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D2.2.8 (HL)—Environmental effects on expression

Environmental factors can alter gene expression through signalling, transcript changes and epigenetic modifications such as DNA methylation in cells and organisms.

Syllabus
First assessment 2025
Objective
D2.2.8
Level
HL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceMay 2018
Most common paperPaper2
Typical marks1–3

Common command terms

  • Compare
  • Suggest
  • Discuss

Scoring notes

Common mistake
Treating altered expression as a necessary DNA base-sequence mutation.

Recent exam appearances

May 2018Paper1 ["HL"] · TZ211[ 1 ]D2.2.8 (HL)—Environmental effects on expression
May 2018Paper2 ["HL"] · TZ21(e)[ 3 ]D2.2.8 (HL)—Environmental effects on expression
May 2017Paper2 ["HL"] · TZ11(f)(ii)[ 2 ]D2.2.8 (HL)—Environmental effects on expression

Coverage 2017–2018 · Updated 16 Jul 2026

Environmental Signals Can Redirect Gene Expression

HL only

An external factor changes gene expression only through a biological sensing and regulatory pathway: signal or exposure → receptor, metabolic sensor or cellular damage response → regulatory proteins or chromatin change → altered transcription or translation.

Examples include:

  • light cycle altering transcription in photosynthetic organisms or seasonal tissues
  • oxygen concentration activating hypoxia-responsive transcription factors
  • steroid hormones binding intracellular receptors
  • nutrient availability regulating bacterial operons
  • diet, drugs, temperature or pollutants altering signalling and sometimes epigenetic tags

The molecular change may alter a cell's proteome; coordinated changes across many cells can alter physiology or phenotype.

The environment does not rewrite genes because an organism needs a trait. It changes regulatory inputs, and some exposures may also damage DNA through separate mutagenic mechanisms.

Air-Pollution Studies Link Exposure, Epigenetic Change and Lung Disease

HL only

Traffic-related pollution includes particulate matter, ozone, carbon monoxide and nitrogen oxides. Exposure has been associated with altered DNA or histone methylation patterns and with respiratory inflammation or disease.

pollutant exposure → oxidative stress or inflammatory signalling in lung tissue → altered regulatory or epigenetic state → changed expression of inflammatory and repair genes → possible contribution to chronic tissue damage

Evidence What it supports What it does not prove alone
exposure correlates with methylation difference pollution may influence regulation that methylation caused the disease
methylation correlates with altered expression the tag may mark a regulatory change that no other pathway contributed
disease risk rises with exposure exposure is a risk factor that every exposed individual will develop disease

A strong conclusion separates observation from mechanism: pollution-related epigenetic change is a plausible contributor, not automatically the only cause of a lung condition.

Epigenetic Regulation Balances Memory with Resetting

HL only

shared genome → cell-specific regulatory and epigenetic state → cell-specific transcriptome → cell-specific proteome → differentiated structure and function

Process What remains continuous? What can change?
mitosis in a differentiated lineage much of the expression state reversible tags and regulatory inputs
environmental response DNA base sequence usually remains signalling, transcription and sometimes epigenetic marks
gamete and early-embryo reprogramming genome and selected imprints most parental epigenetic patterns are reset

For any epigenetic claim, ask where the tag is, how it changes access or protein binding, whether it persists through mitosis or gametes, and whether the evidence shows correlation or causation.

Environmental effects on expression

HL only

[ 3 ]

Using the data in the bar chart, discuss the evidence for Arabidopsis plants adapting to different daylight regimes by changing the pattern of gene expression.

Concept essentials

  • Environmental conditions can alter gene expression.
  • Changes in mRNA abundance show altered transcription or transcript level.
  • DNA methylation patterns can be changed by environmental factors.
  • Different light regimes can produce different transcript patterns in plants.