A tumour is a mass of cells that forms when the cell cycle is no longer controlled. Explain how a fault in the cell cycle leads to the formation of a tumour, and describe what this means for the affected cells.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cancer is a non-communicable disease caused by changes in the cell that lead to uncontrolled growth and division. Normally, cells complete the cell cycle in a highly regulated way, producing two genetically identical daughter cells.
Model answer (5 marks)
A tumour forms when the normal control of the cell cycle is lost. The regulatory checkpoints that normally stop growth and division are bypassed, so cells grow and enter mitosis repeatedly without stopping. Each mitotic event produces two daughter cells, so the number of cells increases rapidly. As these cells keep dividing, they accumulate in a local area, forming a mass of cells – a tumour. Because the faulty gene or mutation is present in the original cell, every daughter cell is genetically identical to it, so the defect is passed on to all new cells.
Examiner tips
- Use the word ‘control’ and ‘bypass’ to show loss of regulation; mention checkpoints (G1, G2, M). Show the chain: loss of control → uncontrolled division → many daughter cells → tumour mass. End with ‘genetically identical’ to cover inheritance of the fault.
Common mistakes
- Mixing up mitosis with meiosis; not linking loss of control to unchecked division; forgetting that all daughter cells inherit the mutation.
Mark scheme (5 marks)
- The cell cycle normally controls when cells grow and divide, but a change/fault in the cell causes this control to be lost
- Cells undergo uncontrolled growth and division / divide repeatedly without stopping
- Mitosis produces large numbers of new daughter cells / cells accumulate in large numbers
- These cells form a mass / tumour as they continue to accumulate
- The daughter cells produced are genetically identical to the original changed/faulty cell, so the fault is passed on to all new cells
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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