Gene Regulation

All somatic cells share the same genes, but which genes are on or off defines the cell type. The lac operon in E. coli makes lactose enzymes only when lactose is present and glucose is absent, saving energy. Eukaryotes regulate at many steps from before transcription through translation, and epigenetics also shapes expression. Toggle the lactose and glucose buttons to see when the lac operon turns ON.

Why are cells different if they share the same DNA?

🤔 Skin cell vs brain cell

①All human somatic cells share the same ~20,000 genes

②Yet skin cells make keratin, red blood cells make hemoglobin

③Secret: which genes are "turned on/off" defines cell identity

④Gene regulation = mechanism that selectively activates needed genes

⑤This is the core principle of "differentiation"

Prokaryotes — lac operon model

🧬 E. coli's smart energy saving

①E. coli prefers glucose (most efficient energy source)

②Makes lactase only when lactose is present — saves energy!

③If glucose is around, no lactase even if lactose is there

④This regulatory system is the "lac operon"

lac operon ON condition

Lactose ✓ + Glucose ✗ → repressor inactive → transcription ON

both conditions must be met for gene expression

lac operon state table

ChartExpression by lactose/glucose combinations

Lactose	Glucose	lac operon	Reason
✗	✗	OFF	lactose unneeded → repressor blocks operator
✗	✓	OFF	glucose used → lactase unneeded
✓	✓	OFF	glucose preferred (catabolite repression)
✓	✗	ON	only lactose available → must digest!

Components of the operon

Operon parts

ListRoles of each lac operon component

Regulator gene

always expressed, makes the repressor protein

codes for repressor

Promoter

RNA polymerase binds here to start transcription

RNA polymerase binding site

Operator

switch — if blocked, no transcription

repressor binding site

Structural genes (Z, Y, A)

Z=β-galactosidase, Y=permease, A=transacetylase

code for lactose enzymes

Eukaryotic gene regulation — multi-level system

🏗️ Prokaryote vs Eukaryote: regulation complexity differs

①Prokaryote: one operon = ON/OFF (simple switch)

②Eukaryote: regulated at DNA → mRNA → protein each step

③Intron/exon structure, histone packaging — extra controls

④This enables complex differentiation in multicellular life

Transcription-level control

transcription factor + promoter → RNA polymerase binding → transcription start

transcription factors are the key switches

Regulation levels

ListEukaryotic gene expression control levels

Pre-transcription

chromatin structure → gene accessibility control

DNA methylation, histone modification

Transcription

most important regulation level

TFs, enhancers, silencers

Post-transcription

intron removal, alternative splicing → many proteins from one gene

mRNA processing (splicing)

Translation

mRNA lifetime → protein output

mRNA degradation by miRNA

Post-translation

protein activity/degradation

protein phosphorylation, ubiquitination

Epigenetics

DNA methylation, histone acetylation → expression change without sequence change

environment can alter gene expression (and inherit!)

Worked Examples and Exam Practice

Example 1

In the E. coli lac operon, under what conditions are the lactose-digesting enzymes made most actively? (Answer by presence of lactose and glucose.)

Lactose must be present so the repressor lets go and transcription can start.

If glucose is present, E. coli uses glucose first, so enzyme production is suppressed (glucose effect).

▸ Lactose present + glucose absent

Both conditions must hold for the lac operon to be ON. With glucose around, the enzymes are barely made even if lactose is there.

Example 2

When lactose enters the medium, what happens at the lac operon so that transcription begins?

Normally the repressor protein binds the operator and blocks RNA polymerase.

Lactose (the inducer) binds the repressor and changes its shape so it leaves the operator; RNA polymerase then transcribes the structural genes.

▸ Lactose binds the repressor → repressor leaves the operator → transcription starts

The operator is a switch and the repressor is a lock sitting on it. Lactose opens the lock.

CSAT-style

Which statement about the prokaryotic lac operon is correct?

①Abundant glucose makes lots of lactose-digesting enzyme

②The operator is where RNA polymerase binds

③The repressor protein is made by the regulatory gene

④Transcription of the structural genes is active when lactose is absent

⑤Eukaryotes also regulate genes with the lac operon

▸ ③ The repressor protein is made by the regulatory gene

The regulatory gene is always expressed, making the repressor protein that blocks the operator.

Abundant glucose suppresses enzyme output (①), RNA polymerase binds the promoter (②), no lactose means transcription is repressed (④), and operon regulation is a prokaryotic feature (⑤).

Summary

Core of gene regulation

same DNA + different gene expression = cell differentiation

why cells share the same genome but become different cells

🎯 Exam Points

①lac operon ON: lactose ✓ + glucose ✗

②Regulator → repressor / Operator = switch

③Lactose binds repressor → leaves operator → transcription begins

④Eukaryotes: TF + promoter regulation is core (multi-level)

⑤Epigenetics: DNA methylation/histone modification → no sequence change, expression changes

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DNA Replication

Biotechnology

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