A scientist is investigating DNA replication. Explain why it is important that DNA replication produces an exact copy of the original DNA, and describe the role of the nucleus in this process.

Eduqas A-Level Biology — 1.5 Nucleic acids and DNA replication · Explain · 5 marks · View as Markdown

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

Before a eukaryotic cell can divide by mitosis, the DNA inside the cell must be replicated so that each daughter cell receives a complete set of genetic information.

Model answer (5 marks)

DNA replication must produce an exact copy of the original DNA so that each daughter cell receives the same genetic information as the parent cell. The DNA contains the complete set of genes that code for all proteins; any change in the sequence could lead to a faulty protein or a loss of function, which may cause the cell to malfunction or die. Therefore, the fidelity of replication is essential for normal cell function and for the maintenance of the organism.
The nucleus is the site of DNA replication. It houses the DNA and all the enzymes (DNA polymerases, helicases, ligases, etc.) required for the process. Within the nucleus the double‑helix is unwound, each strand serves as a template, and new complementary strands are synthesised, restoring the complete double‑helix structure in each daughter nucleus.

Thus, exact replication preserves the genetic code and the double‑helix structure, and the nucleus provides the environment and machinery for this precise copying.

Examiner tips

  • Use the word "exact" or "fidelity" to show understanding of replication accuracy.
  • Mention the nucleus as the site of replication and list key enzymes.
  • Explain the consequence of mutations for cell function.
  • Show that DNA is a double‑stranded polymer and that both strands are copied.

Common mistakes

  • Failing to state that the nucleus is the site of replication.
  • Using vague terms like "copy" without emphasising fidelity.
  • Omitting the link between mutations and cell dysfunction.

Mark scheme (5 marks)

  1. DNA replication occurs in the nucleus
  2. DNA contains all the genetic information / codes for proteins
  3. Each daughter cell must receive an identical copy of the DNA so that they have the same genetic information as the parent cell
  4. If the copy is not exact, the cell may not function correctly / mutations could occur
  5. DNA is a polymer made of two strands twisted into a double helix, and replication ensures both new cells receive this complete double helix structure

Key terms in this question

DNA replication · nucleus

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