Explain how the structure of a nucleosome relates to the packaging of DNA in eukaryotic cells.

IB DP Biology Standard Level (2023 syllabus) — A1.2 Nucleic acids · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A nucleosome is a complex of about 147 base pairs of DNA wrapped 1.65 times around a histone octamer consisting of two each of H2A, H2B, H3 and H4.
This wrapping reduces the length of the DNA by roughly a factor of 10, allowing the long eukaryotic genome to be compacted.
Nucleosomes are linked by linker DNA and further coiled into a 30‑nm fibre and higher‑order structures, giving additional compaction.
This hierarchical packaging is essential because the eukaryotic genome is very large and must fit inside the nucleus.

Examiner tips

  • Mention the histone octamer and 147 bp of DNA; show the 10‑fold reduction; note the 30‑nm fibre; link to the need to fit DNA in the nucleus

Common mistakes

  • Confusing nucleosome with chromatin fibre; omitting the histone core; failing to state the 10‑fold compaction or the 30‑nm fibre

Mark scheme (4 marks)

  1. A nucleosome consists of DNA wrapped around a core of histone proteins
  2. This association with histones allows a long DNA molecule to be compacted/shortened considerably
  3. Nucleosomes are further coiled/folded into higher-order chromatin fibres, increasing the degree of compaction
  4. This packaging is necessary because eukaryotic cells contain very large amounts of DNA that must fit within the nucleus

Key terms in this question

nucleosome · eukaryotic

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