Gene expression and cancer (A-level only)
AQA
A-Level
A tumour is a mass of cells produced by uncontrolled mitosis. Benign and malignant tumours differ in ways you can be asked to compare. Benign tumours grow slowly and remain compact, usually surrounded by a capsule of fibrous tissue. Their cells are differentiated, resembling normal cells with normal-sized nuclei. Because they do not invade, surgical removal is usually a permanent cure. They can cause harm by pressing on an organ, causing blockages, or releasing excess hormones. Malignant tumours grow rapidly, are irregular in shape and lack a capsule, so they invade surrounding tissue. Their cells are undifferentiated, with large, dark nuclei, and can spread (metastasise).
6
Objectives
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Exam Tips
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Pitfalls
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Key Terms
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Mark Points
Subtopics in this area
The main characteristics of benign and malignant tumours.
The role of the following in the development of tumours: tumour suppressor genes and oncogenes abnormal methylation of tumour suppressor genes and oncogenes increased oestrogen concentrations in the development of some breast cancers.
Gene expression and cancer (A-level only) Revision Guide
Learning Objectives
What you need to know and understand
- Compare benign and malignant tumours using growth rate, capsule, differentiation and ability to spread.
- Explain why metastasis makes a malignant tumour far harder to treat than a benign one of the same size.
- Identify, from a description or micrograph, whether a tumour is likely to be benign or malignant and justify the choice.
- Explain how a single mutation in a proto-oncogene produces a protein that drives division without a growth factor.
- Describe two different ways, one genetic and one epigenetic, in which a tumour suppressor gene can stop producing its protein.
- Suggest how a raised oestrogen concentration increases the rate of mitosis in breast tissue, referring to transcriptional factors.
Marking Points
Key points examiners look for in your answers
- one mark for a benign tumour growing slowly and a malignant tumour growing rapidly, both by uncontrolled mitosis
- one mark for benign cells being differentiated, while malignant cells are undifferentiated with large, dark nuclei
- one mark for the benign tumour being surrounded by a capsule so it stays in one place and does not invade surrounding tissue
- one mark for malignant cells breaking off and spreading in the blood or lymph to form secondary tumours, that is metastasis
- one mark for benign tumours usually being cured by removal whereas malignant tumours frequently recur
- one mark for a mutation converting a proto-oncogene into an oncogene whose protein is permanently active or overproduced
- one mark for the outcome being rapid or uncontrollable cell division
- one mark for a mutated or hypermethylated tumour suppressor gene producing no protein or a non-functional protein, so the cell cycle is not stopped
- one mark for increased methylation of DNA inhibiting transcription, and decreased methylation stimulating transcription
- one mark for increased oestrogen concentration stimulating transcription of genes that drive mitosis in breast tissue
Examiner Tips
Expert advice for maximising your marks
- 💡Answer comparison questions in pairs: name the feature, then give both tumour types in the same sentence.
- 💡Use the terms differentiated, capsule and metastasis when comparing tumour types.
- 💡If the question gives a micrograph, comment on nucleus size and cell shape rather than guessing from the caption.
- 💡Both routes end in the same outcome - rapid or uncontrollable cell division - so make that your final point.
- 💡Decide first whether the question is about a mutation or about methylation, as the mechanisms differ.
- 💡When discussing oestrogen, mention that it can stimulate transcription of genes that lead to cell division.
Common Mistakes
Pitfalls to avoid in your exam answers
- stating that benign tumours are harmless, when they can compress an organ, cause blockages or secrete excess hormone
- saying benign tumours 'are not cancerous' without giving a structural or behavioural difference
- describing spread vaguely as 'moving through the body' without naming the blood or lymph
- writing that tumour cells divide by meiosis rather than mitosis
- writing that the oncogene produces no protein or a non-functional protein, when an oncogene's product is overactive or overproduced
- reversing the methylation directions and stating that tumour suppressor genes are hypomethylated in cancer
- answering 'growth' instead of cell division or mitosis
- saying meiosis rather than mitosis when describing the division of tumour cells