression.
Carbohydrate Metabolism Control
- Global Control: The lac operon is part of a larger system that regulates carbon metabolism.
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Glucose Preference: Cells prefer glucose as a carbon source.
- Glucose Repression of Lac Operon: When glucose is present, cAMP levels are low, preventing CAP/CRP from activating the lac operon.
- Lactose Utilization: Only when glucose levels are low will the lac operon be fully activated.
Tryptophan Operon
- Repressible System: The trp operon is transcribed when tryptophan levels are low.
- Repressor Protein: Binds to the operator site in the presence of tryptophan, preventing transcription.
- Corepressor: Tryptophan acts as a corepressor, binding to the repressor and allowing it to bind to the operator.
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Negative Regulation: The tryptophan operon is an example of negative regulation.
- Repression: The repressor protein blocks the expression of the operon when tryptophan is present.
- Derepression: The removal of tryptophan allows the operon to be transcribed.
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Description
Test your knowledge on bacterial gene expression, focusing on the concepts of constitutive, repressible, and inducible enzymes. Explore operon structures like the Lactose and Tryptophan operons and their regulatory mechanisms. This quiz will help reinforce your understanding of how genes are expressed in bacteria.
