CourseCourseQ BankQuestion BankDocsDocuments

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
Practice this objective

Coverage 2019–2025 · Updated 16 Jul 2026

Epigenetic Tags Alter Access to DNA

HL only

Methyl groups added to promoter DNA or to histones act as epigenetic tags that alter transcription without changing DNA base sequence.

Tag location Typical expression effect
Cytosine in a DNA promoter Methylation represses transcription of the downstream gene
Amino acids in histone proteins Methylation can repress or activate transcription, depending on the site and context

The tags change which regulatory proteins can associate with chromatin and how accessible the promoter is to transcription machinery.

Extensive methylation of promoter cytosines can make a gene less accessible, reducing its mRNA and protein production.

Do not apply one universal rule to all methylation: promoter DNA methylation is generally repressive, while histone methylation may activate or repress.

Epigenetic tags

HL only

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through structured response, commonly using Outline / Suggest / Explain.

Command terms

Outline / Suggest / Explain

What earns marks

Build the answer around this relationship: Promoter DNA methylation usually represses transcription.

Watch for

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

Representative question

Question 1

[Maximum number: 2]

Explain how methylation of nucleosomes affects DNA transcription.

HL Gene Expression Control

HL only
  • Transcription factors, promoters, enhancers, activators and repressors control RNA polymerase activity.
  • mRNA lifetime limits translation; poly-A shortening and nucleases help remove transcripts.
  • DNA methylation and histone modification alter chromatin access without changing base sequence, creating epigenetic patterns during differentiation.
  • The genome is all genetic information; the transcriptome and proteome vary with cell type, time and environment.
  • Some epigenetic marks persist through cell division or inheritance, although most are reset during gamete formation; genomic imprinting is an exception.
  • Hormones regulate eukaryotic transcription through receptors and transcription factors; lac and trp operons illustrate bacterial control.
  • Twin studies and environmental exposures help separate genetic, epigenetic and environmental effects on phenotype.

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.