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A3.2.9 (HL)—Three domains classification

Three-domain classification separates Archaea, Eubacteria, and Eukaryotes using rRNA sequence evidence plus cell wall, membrane, ribosome, and DNA associated features.

Syllabus
First assessment 2025
Objective
A3.2.9
Level
HL

Exam analysis

Chance of appearing7%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1
Typical marks1

Common command terms

  • State
  • Explain
  • Distinguish
  • Suggest

Scoring notes

Common mistake
Confusing domain with kingdom, phylum, class, or ecological role.

Recent exam appearances

November 2025Paper1A ["HL"] · TZ321[ 1 ]A3.2.9 (HL)—Three domains classification
May 2025Paper1A ["HL"] · TZ222[ 1 ]A3.2.9 (HL)—Three domains classification
May 2023Paper1 ["HL"] · TZ117[ 1 ]A3.2.9 (HL)—Three domains classification
November 2020Paper1 ["HL"] · TZ019[ 1 ]A3.2.9 (HL)—Three domains classification
November 2019Paper2 ["HL"] · TZ01(a)[ 1 ]A3.2.9 (HL)—Three domains classification
Practice this objective

Coverage 2015–2025 · Updated 15 Jul 2026

Concept essentials

  • The three domains are Archaea, Eubacteria, and Eukaryotes.
  • rRNA sequence evidence was central to establishing the three-domain system.
  • Eubacteria have peptidoglycan cell walls, unlike Archaea and Eukaryotes.
  • Eukaryotes have nuclei and membrane-bound organelles.
  • Archaea and Eubacteria are both prokaryotic but differ in rRNA, membranes, cell walls, and DNA-associated proteins.
ConceptIB Biology HL