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D2.2.2 (HL)—Regulation of transcription

Transcription is regulated by DNA binding proteins, promoters, enhancers, repressors, chromatin state and chemical signals in living cells and tissues.

Syllabus
First assessment 2025
Objective
D2.2.2
Level
HL

Exam analysis

Chance of appearing6%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper2
Typical marks1–7

Common command terms

  • Suggest
  • Explain

Scoring notes

Common mistake
Treating promoters as translation start sites instead of RNA polymerase binding regions.

Recent exam appearances

May 2025Paper2 ["HL"] · TZ19(c)[ 7 ]D2.2.2 (HL)—Regulation of transcription
November 2022Paper2 ["HL"] · TZ04(d)[ 3 ]D2.2.2 (HL)—Regulation of transcription
May 2022Paper1 ["HL"] · TZ227[ 1 ]D2.2.2 (HL)—Regulation of transcription
May 2021Paper2 ["HL"] · TZ27(c)[ 7 ]D2.2.2 (HL)—Regulation of transcription
May 2017Paper2 ["HL"] · TZ28(c)[ 8 ]D2.2.2 (HL)—Regulation of transcription

Coverage 2014–2025 · Updated 16 Jul 2026

The Promoter Organizes Transcription Initiation

HL only

A promoter is a non-coding DNA region near the start of a gene where general transcription factors and RNA polymerase assemble to initiate transcription.

Specific DNA sequences are recognized by proteins → a transcription-initiation complex assembles at the promoter → RNA polymerase is positioned on the template strand → transcription begins at the correct site and direction.

Proteins that stabilize or recruit the initiation complex increase transcription. Proteins that block assembly, polymerase recruitment or promoter access decrease transcription.

The promoter is DNA; a transcription factor is a protein that binds DNA. Neither should be called a gene that codes for the protein being regulated.

Enhancers and Repressors Supply Gene-Specific Inputs

HL only

Sequence-specific transcription factors bind regulatory DNA only when their DNA-binding region matches the target base sequence.

Regulatory input Binding site Usual effect
activator enhancer helps recruit or stabilize the transcription machinery, increasing transcription
repressor silencer or other regulatory sequence interferes with activation, promoter access or initiation, decreasing transcription

An enhancer may lie thousands of base pairs upstream or downstream of its gene. DNA looping can bring enhancer-bound activators into contact with proteins at the promoter.

A gene's transcription rate reflects the combination of regulatory proteins present in that cell. One enhancer does not make a gene permanently active in every tissue.

Expression Output Is the Result of a Regulated Flow

HL only

regulatory proteins and chromatin access → transcription initiation → mature mRNA abundance and lifetime → translation → protein activity and lifetime → cellular effect

Observation First control question to ask
little or no mRNA is transcription initiation repressed or is mRNA rapidly degraded?
abundant mRNA but little protein is translation inefficient or is protein rapidly removed?
protein present but no expected effect is the protein inactive, misplaced or missing a required partner?

Gene expression is not simply gene present versus gene absent. Explain a change by naming the regulated stage and tracing its consequence to functional protein and phenotype.

Steroid Hormones Turn an External Signal into Transcriptional Control

HL only
1

A lipid-soluble steroid hormone can cross the plasma membrane and bind an intracellular receptor. Ligand binding changes the receptor into an active transcription-regulating complex.

The hormone enters a target cell and binds its specific receptor. The receptor changes conformation, may dimerize and becomes able to regulate transcription.

2

The activated complex enters or acts in the nucleus, binds a hormone-response DNA sequence and recruits coactivators or corepressors, changing target-gene transcription.

Only cells with the required receptor and accessible target regulatory sequences respond directly. The same hormone can therefore cause different expression changes in different tissues.

The hormone is the signal, not the transcription factor's DNA target. The activated receptor complex regulates transcription by binding specific DNA sequences with other regulatory proteins.

Signals Change Expression by Altering Regulatory Protein Activity

HL only

external molecule → binds a receptor or regulatory protein → protein conformation, location or DNA binding changes → transcription changes → mRNA and protein abundance change → cell response

System Signal sensor DNA-control event Expression result
steroid-hormone target cell intracellular receptor activated receptor complex binds response sequence selected eukaryotic genes increase or decrease transcription
lac operon repressor binds lactose-derived inducer repressor leaves operator lactose-use genes are transcribed
trp operon repressor binds tryptophan corepressor active repressor occupies operator tryptophan-synthesis genes are repressed

Gene regulation is specific because the responding cell must contain the matching receptor or regulator and the target DNA sequence. The same external condition therefore does not activate every gene or every cell.

Regulation of transcription

HL only

[ 7 ]

Explain how gene expression can be regulated during transcription to determine an organism's phenotype.

Concept essentials

  • Promoters help position RNA polymerase for transcription.
  • Transcription factors can activate or repress gene expression.
  • Enhancers and silencers alter transcription rate.
  • Chromatin structure and epigenetic marks affect access to DNA.