Gene expression and cancer (A-level only) — AQA A-Level Biology
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Gene expression and cancer (A-level only) explained
Gene expression and cancer (A-level only) covers 2 specification points, including The main characteristics of benign and… and Tumour suppressor genes and oncogenes abnormal….
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Open a point below for its explanation and revision guidance.
The main characteristics of benign and malignant tumours.
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).
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.
Two classes of gene normally keep cell division in balance. Proto-oncogenes code for proteins that stimulate division. A mutation can turn a proto-oncogene into an oncogene, whose protein is permanently active or overproduced, so the cell divides uncontrollably. Tumour suppressor genes (TSGs) code for proteins that slow the cell cycle, repair DNA or trigger apoptosis. A mutation can inactivate a TSG, so damaged cells survive and divide. These outcomes also arise epigenetically: hypermethylation of a TSG's promoter prevents its transcription, while reduced methylation of an oncogene increases it. Increased oestrogen concentrations can also stimulate mitosis in some breast cancers.
Your focus
- 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.
Show all 6 objectives
- 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.
Gene expression and cancer (A-level only) exam tips
Marking Points
- 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
- 💡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
- 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