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D2.2.6 (HL)—Epigenetic tags

Epigenetic tags such as DNA methylation and histone methylation alter chromatin access and regulate transcription in cells and tissues over time.

Syllabus
First assessment 2025
Objective
D2.2.6
Level
HL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper2
Typical marks1–2

Common command terms

  • Outline
  • Suggest
  • Explain

Scoring notes

Common mistake
Describing histone methylation when the question specifically asks about DNA methylation.

Recent exam appearances

November 2025Paper1B ["HL"] · TZ11(c)[ 2 ]D2.2.6 (HL)—Epigenetic tags
May 2025Paper1A ["HL"] · TZ220[ 1 ]D2.2.6 (HL)—Epigenetic tags
May 2019Paper2 ["HL"] · TZ22(b)(ii)[ 1 ]D2.2.6 (HL)—Epigenetic tags
May 2019Paper2 ["HL"] · TZ22(b)(i)[ 2 ]D2.2.6 (HL)—Epigenetic tags

Coverage 2019–2025 · Updated 16 Jul 2026

Promoter DNA Methylation Usually Reduces Transcription

HL only

DNA methylation adds methyl groups to particular cytosines without changing the DNA base sequence. Dense methylation in a promoter is usually associated with reduced transcription of the downstream gene.

promoter cytosines become methylated → proteins that recognize methylated DNA can bind and recruit chromatin-compacting machinery → transcription factors and RNA polymerase gain less access → transcription decreases

Because methyl groups can be added, maintained or removed by enzymes, promoter methylation is potentially reversible even when the underlying gene sequence is unchanged.

The regulatory effect of methylation depends on its location. A methyl group anywhere in a gene does not automatically switch that gene off.

Histone Tags Change Chromatin Access in Different Ways

HL only

DNA wraps around histone proteins in nucleosomes. Chemical modification of histone tails changes which proteins associate with chromatin and how accessible nearby DNA is to transcription machinery.

Histone modification Typical interpretation Important condition
acetylation often loosens chromatin and supports transcription the effect reflects recruitment of acetyl-binding and chromatin-remodelling proteins
deacetylation often supports compact, less accessible chromatin reduced access can lower transcription
methylation may activate or repress transcription the effect depends on the amino acid and methylation state

Promoter DNA methylation is usually taught as repressive, but histone methylation is not automatically repressive. Its meaning depends on the modified histone site.

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.

Epigenetic tags

HL only

[ 2 ]

Explain how methylation of nucleosomes affects DNA transcription.

Concept essentials

  • Promoter DNA methylation usually represses transcription.
  • Histone methylation can alter chromatin accessibility.
  • Different methylation patterns can be associated with tumour cells.
  • Epigenetic tags change gene expression without changing DNA sequence.