Explain how the structure of the nucleus is related to its role in controlling cell activity.

IB DP Biology Standard Level (2023 syllabus) — A2.2 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)

The nuclear envelope is a double‑membrane that encloses the nucleus, separating transcription from translation and protecting DNA from cytoplasmic enzymes.
Nuclear pores in the envelope allow selective passage of molecules such as mRNA and proteins between the nucleus and cytoplasm, enabling the nucleus to regulate gene expression.
The nucleus contains DNA organised into chromosomes, which carry genes that code for proteins; by controlling which genes are transcribed, the nucleus determines the proteins produced and therefore cell activity.
The nucleolus within the nucleus synthesises ribosomal RNA (rRNA) and assembles ribosome subunits, which are then exported through nuclear pores to carry out protein synthesis in the cytoplasm.

Examiner tips

  • Use the exact phrases from the mark scheme (e.g., "double‑membrane", "selective passage", "DNA organised into chromosomes").
  • Show the link between structure (envelope, pores, nucleolus) and function (transcription, export, ribosome assembly).
  • Keep each point short and to the point – 4 marks, 1 point each.

Common mistakes

  • Confusing the nucleus with the cytoplasm or ribosomes; forgetting the role of the nuclear envelope.
  • Using vague terms like "DNA" without mentioning chromosomes or genes.

Mark scheme (4 marks)

  1. The nuclear envelope is a double membrane that encloses the nucleus, separating transcription from translation and protecting DNA from damage by cytoplasmic enzymes.
  2. Nuclear pores in the nuclear envelope allow selective passage of molecules such as mRNA and proteins between the nucleus and cytoplasm, enabling the nucleus to regulate gene expression.
  3. The nucleus contains DNA organised into chromosomes, which carry genes that code for proteins; by controlling which genes are transcribed, the nucleus determines the proteins produced and therefore cell activity.
  4. The nucleolus within the nucleus synthesises ribosomal RNA (rRNA) and assembles ribosome subunits, which are then exported through nuclear pores to carry out protein synthesis in the cytoplasm.

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