gene expression

1⃣ Operons — the INTUITION (plain-language first)

The core idea

Bacteria do not waste energy.
They only make proteins when they need them.

An operon is basically:

a single on/off switch that controls multiple related genes at once

Think of it like a power strip:

  • One switch

  • Several appliances plugged in

  • Flip it on → everything runs

  • Flip it off → nothing runs

2⃣ What an Operon Actually Is

An operon has three main DNA parts:

🧬 Promoter

  • Where RNA polymerase binds

  • “Start transcription here”

🧬 Operator

  • The switch

  • Determines whether RNA polymerase can move forward

🧬 Structural genes

  • The genes that code for enzymes or proteins

📌 Important:

Operons exist ONLY in prokaryotes

3⃣ Negative vs Positive Gene Regulation (this is where people get lost)

🔴 Negative regulation

  • A repressor protein blocks transcription

  • Default state = OFF

🟢 Positive regulation

  • An activator or inducer allows transcription

  • Default state = OFF until activated

4⃣ Two Classic Operons You MUST Know

🟦 trp Operon (REPRESSIBLE)

Purpose

  • Makes tryptophan

  • Tryptophan is expensive to make → don’t overproduce it

Logic

If tryptophan is already present → STOP making it

How it works

  • Tryptophan = corepressor

  • Tryptophan binds to the trp repressor

  • Repressor becomes active

  • Repressor binds operator

  • RNA polymerase blocked

  • Transcription OFF

If tryptophan is ABSENT

  • Repressor cannot bind operator

  • RNA polymerase transcribes genes

  • Transcription ON

🧠 Key phrase for AP:

Repressible operons are usually ON

🟨 lac Operon (INDUCIBLE)

Purpose

  • Breaks down lactose

  • Lactose is not always present

Logic

Only make lactose-digesting enzymes when lactose is present

How it works

  • Without lactose:

    • Repressor is active

    • Binds operator

    • Transcription OFF

  • With lactose:

    • Lactose → converted to allolactose (inducer)

    • Allolactose binds repressor

    • Repressor changes shape

    • Falls off operator

    • RNA polymerase transcribes genes

    • Transcription ON

🧠 Key phrase for AP:

Inducible operons are usually OFF

5⃣ Catabolic vs Anabolic (this ties it together)

Operon

Pathway Type

Function

lac

Catabolic

Breaks lactose down

trp

Anabolic

Builds tryptophan

6⃣ Eukaryotes vs Prokaryotes (EXAM FAVORITE)

Prokaryotes

  • Operons

  • Regulation at transcription only

Eukaryotes

  • No operons

  • Regulation at:

    • Transcription

    • RNA processing

    • mRNA stability

    • Translation

    • Post-translational modification

🧠 Key idea:

Same DNA, different cell types → different gene expression

7⃣ Eukaryotic Gene Regulation (simple version)

Transcription level

  • Enhancers + activators → increase transcription

  • Silencers + repressors → decrease transcription

RNA processing

  • 5’ cap

  • Poly-A tail

  • Introns removed

  • Alternative splicing → different proteins from same gene

mRNA stability

  • Longer mRNA life → more protein made

Translation control

  • Some mRNAs translated only under certain conditions

Post-translational modification

  • Proteins activated by:

    • Cleavage

    • Adding sugars, lipids, or chemical groups

📘 AP BIO NOTES (CONDENSED)

Operons

  • Found only in prokaryotes

  • Coordinate gene expression

  • Components: promoter, operator, structural genes

Repressible Operon (trp)

  • Default ON

  • Corepressor activates repressor

  • Product inhibits its own synthesis

Inducible Operon (lac)

  • Default OFF

  • Inducer inactivates repressor

  • Substrate induces enzyme production

Eukaryotic Regulation

  • No operons

  • Regulation at multiple levels

  • Enables cell specialization