Theme 3 M1/2

What are constitutive genes?

Genes that are always on, continuously expressed, and needed for basic cell functions

What are regulated genes?

Genes that are turned on or off in response to environmental conditions such as nutrient levels or stress.

What are the 3 levels at which gene expression can be regulated?

• Transcriptional control – controls DNA → mRNA.

• Translational control – controls mRNA → protein.

• Post-translational control – controls activation of proteins

Why do cells regulate genes?

To respond to environmental changes efficiently and save energy by only producing proteins when needed.

What machinery do prokaryotic cells have for growth and division?

They have all cellular components needed to replicate DNA, divide, and synthesize proteins.

When do prokaryotic cells grow rapidly?

Under favorable conditions (adequate nutrients and space).

Q: What limits prokaryotic growth?

Which nutrients are essential for bacterial growth?

Amino acids, vitamins, nucleotides, and carbohydrates.

How do gene regulation mechanisms help prokaryotes survive environmental changes?

By turning certain genes on or off depending on nutrient availability, temperature, or stress, allowing adaptation when conditions are suboptimal.

What are the two main categories of bacterial genes?

1. Housekeeping genes: Always on; maintain essential cellular functions.

2. Regulated genes: Turned on/off depending on environmental needs.

What are examples of housekeeping genes?

Genes coding for ribosomal proteins, RNA polymerase, and DNA polymerase.

Why must enzyme production in bacteria be regulated?

To conserve energy and resources — enzymes should only be produced when their substrates (nutrients) are available.

What is the preferred energy source for E. coli?

Glucose.What happens when glucose is depleted in E. coli?

What happens when glucose is depleted in E. coli?

Cells undergo a metabolic shift — they switch to metabolizing lactose as an alternative energy source.

How does E. coli metabolize lactose?

Using the enzyme β-galactosidase, which breaks lactose (a disaccharide) into glucose and galactose.

When is the β-galactosidase gene turned on?

Only when lactose is present and glucose is absent — an example of inducible gene expression.

What did the 1960s E. coli lactose experiment show?

• β-galactosidase was produced only when lactose was added to the culture medium.

• When lactose was removed, enzyme production stopped.

• Conclusion: Lactose induces β-galactosidase gene expression.

What are the three stages of gene expression control?

• Transcriptional – regulates mRNA production.

• Translational – controls protein synthesis from mRNA.

• Post-translational – modifies proteins to activate/deactivate them.

What happens if any step of gene expression fails?

No functional protein is produced or activated.

What is transcriptional regulation?

Control of how much mRNA is transcribed from DNA. It involves regulatory proteins binding near promoters to help or block RNA polymerase.

Why is transcriptional regulation energy-efficient?

Because proteins are made only when needed, saving energy by avoiding unnecessary transcription and translation.

Give an example of transcriptional regulation.

E. coli activates transcription of the β-galactosidase gene when lactose is present and glucose is absent.

What determines translational regulation efficiency?

How efficiently ribosomes bind to mRNA and how stable the mRNA is.

What are the key ribosome binding sites in eukaryotes and prokaryotes?

• Eukaryotes: 5’ CAP of mRNA

• Prokaryotes: Shine-Dalgarno sequence

What happens if mRNA is unstable or degraded quickly?

Less protein is made because translation stops early.

What is post-translational regulation?

Regulation after translation, involving protein folding or chemical modifications (e.g., phosphorylation) to activate or deactivate proteins.

Why is post-translational regulation the fastest response mechanism?

Because cells already have inactive proteins stored — activation just needs a quick modification.

Compare the speed and efficiency of transcriptional, translational, and post-translational regulation.

Level	Speed	Efficiency (Energy Cost)	Example

Post-translational

Fastest ⚡️

Intermediate

Quick activation via phosphorylation

Translational

Intermediate

Intermediate

Controlled by ribosome binding/mRNA stability

Transcriptional

Slowest 🐌

Most efficient

β-galactosidase gene activation in E. coli

Why is transcriptional regulation slower than the others?

Because it requires completing transcription, translation, and post-translational modification before a functional protein is made.

Why is transcriptional regulation considered the most energy-efficient?

It prevents the cell from producing unnecessary mRNA or proteins, conserving energy.

How do bacteria like E. coli respond to environmental nutrient changes

They can switch metabolism based on available sugars.

• Prefer glucose (simpler sugar, faster ATP yield).

• When glucose is depleted → switch to lactose metabolism by activating genes that produce lactose-metabolizing enzymes.

What two conditions must be met for E. coli to switch from glucose to lactose metabolism?

• Low glucose → triggers the metabolic shift.

• Presence of lactose → induces expression of enzymes needed to break down lactose.

Which two proteins increase during lactose metabolism, and what do they do?

• Lactose permease (lacY): transports lactose into the cell membrane.

• β-galactosidase (lacZ): cleaves lactose into glucose + galactose for energy.

Why are lacZ and lacY expressed together?

Because they are functionally related and clustered in the same operon, allowing coordinated expression from a single promoter.

How do prokaryotic and eukaryotic gene regulation differ?

• Prokaryotes: Related genes are grouped in operons → transcribed together as one mRNA.

• Eukaryotes: Each gene has its own promoter/enhancers → co-regulation occurs indirectly through shared transcription factors, not operons.

Who discovered the operon model and when

François Jacob and Jacques Monod in 1961.

What is an operon?

A cluster of functionally related genes regulated together by a single promoter and an operator (the “on/off switch”).

What are the main parts of an operon?

1. Promoter: RNA polymerase binding site.

2. Operator: DNA region that controls access to the promoter (on/off switch).

3. Gene cluster: Codes for proteins in the same pathway.

What happens if the operator is absent or unbound by a repressor?

RNA polymerase can bind the promoter freely → transcription of all genes in the operon proceeds.

What does polycistronic mRNA mean?

A single mRNA molecule that contains multiple coding regions (each with its own start/stop codons) and can produce several different proteins

Which genes belong to the lac operon?

• lacZ: codes for β-galactosidase.

• lacY: codes for lactose permease.

• (Sometimes lacA is also included, for transacetylase.)

What is the regulatory gene for the lac operon and what does it produce?

lacI → encodes the repressor protein that binds the operator and blocks transcription.

When the lac repressor is bound to the operator, what happens?

RNA polymerase cannot move forward → transcription of lacZ and lacY stops → enzymes are not produced.
➡ This is negative regulation.

What happens when lactose is absent?

• The lacI repressor binds tightly to the operator.

• RNA polymerase is blocked.

• lacZ and lacY are not transcribed.

• The cell conserves energy (no unnecessary enzyme production).

What happens when lactose is present?

• Lactose (actually its isomer allolactose) binds to the repressor → conformational change.

• Repressor can no longer bind operator.

• RNA polymerase can transcribe lacZ and lacY.
  ➡ Enzymes are made, and lactose metabolism starts.

Describe the negative regulation mechanism of the lac operon.

• Repressor (tetramer) binds operator → DNA forms loop.

• RNA polymerase cannot bind promoter → transcription off.

• When lactose binds repressor → repressor detaches → transcription begins.

What kind of protein is the lac repressor?

A tetramer — made of four identical subunits that bind tightly to operator DNA sequences.

What is the relationship between glucose presence and lac operon activity?

• Glucose present: lac operon OFF (cell uses glucose first).

• Glucose absent: lac operon can be turned ON (if lactose is available).

What is positive regulation in the lac operon?

Activation of transcription by a positive control protein (CRP / CAP) that helps RNA polymerase bind efficiently to the promoter.

What molecule signals low glucose levels in E. coli?

cAMP (cyclic AMP), produced by the enzyme adenylyl cyclase when glucose is scarce.

How does cAMP activate transcription?

1. cAMP binds to CRP (CAP) → forms CRP–cAMP complex.

2. This complex binds to a site near the lac promoter.

3. It helps RNA polymerase attach more strongly → increases transcription of lacZ and lacY.

Why is CRP–cAMP activation called positive regulation?

Because the activator protein (CRP–cAMP) increases transcription instead of repressing it.

What happens to cAMP levels when glucose is high?

Adenylyl cyclase is inhibited → cAMP levels remain low → CRP–cAMP cannot form → weak or no activation of lac operon.

Fill in the lac operon control table:

Glucose	Lactose	cAMP	CRP–cAMP	Repressor bound?	Transcription
High	Absent	Low	No	Yes	None
High	Present	Low	No	No	Low (weak)
Low	Absent	High	Yes	Yes	None
Low	Present	High	Yes	No	High (ON)

In which condition is the lac operon fully active?

When glucose is low and lactose is present.
→ cAMP high → CRP–cAMP formed → repressor inactive → RNA polymerase efficiently transcribes lac genes.

Why does E. coli use glucose first even if lactose is present?

Glucose is the preferred carbon source because it requires fewer enzymatic steps and yields energy more efficiently.
Only when glucose is gone does it induce lac operon expression to use lactose.

What does the growth curve (points A → C) show?

• A: High glucose → lac operon off, no mRNA.

• B: Glucose nearly used → transition phase → low lac mRNA.

• C: Glucose gone, lactose used → high lac mRNA, operon fully on.
  This demonstrates how gene expression dynamically changes with environment.

Why is the lac operon model important in biology?

It was the first clear example showing how genes can be turned on/off in response to environmental signals — forming the foundation of molecular genetics and gene regulation studies.

What’s the advantage of operon-based gene control in bacteria?

Energy efficiency and coordination — bacteria can rapidly regulate several related genes together in response to nutrient availability.