Chapter 18 !!!

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Questions and Answers

What is the function of lacI in the lac operon?

To act as an inducer of lactose metabolism

To transcribe the lac genes into separate mRNAs

To bind to the promoter and initiate transcription

To bind to the operator and block RNA polymerase (correct)

What type of regulation is involved in the lac operon when lactose is present?

Negative feedback

Positive feedback

Constitutive expression

Allosteric regulation (correct)

What is the term for the group of genes involved in lactose metabolism?

Lac gene cluster

Lac operon (correct)

Polycistronic transcript

RNA polymerase complex

What is the result of the lacI repressor binding to the operator?

RNA polymerase is blocked from transcribing the lac genes Signup and view all the answers

What is the role of the promoter in the lac operon?

To initiate transcription of the lac genes Signup and view all the answers

What is the characteristic of the lac genes in the lac operon?

They are transcribed together into one mRNA Signup and view all the answers

What is the role of an inducer in gene expression?

It increases expression by binding to a repressor Signup and view all the answers

What is the term for the coordinated regulation of many genes?

Global gene regulation Signup and view all the answers

What is the role of activator proteins in gene transcription?

They enhance binding of RNA polymerase to the promoter Signup and view all the answers

What happens to lactose transport into the cell when glucose is high?

It is inhibited Signup and view all the answers

What is the binding site for a repressor?

Operator Signup and view all the answers

What is the effect of low intracellular lactose on lacI?

It does not bind to lacI Signup and view all the answers

What type of regulation occurs when a protein is bound and expression increases?

Positive regulation Signup and view all the answers

What is the term for a set of separate genes or operons that contain the same regulatory sequences?

Regulon Signup and view all the answers

What is the role of the lac operon model in scientific research?

It has been an immensely important model system Signup and view all the answers

What is the role of a repressor in gene expression?

It reduces gene expression by binding to DNA Signup and view all the answers

What is the mechanism by which glucose regulates the lac operon?

Inducer exclusion by glucose Signup and view all the answers

What type of control does the lac operon exhibit?

Negative control by repressor proteins Signup and view all the answers

What is the condition required for CAP to bind to the regulatory sequence upstream of the promoter?

Binding to cyclic AMP (cAMP) Signup and view all the answers

What is the effect of CAP-cAMP complex on RNA polymerase binding to the lac operon?

Stimulates RNA polymerase binding Signup and view all the answers

What is the result of high glucose levels on cAMP synthesis?

Inhibits cAMP synthesis Signup and view all the answers

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

It stimulates RNA polymerase binding Signup and view all the answers

What is the effect of glucose availability on lac operon expression?

It inhibits lac operon expression Signup and view all the answers

What is the purpose of the CAP binding site in the lac operon?

To stimulate RNA polymerase binding Signup and view all the answers

Why do bacteria prefer glucose as a fuel source?

Because it is the most energy-efficient fuel Signup and view all the answers

What type of control provides the most rapid response?

Post-translational control Signup and view all the answers

What is the primary reason that E. coli uses lactose only when glucose is depleted?

Because E. coli has a strong preference for glucose as a fuel Signup and view all the answers

What is required for E. coli to use lactose?

Both transporting it into the cell with galactoside permease and cleaving it with enzyme β-galactosidase Signup and view all the answers

Why are the enzymes galactoside permease and β-galactosidase not made unless the cell needs to use lactose?

Because the cell prioritizes energy efficiency Signup and view all the answers

What is the primary level of control used to regulate gene expression in E. coli when glucose is absent?

Transcriptional control Signup and view all the answers

Study Notes

Control of the lac Operon

Glucose prevents lac operon expression through two mechanisms: inducer exclusion and CAP regulation.
High glucose levels inhibit lactose transport, preventing lactose from entering the cell.
Low intracellular lactose means it cannot bind to the repressor, allowing repression to continue.

Importance of the Lac Operon Model

The lac operon serves as a key model for understanding:
- Bacterial genes regulated by repressor proteins.
- Activator proteins that enhance RNA polymerase binding to promoters.
- Regulation of gene transcription via physical interactions between proteins and DNA sequences.

Structure of the lac Operon

Co-coined by Jacob and Monod, the term operon refers to coordinately regulated bacterial genes transcribed together into one polycistronic mRNA.
The lac operon includes the genes lacZ, lacY, and lacA involved in lactose metabolism, alongside a regulatory gene, lacI.

Central Ideas of the Operon Model

lacZ, lacY, and lacA are adjacent and transcribed as a single mRNA from the lac operon promoter.
The lacI gene expresses a repressor that binds to the operator, blocking RNA polymerase.
Lactose acts as an inducer, altering the shape of lacI and releasing it from the operator, facilitating transcription.

Glossary of Terms

Operator: DNA sequence where repressor proteins bind.
Inducer: Small molecule like lactose that increases operon expression.
Repressor: Protein that decreases operon expression when bound to DNA.
Activator: Protein that increases operon expression upon binding to DNA.
Co-repressor: Small molecule aiding repressor binding to DNA.
Positive Regulation: Binding of a protein increases expression (e.g., CAP/cAMP).
Negative Regulation: Binding of a protein decreases expression (e.g., lacI, tryptophan repressor).

Global Gene Regulation

Global gene regulation coordinates expression of numerous genes as needed.
A regulon consists of separate genes or operons with shared regulatory sequences, under negative control by repressors or positive control by activators.

Positive Control Mechanisms

CAP positively regulates the lac operon; it binds upstream of the promoter to enhance transcription.
The CAP-cAMP complex forms under low glucose, stimulating RNA polymerase binding to enhance transcription.
High glucose levels inhibit cAMP synthesis, preventing formation of the CAP-cAMP complex and halting transcription.

Post-Translational Control

Post-translational mechanisms provide rapid responses, with gene expression modulated by chemical modifications.

Energy Efficiency in Bacteria

Bacteria prefer glucose as fuel but can switch to other sugar sources when glucose is depleted, requiring new enzymes for metabolism.

Lactose Metabolism in E. coli

E. coli utilizes lactose only when glucose is low.
Lactose must first be transported into the cell by galactoside permease and then cleaved by β-galactosidase to yield glucose and galactose.
These enzymes are only synthesized when the cell requires lactose for energy.

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Description

This quiz covers the genes involved in lactose metabolism, including the lac operon, and the operon model coined by Jacob and Monod. Learn about the regulation of bacterial genes and polycistronic transcription.