Explain how the structural features of the nucleus enable it to regulate gene expression whilst maintaining the integrity of DNA.

IB DP Biology Higher Level (2023 syllabus) — B2.2 Organelles and compartmentalisation · 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 nuclear envelope, a double‑membrane, isolates DNA from the cytoplasm, shielding it from cytoplasmic enzymes and ribosomes that could damage it.
2. Nuclear pores permit selective export of mRNA and other molecules, so only specific gene products reach the cytoplasm, allowing regulation of gene expression.
3. DNA is wrapped around histone proteins to form nucleosomes and chromatin; compaction into heterochromatin or relaxation into euchromatin controls access of transcription machinery, regulating which genes are expressed.
4. The nucleolus, a specialised nuclear region, synthesises rRNA and assembles ribosomal subunits; by controlling ribosome production the cell regulates the overall rate of protein synthesis.

Examiner tips

  • Use the exact terms: nuclear envelope, nuclear pores, chromatin, heterochromatin/euchromatin, nucleolus. Show how each structure contributes to protection or regulation. Keep each point short and focused to fit the 4‑mark format.

Common mistakes

  • Confusing the nuclear envelope with the plasma membrane. Failing to mention selective transport via nuclear pores. Mixing up the nucleolus with the nucleoplasm or cytoplasm.

Mark scheme (4 marks)

  1. The nuclear envelope (double membrane) physically separates DNA from the cytoplasm, protecting it from cytoplasmic enzymes/ribosomes that could damage it.
  2. Nuclear pores allow selective transport of mRNA (and other molecules) out of the nucleus into the cytoplasm, enabling regulation of which gene products reach the cytoplasm.
  3. DNA is associated with histone proteins forming chromatin/nucleosomes, allowing compaction and enabling gene expression to be regulated by condensing/relaxing chromatin (heterochromatin vs euchromatin).
  4. The nucleolus is a specialised region within the nucleus where rRNA is synthesised and ribosomal subunits are assembled, allowing the cell to regulate the rate of protein synthesis by controlling ribosome production.

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