Operon Concept in Genetics

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Questions and Answers

What happens to the lac operon when lactose is present in the cell?

The repressor binds to the operator, blocking transcription.

RNA polymerase is inhibited by glucose.

Allolactose binds to the repressor, inactivating it. (correct)

The lac operon remains off due to high cAMP levels.

What role does the Catabolite Activator Protein (CAP) play in the transcription of the lac operon?

It decreases the affinity of RNA polymerase for the promoter.

It binds to glucose to inhibit operon activation.

It acts as a repressor to block transcription.

It enhances the binding of RNA polymerase to the promoter. (correct)

How does the presence of glucose affect the transcription of the lac operon?

It activates the repressor to block transcription.

It increases cAMP levels, enhancing transcription.

It decreases cAMP levels, leading to lower transcription. (correct)

It has no effect on the lac operon.

What is the function of the repressor in the Trp operon?

It decreases the transcription when tryptophan is abundant. Signup and view all the answers

What is the primary method used by the Trp operon for attenuation?

Premature transcription termination. Signup and view all the answers

What is the result of high levels of tryptophan in relation to the Trp operon?

The repressor activates and binds the operator, blocking transcription. Signup and view all the answers

What molecule binds to the repressor in the lac operon to deactivate it?

Allolactose Signup and view all the answers

What happens to RNA polymerase's affinity for the lac operon when glucose is present?

Affinity decreases, leading to negligible transcription. Signup and view all the answers

What is the primary function of the operator in an operon?

To act as an 'on-off' switch for transcription Signup and view all the answers

Which of the following is true about repressible operons?

They use a co-repressor to inhibit transcription Signup and view all the answers

Which regulatory mechanism is utilized by the lac operon when lactose is present?

Positive induction via cyclic AMP Signup and view all the answers

What role does allolactose play in the regulation of the lac operon?

It binds to the repressor to induce transcription Signup and view all the answers

What distinguishes an inducible operon from a repressible one?

Inducible operons require activators, while repressible operons use repressors Signup and view all the answers

Which of the following statements about positive regulation in operons is correct?

Positive regulation involves binding of an activator to a promoter Signup and view all the answers

What does the lacZ gene within the lac operon encode?

An enzyme that breaks down lactose into glucose and galactose Signup and view all the answers

Why is glucose considered a preferred substrate over lactose in the regulation of the lac operon?

Glucose inhibits cyclic AMP production Signup and view all the answers

Study Notes

Operon Concept

Proposed by Jacob and Monod in the 1960s: They studied lactose metabolism in E.coli and found that three genes were induced together by galactosides. This observation led to the development of the operon concept.
Operons are units of genetic regulation in prokaryotes: They consist of a cluster of genes transcribed together as a single mRNA transcript (polycistronic).
Components of an operon:
- Operator: Acts as an "on-off" switch, regulating gene expression.
- Promoter: Where RNA polymerase binds to initiate transcription.
- Genes: Code for metabolic enzymes related to a specific function.
Regulation by Regulatory Proteins:
- Positive Regulation: An activator protein binds to the promoter and increases transcription.
- Negative Regulation: A repressor protein binds to the operator and prevents transcription.

Operon Types: Repressible and Inducible

Repressible Operons: The operon is usually ON but can be turned OFF by binding a co-repressor (metabolite/end-product) to the repressor.
Inducible Operons: The operon is usually OFF but can be turned ON by binding an inducer (metabolite/starting substrate) to the repressor.

Operon Combinations

Negative Repressible Operon: Operon is repressed by a repressor that binds a co-repressor to block transcription. Examples include the Trp operon.
Negative Inducible Operon: Operon is induced by a repressor that binds an inducer to activate transcription. Example: Lac operon.
Positive Inducible Operon: Operon is induced by an activator that binds an inducer to activate transcription. This is not found in nature.

The Lac Operon

First discovered operon: Regulates lactose metabolism in E.coli.
Contains 3 genes:
- LacZ: Encodes beta-galactosidase, which breaks down lactose into glucose and galactose. It also isomerizes lactose to allolactose, an inducer.
- LacY: Encodes permease, a protein responsible for transporting lactose across the cell membrane.
- LacA: Encodes transacetylase, its function is not fully understood.

Lac Operon Control Circuits:

Negative Inducible Control: The repressor, in the absence of lactose, prevents transcription. When lactose is present, allolactose binds to the repressor, inactivating it, and allowing transcription.
Positive Inducible Control: Requires the presence of cAMP, which binds to the Catabolite Activator Protein (CAP). CAP-cAMP complex binds to the promoter, enhancing RNA polymerase binding and transcription. Glucose inhibits cAMP production, leading to reduced transcription due to decreased CAP-cAMP formation.

Trp Operon

The Trp operon codes for enzymes involved in tryptophan biosynthesis.
It is a negative repressible system. In the absence of tryptophan, the repressor is inactive, and transcription occurs. When tryptophan is present, it binds to the repressor, activating it, and repressing transcription.
Attenuation: A second control mechanism in the Trp operon involves premature transcription termination, which is regulated by tryptophan levels. When tryptophan levels are high, transcription is terminated, leading to reduced production of the tryptophan biosynthesis enzymes.

Summary Points

Operons are efficient gene regulation systems in prokaryotes: They ensure genes involved in a specific pathway are expressed together.
Regulation can be achieved through repressors, activators, inducers, and co-repressors: These molecules interact with the operon components to control transcription.
The Lac and Trp operons are well-characterized examples: They demonstrate the complexity of gene regulation in prokaryotes and highlight the importance of resource allocation and metabolic regulation in bacterial cells.

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Description

Explore the operon concept introduced by Jacob and Monod, focusing on the regulation of gene expression in prokaryotes. Learn about the key components such as the operator, promoter, and the role of regulatory proteins. This quiz delves into the types of operons, including repressible and inducible operons, providing a comprehensive overview of genetic regulation.