(5) Chapter 8: Microbial Genetics

What is gene regulation?

The control of when and how much a gene is expressed.

Why do cells regulate gene expression?

To avoid wasting energy by making proteins only when they are needed.

What are constitutive genes?

Genes that are continuously expressed at a relatively constant rate.

What percentage of bacterial genes are constitutive according to the lecture?

About 60–80%.

Give an example of constitutive genes.

Genes encoding enzymes needed for glycolysis.

What are regulated genes?

Genes that are turned on or off depending on the cell's needs.

What are the two major types of gene regulation discussed in bacteria?

Induction and repression.

What is an operon?

A group of related genes regulated together under the control of a single promoter and operator.

What is the operon model?

A model describing how bacteria regulate groups of genes together.

Who proposed the operon model?

François Jacob and Jacques Monod.

What components make up an operon?

Promoter, operator, and one or more structural genes.

What is the promoter?

The DNA sequence where RNA polymerase binds to begin transcription.

What is the operator?

The DNA sequence that acts as an on/off switch by controlling RNA polymerase access to the structural genes.

What are structural genes?

Genes that encode proteins or enzymes with related functions.

What is a regulatory gene?

A gene located outside the operon that produces a regulatory (repressor) protein.

What protein does the regulatory gene produce?

A repressor protein.

Does the regulatory gene belong to the operon?

No. It is separate from the operon.

What is a repressor protein?

A protein that can bind the operator and block transcription.

What happens when a repressor binds the operator?

RNA polymerase cannot transcribe the structural genes.

What happens if the operator is not blocked?

RNA polymerase transcribes the structural genes.

What is a repressible operon?

An operon that is normally ON but can be turned OFF.

Which operon is the classic repressible operon?

The trp (tryptophan) operon.

Why is the trp operon normally ON?

The repressor protein is inactive and cannot bind the operator.

What product is made by the trp operon?

Enzymes required to synthesize tryptophan.

What happens when tryptophan levels are low?

The trp operon remains ON so more tryptophan is synthesized.

What happens when tryptophan accumulates in the cell?

It binds the inactive repressor protein.

What role does tryptophan play in the trp operon?

It acts as a corepressor.

What is a corepressor?

A molecule that activates a repressor protein.

What happens after tryptophan binds the repressor?

The repressor becomes active and binds the operator.

What is the result when the activated repressor binds the operator?

Transcription stops.

Why does the trp operon shut off when tryptophan is abundant?

The cell no longer needs to synthesize more tryptophan.

Complete the statement: Repressible operon = normally _____.

ON.

What is an inducible operon?

An operon that is normally OFF but can be turned ON.

Which operon is the classic inducible operon?

The lac (lactose) operon.

Why is the lac operon normally OFF?

The active repressor protein is bound to the operator.

What product does the lac operon produce?

Enzymes needed to metabolize lactose.

How many structural genes are found in the lac operon discussed in lecture?

Three.

What must be present for the lac operon to turn ON?

Lactose.

What is the true inducer of the lac operon?

Allolactose.

How is allolactose produced?

It is formed from lactose inside the cell.

What does allolactose do?

It binds and inactivates the repressor protein.

What happens after the repressor is inactivated?

It releases the operator.

What happens after the operator is unblocked?

RNA polymerase transcribes the lac operon.

What proteins are produced when the lac operon is ON?

Enzymes needed to break down lactose.

Complete the statement: Inducible operon = normally _____.

OFF.

Compare the trp and lac operons.

trp = repressible, normally ON; lac = inducible, normally OFF.

What is catabolite repression?

The inhibition of the lac operon when glucose is available.

Which sugar do bacteria prefer?

Glucose.

Why do bacteria use glucose before lactose?

Glucose is a more efficient energy source.

What happens to bacterial growth when both glucose and lactose are present?

Bacteria use glucose first, then switch to lactose after glucose is depleted.

What is the lag phase seen after glucose is depleted?

The time needed for bacteria to activate genes required to metabolize lactose.

What signaling molecule increases when glucose is scarce?

cAMP (cyclic AMP).

What does cAMP indicate?

Low glucose availability.

What protein binds cAMP?

CAP (catabolite activator protein), also called the cAMP receptor protein.

What forms when cAMP binds CAP?

The CAP-cAMP complex.

Where does the CAP-cAMP complex bind?

Near the promoter of the lac operon.

What is the function of the CAP-cAMP complex?

It helps RNA polymerase bind efficiently and stimulates transcription.

When is the lac operon expressed at high levels?

When lactose is present AND glucose is absent.

Why isn't lactose alone enough for maximum lac operon expression?

Low glucose is also required to produce cAMP and activate CAP.

List the two conditions required for maximal lac operon expression.

Lactose present and glucose absent.

What happens to cAMP levels when glucose is abundant?

cAMP levels decrease.

What happens to CAP activity when cAMP levels are low?

CAP cannot activate transcription.

What happens to lac operon expression when glucose is high?

It remains low even if lactose is present.

What is diauxic growth?

Bacterial growth in two phases: first on glucose, then on lactose after a lag phase.

What is epigenetic regulation?

Control of gene expression without changing the DNA nucleotide sequence.

How is epigenetic regulation commonly achieved in bacteria?

DNA methylation.

What is DNA methylation?

The addition of methyl groups to specific DNA bases.

Does DNA methylation change the DNA sequence?

No.

Why is DNA methylation not considered a mutation?

The nucleotide sequence remains unchanged.

How can DNA methylation affect gene expression?

It can prevent certain genes from being expressed.

What is the major difference between mutation and DNA methylation?

Mutation changes the DNA sequence; methylation changes gene expression without altering the sequence.

Exam tip: Promoter vs. Operator

Promoter = RNA polymerase binding site; Operator = regulatory switch where the repressor binds.

Exam tip: Repressible vs. Inducible

trp operon = repressible (normally ON); lac operon = inducible (normally OFF).

Exam tip: Corepressor vs. Inducer

Corepressor activates a repressor (trp); inducer inactivates a repressor (lac).

Exam tip: When is the lac operon maximally active?

Only when lactose is present and glucose is absent.

Exam tip: Function of cAMP-CAP

Enhances RNA polymerase binding and increases lac operon transcription during low glucose conditions.