Explain why DNA replication must occur before mitosis and describe one consequence if replication failed to occur.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
DNA replication must occur before mitosis because it creates a complete copy of the genome; without this step the cell would have only half the DNA needed for each daughter cell. During mitosis the duplicated chromosomes are separated so that each daughter cell receives one full set of chromosomes. If replication failed, the daughter cells would be missing essential genes, preventing the synthesis of vital proteins, disrupting cellular function and likely leading to cell death or failure of tissue growth.
Examiner tips
- State that replication precedes mitosis to provide a full set of chromosomes for each daughter cell
- Explain that mitosis distributes duplicated chromosomes
- Mention a specific consequence such as loss of essential genes or cell death
- Use clear, concise language and correct terminology
Common mistakes
- Confusing replication with mitosis or describing mitosis as the copying step
- Failing to link the lack of replication to missing genes or protein synthesis
- Using vague terms like ‘damage’ instead of ‘missing essential genes’
Mark scheme (4 marks)
- DNA replication ensures each daughter cell receives a complete / full copy of the genetic information
- Mitosis separates the duplicated chromosomes so one copy goes to each daughter cell; without prior replication there is insufficient DNA to distribute
- If replication failed, daughter cells would lack essential genes / would be missing genetic information
- This would prevent production of essential proteins / disrupt cellular function / cause cell death or inability of the organism to grow and repair tissues
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →