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A1.2.13 (HL)—Structure of a nucleosome

A nucleosome consists of DNA wrapped around histone proteins, packaging eukaryotic DNA while still allowing regulated access to genes in IB Biology exam contexts.

Syllabus
First assessment 2025
Objective
A1.2.13
Level
HL

Exam analysis

Chance of appearing11%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper2
Typical marks1–2

Common command terms

  • Identify
  • State
  • Outline
  • Describe

Scoring notes

Common mistake
Treating a nucleosome as DNA alone instead of a DNA-protein packaging structure.

Recent exam appearances

November 2025Paper1B HL · TZ11(b)[ 3 ]A1.2.13 (HL)—Structure of a nucleosome
May 2025Paper1B HL · TZ12(a)(ii)[ 2 ]A1.2.13 (HL)—Structure of a nucleosome
May 2025Paper1B HL · TZ12(a)(i)[ 1 ]A1.2.13 (HL)—Structure of a nucleosome
May 2025Paper1A HL · TZ32[ 1 ]A1.2.13 (HL)—Structure of a nucleosome
May 2025Paper1A HL · TZ22[ 1 ]A1.2.13 (HL)—Structure of a nucleosome
Practice this objective

Coverage 2012–2025 · Updated 14 Jul 2026

Nucleosomes Package DNA around Histones

HL only

A nucleosome consists of DNA wrapped around a core of eight histone proteins, called a histone octamer.

Linker DNA connects neighbouring nucleosomes. An additional histone, H1, attaches to linker DNA and helps hold the packaged structure together.

Identify four parts in a model: wrapped DNA, the eight-protein histone core, linker DNA, and the additional H1 histone.

Repeated nucleosomes give chromatin a beads-on-a-string appearance: each bead is DNA around a histone octamer and each connecting segment is linker DNA.

H1 is additional to the eight core histones; do not count it as one of the histone octamer.

Structure of a nucleosome

HL only

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify / State / Outline.

Command terms

Identify / State / Outline / Describe

What earns marks

Build the answer around this relationship: Structure of a nucleosome should be described using precise molecular vocabulary.

Watch for

Treating a nucleosome as DNA alone instead of a DNA-protein packaging structure.

Representative question

Question 1

[Maximum number: 4]

Outline the structure and functions of nucleosomes.

Retrieve The HL Extensions

HL only

HL adds directionality, helix geometry, DNA packaging, and classic evidence. These ideas stay separate: 5′/3′ explains strand direction; purine-pyrimidine pairing explains width; nucleosomes explain packaging; Hershey-Chase and Chargaff explain evidence for DNA and base pairing.

  • Directionality: phosphodiester bonds create 5′ and 3′ ends, and DNA strands are antiparallel.
  • Helix stability: purine-pyrimidine pairing keeps width constant and hydrogen bonds stabilize base pairs.
  • Packaging: DNA wraps around histone octamers and H1 binds linker DNA.
  • Evidence: Hershey-Chase supports DNA as genetic material; Chargaff supports complementary pairing.
  • Chargaff found purines equal pyrimidines across DNA samples.

Concept essentials

  • Structure of a nucleosome should be described using precise molecular vocabulary.
  • Structure and information must be kept separate: backbones provide continuity while bases carry sequence meaning.
  • Exam answers should match the command term and avoid adding unsupported extra claims.