Grade 11-12 (age 16-18)

high school Gene Regulation

Every somatic cell in the body carries the same genes, yet a skin cell and a nerve cell look and work nothing alike, because each one switches on a different set of those genes. E. coli controls its lactose enzymes with the lac operon: without lactose, a repressor sits on the operator and blocks transcription, and even with lactose, transcription stays very weak as long as glucose is around. Eukaryotes regulate expression at many stages, from how tightly the chromatin is packed, through transcription and mRNA processing, to translation and changes made to the finished protein. In the diagram, the lactose and glucose buttons give four combinations, and you can watch the repressor and CAP come and go as the transcription arrow disappears, fades, or runs at full strength.

How one genome makes different cells

🤔 What separates a skin cell from a nerve cell

①Every somatic cell descends from a single fertilized egg, so each carries the same set of about 20,000 protein-coding genes

②Yet skin cells make large amounts of keratin, while cells on their way to becoming red blood cells make hemoglobin

③The difference lies not in which genes a cell has but in which ones it expresses, when, and how strongly

④Switching on only the genes a cell needs is called regulation of gene expression

⑤As a cell's pattern of active genes becomes fixed, it takes on a specific shape and function, and that process is cell differentiation

Prokaryotic control: the lac operon

🧬 When E. coli makes lactose enzymes

①E. coli uses glucose first and turns to lactose only when glucose runs out

②Without lactose, the repressor made by the regulator gene binds the operator, so RNA polymerase cannot transcribe the structural genes

③When lactose enters the cell, a derivative of it (allolactose) binds the repressor, changes its shape, and pulls it off the operator

④If glucose is also present, cAMP levels are low, CAP cannot bind next to the promoter, and transcription stays very weak

⑤Only with lactose present and glucose absent does the cAMP-CAP complex help RNA polymerase bind, so the cell makes lactose enzymes in quantity

Condition for strong lac operon transcription

lactose present + glucose absent → repressor released + cAMP-CAP bound → transcription ON

Lactose lifts the repression, and the absence of glucose lets CAP boost transcription. Both conditions must hold for the cell to make plenty of enzyme.

lac operon states

Expression for each lactose/glucose combination

Lactose	Glucose	lac operon	Why
−	−	OFF	no lactose, so the repressor stays on the operator (CAP may bind, but transcription is blocked)
−	+	OFF	no lactose, so the repressor blocks the operator, and glucose keeps CAP off as well
+	+	LOW	the repressor is released, but CAP cannot bind, so transcription is very weak
+	−	ON	the repressor is released and cAMP-CAP boosts transcription

Parts of the lac operon

Operon parts

Roles of the lac operon and its regulator gene

Regulator gene

sits outside the operon and is always expressed, so the repressor is made all the time

codes for the repressor

Promoter

RNA polymerase binds here to start transcription, and the CAP binding site lies just upstream

where RNA polymerase binds

Operator

while the repressor sits here, RNA polymerase cannot move past and transcription stops

where the repressor binds

Structural genes (Z, Y, A)

Z = β-galactosidase (breaks down lactose), Y = permease (brings lactose in), A = transacetylase. All three are transcribed together on one mRNA

code for the enzymes that use lactose

Eukaryotic control: many stages

🏗️ How prokaryotic and eukaryotic control differ

①Prokaryotes group related genes into one operon and switch them on or off together

②In eukaryotes, each gene has its own promoter, and combinations of transcription factors control its transcription

③Before transcription, how loosely the chromatin is packed matters; after it, how the mRNA is processed and how long it lasts change how much protein is made

④Because control is possible at every stage, a multicellular organism builds hundreds of cell types from one genome

Control at transcription

transcription factors bind control regions → RNA polymerase binds the promoter → transcription begins

Each cell type holds a different mix of transcription factors, so the same gene can be on in one cell and off in another.

Control stages

Stages of eukaryotic gene expression control

Before transcription

tightly packed chromatin keeps transcription factors away from the DNA, and loosened chromatin makes transcription easier

DNA methylation, histone modification

Transcription

the most important stage for setting expression levels, with a different combination of factors for each gene

transcription factors and control regions such as enhancers

After transcription

introns are removed and exons are joined in different ways, so one gene can yield several proteins

mRNA processing (alternative splicing)

Translation

how long an mRNA survives sets how much protein it makes

miRNAs block translation or trigger mRNA breakdown

After translation

controls when a protein is switched on or broken down

phosphorylation, ubiquitin tagging

Epigenetics

DNA methylation, histone acetylation → same base sequence, different expression

These marks are passed on to daughter cells when a cell divides, and they can shift with the environment.

Worked Examples and Exam Practice

Example 1

In the lac operon of E. coli, under which condition are lactose enzymes made in the largest amounts? Answer in terms of whether lactose and glucose are present.

Lactose must be present so the repressor comes off the operator and RNA polymerase can reach the structural genes.

If glucose is also present, cAMP is low and CAP cannot bind in front of the promoter, so transcription stays very weak even though the repressor is gone.

▸ lactose present + glucose absent

Lactose is the condition that lifts the repression, and the lack of glucose is the condition that boosts transcription. With lactose + and glucose − in the diagram, CAP appears and the transcription arrow is at its strongest.

Example 2

Lactose is added to a medium with no glucose. In what order do events in the lac operon lead to transcription?

Before lactose arrives, the repressor sits on the operator and RNA polymerase cannot transcribe the structural genes.

Once lactose arrives, a lactose derivative binds the repressor, changes its shape, and releases it from the operator. With no glucose around, cAMP-CAP binds as well, and RNA polymerase transcribes the structural genes at a high rate.

▸ lactose derivative binds the repressor → repressor leaves the operator → transcription starts with help from cAMP-CAP

The repressor decides whether transcription happens at all, and CAP decides how strongly it runs. That is why the lac operon responds to two signals at once.

CSAT-style problem

Which statement about the lac operon of prokaryotes is correct?

①When glucose is plentiful, lactose enzymes are made in large amounts

②The operator is where RNA polymerase binds

③The repressor is made from the regulator gene

④Without lactose, the structural genes are actively transcribed

⑤Eukaryotes usually control many genes together with a single operon

▸ ③ The repressor is made from the regulator gene

The regulator gene is always expressed and makes the repressor, which binds the operator when lactose is absent.

With plenty of glucose, CAP cannot bind and almost no enzyme is made (①); RNA polymerase binds the promoter (②); without lactose, transcription is repressed (④); and in eukaryotes, each gene usually has its own promoter (⑤).

Summary

The core of gene regulation

same genome + different expression in each cell = cell differentiation

This is why cells that share one genome still become different kinds of cells.

🎯 Exam Points

①The lac operon consists of a promoter, an operator, and structural genes, and the regulator gene lies outside it

②Without lactose, the repressor binds the operator and no transcription occurs

③When lactose arrives, a lactose derivative binds the repressor and pulls it off the operator

④With glucose also present, CAP cannot bind and transcription stays very weak; transcription peaks with lactose alone

⑤Eukaryotes control transcription with combinations of transcription factors, and epigenetic marks change expression without changing the base sequence

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DNA Replication & Gene Expression

Biotechnology

Last updated 2026-10-17

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