Gene Regulation and the Lac Operon

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Questions and Answers

What role do repressor proteins play in gene expression regulation?

They prevent transcription by binding to the operator. (correct)

They enhance the stability of mRNA in the cytoplasm.

They assist RNA polymerase in unwinding the DNA.

They stimulate transcription by enhancing RNA polymerase binding.

Which component of an operon is directly responsible for binding RNA polymerase?

Regulatory gene

Promoter (correct)

Operator

Structural gene

What effect does lactose have on the metabolism of E. coli?

It completely inhibits lactose metabolism.

It acts as an inducer by stimulating enzyme production. (correct)

It inhibits the transcription of related enzymes.

It functions as a repressor of the operon.

In a gene regulatory system, what is the role of an operator?

To function as a regulatory switch controlled by repressor proteins. Signup and view all the answers

Which of the following is NOT a component of an operon?

Inducer Signup and view all the answers

How does positive regulation differ from negative regulation in gene expression?

Positive regulation enhances transcription through activator proteins, unlike negative regulation. Signup and view all the answers

What is the main function of structural genes in an operon?

To code for proteins that perform specific functions. Signup and view all the answers

What is the primary role of the lacZ gene in the lac operon?

To encode β-galactosidase, which breaks down lactose Signup and view all the answers

What triggers the release of the repressor protein from the operator?

The presence of lactose binding to the repressor Signup and view all the answers

Which component of the lac operon is typically responsible for binding RNA polymerase?

Promoter Signup and view all the answers

What type of regulatory element is the repressor protein considered in the operon model?

Allosteric regulator Signup and view all the answers

What is the function of the lacA gene within the lac operon?

To encode transacetylase, which detoxifies compounds Signup and view all the answers

How does the repressor protein inhibit transcription when bound to the operator?

By obstructing the RNA polymerase binding site Signup and view all the answers

In the absence of lactose, what is the typical state of the lac operon?

Inactive and repressed Signup and view all the answers

What characteristic of the repressor protein allows it to interact with lactose?

It exhibits allosteric properties Signup and view all the answers

Which structural gene encodes the protein responsible for transporting lactose into the E. coli cell?

lacY Signup and view all the answers

What prevents the transcription of the lac operon in the absence of an inducer?

The binding of the lac repressor to the operator DNA Signup and view all the answers

What is the primary function of allolactose in the lac operon?

To induce conformational change in the repressor Signup and view all the answers

What is the role of β-galactosidase in the lac operon?

To cleave lactose into glucose and galactose Signup and view all the answers

Which condition leads to catabolite repression in the lac operon?

High levels of glucose and low lactose Signup and view all the answers

How does the presence of lactose affect the activity of the lac operon?

It prevents the repressor from binding to the operator Signup and view all the answers

In the lac operon, what is the consequence of the repressor molecule changing shape?

It prevents the repressor from binding to the operator Signup and view all the answers

What is NOT part of the lac operon structure?

Regulator gene Signup and view all the answers

What molecule signals the lac operon to be active when lactose is available?

Allolactose Signup and view all the answers

What happens when the repressor is bound to allolactose?

Transcription of structural genes is activated Signup and view all the answers

What happens to the transcription rate of the lac operon in the presence of high lactose and low glucose?

Transcription is elevated because CAP is bound and the lac repressor is inactive. Signup and view all the answers

Why is it energetically advantageous for E.coli to repress expression of tryptophan synthesis genes in the presence of tryptophan?

To conserve cellular resources by not producing unnecessary enzymes. Signup and view all the answers

What is the role of the allosteric repressor in the tryptophan operon?

It undergoes a conformational change upon binding with tryptophan to repress transcription. Signup and view all the answers

In which of the following conditions is transcription of the lac operon maintained at a low rate?

High lactose and high glucose. Signup and view all the answers

What condition must be met for the tryptophan operon to be repressed?

Sufficient tryptophan must be present to form a receptor-repressor complex. Signup and view all the answers

What is the role of CAP in the regulation of the lac operon?

It facilitates the binding of RNA polymerase to the promoter. Signup and view all the answers

How does glucose affect the cAMP levels in a cell?

Glucose decreases cAMP levels by inhibiting adenylyl cyclase. Signup and view all the answers

What must occur for maximal transcription of structural genes in the lac operon?

Lactose must bind to the repressor and CAP must bind to the CAP-binding site. Signup and view all the answers

What does cAMP bind to in order to initiate the positive control of transcription in the lac operon?

CAP (cyclic AMP receptor protein). Signup and view all the answers

What effect does the cAMP-CAP complex have on DNA structure?

It bends the DNA to enhance RNA polymerase binding. Signup and view all the answers

What is the consequence of CAP not binding to the CAP-binding site?

Transcription of the lac operon is inhibited. Signup and view all the answers

Which enzyme is responsible for converting ATP to cAMP?

Adenyl cyclase. Signup and view all the answers

In what condition is CAP able to exert positive control over transcription?

When cAMP is bound to CAP. Signup and view all the answers

What happens to the lac operon when glucose is present?

It remains inactive due to low cAMP levels. Signup and view all the answers

What is the main function of the CAP-binding site?

To facilitate the binding of the CAP-cAMP complex. Signup and view all the answers

Study Notes

Prokaryotic Gene Expression Regulation

Gene expression begins at the promoter, where RNA polymerase binds to initiate transcription.
Gene transcription must be selective.
Two types of regulatory proteins bind to DNA (repressor and activator) to regulate gene expression.
Negative regulation: Binding of a repressor protein prevents transcription.
Positive regulation: An activator protein binds to DNA to stimulate transcription.

Lactose Metabolism in E. coli (Lac Operon)

An operon is a functional unit of DNA containing a cluster of genes under the control of a single promoter.
The lac operon in E. coli controls lactose breakdown.
The operon consists of:
- Promoter: DNA sequence where RNA polymerase binds to initiate transcription.
- Operator: A segment of DNA that acts as a regulatory switch, controlled by a repressor protein (it's a regulatory region).
- Structural genes: Genes that code for proteins (LacZ, LacY, and LacA).
LacZ encodes β-galactosidase, an enzyme that converts lactose to glucose and galactose.
LacY encodes lactose permease, which transports lactose into the cell.
LacA encodes transacetylase, involved in detoxifying certain compounds related to lactose metabolism.
Genes are transcribed together into a single mRNA strand and translated together in the cytoplasm.
Negative Control: The LacI repressor protein binds to the operator region preventing RNA polymerase from transcribing the structural genes when lactose is absent.
Allolactose binds to the repressor in the presence of lactose, causing a conformational change in the repressor, the repressor is then released from the operator allowing RNA Polymerase to transcribe the operon and produce lac proteins.
Positive Control: A regulatory protein called CAP (catabolite activator protein) is involved in regulating the operon.
- When glucose levels are low, cAMP levels are high.
- cAMP binds to CAP which binds to the promoter making it easier for RNA polymerase to bind, and increasing the rate of transcription of lac operon.
When glucose is present, cAMP levels are low.
- CAP is unbound and cannot greatly enhance the rate of transcription.
Summary of control: The lac operon is regulated by both negative and positive regulation: absence of lactose results in the repressor protein binding to the operator site blocking transcription; low glucose results in cAMP binding to CAP, enhancing binding of RNA polymerase, which increases transcription.

Tryptophan Metabolism in E. coli (Trp Operon)

The trp operon controls tryptophan synthesis.
The operon contains five structural genes (trpE, trpD, trpC, trpB, and trpA) that code for enzymes involved in tryptophan synthesis, and a regulatory gene trpR encoding a repressor protein that binds to the operator.
Repressible operon:
- When tryptophan is present in sufficient amounts, the operon is repressed and tryptophan synthesis is stopped.
- Tryptophan binds to the repressor protein, causing a conformational change that enables the repressor to bind to the operator, preventing transcription.
- This is called repressible because the process can be switched from "on" to "off" (synthesizing/not synthesizing enzymes) based on the repressor.
When tryptophan is absent, the repressor is inactive, and transcription occurs.

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Description

Test your knowledge on gene expression regulation, specifically focusing on the lac operon in E. coli. This quiz covers topics such as repressor proteins, operator functions, and the roles of various genes within the operon. Challenge yourself with questions about metabolic effects and regulatory mechanisms.