A student examined two types of cell under an electron microscope and noted that one cell contained a nucleus, mitochondria, and ribosomes, while the other contained only ribosomes and a circular DNA molecule. Explain why the cell containing only ribosomes and a circular DNA molecule is classified as prokaryotic rather than eukaryotic.

OCR A-Level Biology A (H420) — 2.1 Cell structure · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. The prokaryotic cell lacks a membrane‑bound nucleus, so its DNA is free in the cytoplasm.
2. Its DNA is circular and not wrapped around histone proteins, unlike the linear, histone‑associated DNA of eukaryotes.
3. It contains no other membrane‑bound organelles such as mitochondria or endoplasmic reticulum.
4. The ribosomes are 70S, smaller than the 80S ribosomes found in eukaryotic cells.

Examiner tips

  • Use the exact terms "membrane‑bound nucleus", "circular DNA", "membrane‑bound organelles", "70S ribosomes".
  • Show each point as a separate sentence or numbered point to match the 4‑mark structure.

Common mistakes

  • Confusing the size of ribosomes or omitting the 70S detail.
  • Claiming the cell has a nucleus or other organelles, or describing the DNA as linear.

Mark scheme (4 marks)

  1. Absence of a membrane-bound nucleus in the prokaryotic cell
  2. DNA is circular and not associated with histone proteins
  3. Absence of membrane-bound organelles in prokaryotic cells
  4. Prokaryotic ribosomes are smaller (70S) than eukaryotic ribosomes (80S)

Key terms in this question

electron microscope · circular DNA molecule · prokaryotic · eukaryotic · ribosomes

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