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Practice · Biology · 2025 syllabus · A3.2

IB Biology A3.2 Classification and cladistics: practice questions

4 exam-style questions on A3.2 Classification and cladistics, HL only: 3 multiple choice and 1 short answer. Each one comes with the mark scheme, a model answer and the reasoning behind every mark. Free with your email.

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A3.2-02 · Paper 1A style · multiple choice · 1 mark · HL

Evidence for the three domains

A newly discovered prokaryote from a hot spring was placed in the domain Archaea rather than in the domain Bacteria. Which evidence would justify this classification? [1]

  1. It has 70S ribosomes and a single loop of DNA that is not enclosed in a nucleus.
  2. It grows and reproduces at temperatures above 80 °C.
  3. Its cell is surrounded by a cell wall and it has no membrane-bound organelles.
  4. The base sequence of its ribosomal RNA is more similar to that of archaea than to that of bacteria.
Show the answer and the reasoning

Answer: D. The division of living things into three domains, Bacteria, Archaea and Eukarya, was based on comparing the base sequences of ribosomal RNA, a molecule found in every organism. A new prokaryote is assigned to a domain the same way. A and C describe features that archaea and bacteria share, so they cannot separate the two domains. B is not reliable evidence: some bacteria also live in very hot water, and many archaea live in ordinary habitats.

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The 4 questions in A3.2

  1. A3.2-01 Recognizing a cladePaper 1A style · multiple choice · 1 mark · HL
  2. A3.2-02 Evidence for the three domainsPaper 1A style · multiple choice · 1 mark · HL
  3. A3.2-03 Relationships from base sequencesPaper 1A style · multiple choice · 1 mark · HL
  4. A3.2-04 A molecular clockPaper 2 style · short answer · 6 marks · HL

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Every Biology and Chemistry question here is original, written for practice in the style of the papers set since the 2025 syllabus, with invented data. Developed independently from the International Baccalaureate Organization, which has not endorsed it. Found a mistake? Email vytis@limeadmissions.com with the question id.