AQA A-level Biology: Regulation of Gene Expression and Cancer (7402) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions follow AQA A-level Biology (7402), sections 3.8.2.2, Regulation of transcription and translation, and 3.8.2.3, Gene expression and cancer, in the AQA AS and A-level Biology specification for first teaching 2015. Both sections are A-level only. Explanations are in the study guide and the revision notes. Course pages: AQA A-level Biology hub, the printable checklist and the free diagnostics. All data are invented for practice. The marks shown are for practice and are not an official tariff.

Questions

1. Define epigenetics, and explain why a change in the methylation of DNA is not a gene mutation. [3]

2. Explain how the steroid hormone oestrogen causes the transcription of a target gene in a cell that has the right receptor. [5]

3. Explain how decreased acetylation of histones inhibits the transcription of a gene. [4]

4. Four cell lines were tested for methylation of the promoter of gene K and for the amount of mRNA from gene K.

Cell line	Promoter sites methylated (%)	mRNA (arbitrary units)
A	5	210
B	12	190
C	81	14
D	88	9

(a) Calculate the mean mRNA level for lines with low methylation (A and B) and for lines with high methylation (C and D). [2] (b) Calculate the percentage fall in mean mRNA from low to high methylation. [1] (c) Describe the relationship shown by the table. [1] (d) Explain why the table does not prove that methylation lowers the mRNA level, and suggest one investigation that would test the idea. [2]

5. A researcher wants to reduce the translation of an mRNA that contains the sequence GAUCGGAUUCAG. (a) Write the sequence of the siRNA strand that would pair with this region. [2] (b) Explain how the siRNA stops the protein being made. [3]

6. A tumour grows from one cell. (a) State three differences between a benign and a malignant tumour. [3] (b) The cells of a tumour divide once every 24 hours and none die. Calculate the number of cells formed from one cell after 10 days. [2]

7. (a) Explain how a mutation in a proto-oncogene can lead to the formation of a tumour. [3] (b) Explain how a tumour could form because of a change to a tumour suppressor gene that does not involve a change in the base sequence. [3]

8. In a follow-up study, 600 women with a high lifetime exposure to oestrogen and 700 women with a low exposure were observed. Breast cancer developed in 54 of the high-exposure women and 21 of the low-exposure women. (a) Calculate the percentage of each group that developed breast cancer. [2] (b) Calculate how many times greater the risk is in the high-exposure group. [1] (c) Explain how raised oestrogen concentrations could increase the chance of some breast cancers. [3] (d) Give two reasons the data are not enough to show that oestrogen causes the difference. [2]

9. A variant allele of a tumour suppressor gene was found in 90 people. Cancer developed in 18 of them. In 2400 people without the variant, cancer developed in 120. (a) Calculate the percentage in each group who developed cancer. [2] (b) Calculate the relative risk for people with the variant. [1] (c) A newspaper states that “everyone with this variant will get cancer”. Evaluate the statement using the data. [4]

10. In a study of twins, 80 identical pairs and 80 non-identical pairs were each selected because one twin had a certain cancer. The other twin also had it in 30 identical pairs and 12 non-identical pairs. (a) Calculate the percentage of pairs in which both twins had the cancer for each type of twin. [2] (b) What do the figures suggest about genetic and environmental influences? [2] (c) Suggest two reasons why not all the identical pairs had the cancer in both twins. [2]

11. A cancer cell line over-expresses an oncogene, and its promoter is hypomethylated. Each of two flasks was seeded with 1.2 × 10⁵ cells. One flask received siRNA matching the oncogene mRNA. After 48 hours the control flask held 4.8 × 10⁵ cells and the siRNA flask 1.9 × 10⁵ cells. (a) Calculate how many times the cell number increased in each flask, and the percentage by which the final siRNA count is lower than the control. [3] (b) Explain how the siRNA reduced cell numbers. [3] (c) Explain how hypomethylation could lead to over-expression of the oncogene. [2] (d) Suggest two ways the investigation could be improved. [2]

Answers

1. Epigenetics is a heritable change in gene function [1] without a change in the base sequence of DNA [1]. Methylation adds groups to DNA but no base is substituted or deleted, so it is not a mutation [1]. [3] Examiner insight: use the phrase “base sequence” for the second part; saying only that it is “not permanent” does not distinguish it from a mutation.

2. Oestrogen is lipid-soluble and diffuses through the cell-surface membrane [1]. It binds to a complementary receptor in the cytoplasm [1]. The receptor changes shape and the complex acts as a transcription factor [1]. The complex enters the nucleus through a nuclear pore and binds to a specific base sequence near the gene [1]. This lets RNA polymerase bind, so transcription is stimulated [1]. [5] Examiner insight: give the sequence of events in order and say what moves into the nucleus; “oestrogen switches on the gene” gives the result without the mechanism.

3. Histones are positively charged and attract the negatively charged phosphate groups of DNA [1]. Acetylation reduces this attraction, so decreased acetylation means the histones hold DNA more strongly [1]. DNA is more tightly wound [1]. Transcription factors and RNA polymerase cannot reach the gene, so transcription is reduced [1]. [4] Examiner insight: link the change in charge to the strength of binding; reversing the direction (acetylation increasing the hold) loses the explanation.

4. (a) Low: (210 + 190) / 2 = 200 [1]; high: (14 + 9) / 2 = 11.5 [1]. [2] (b) (11.5 − 200) / 200 × 100 = −94.25, so a fall of 94% [1]. [1] (c) Higher methylation is associated with lower mRNA (a negative correlation) [1]. [1] (d) Correlation does not show cause, and only four lines were studied [1]. Remove the methyl groups from lines C and D, for example with a drug, and see whether the mRNA level rises [1]. [2] Examiner insight: calculate percentage change from the low-methylation value and quote the means; an answer to (d) needs a stated test, not only “do more repeats”.

5. (a) Pair A with U and G with C [1]: CUAGCCUAAGUC [1]. [2] (b) The siRNA strand binds to the mRNA by complementary base pairing [1]. The mRNA is then hydrolysed (broken down) [1], so it cannot be translated and the polypeptide is not made [1]. [3] Examiner insight: a partner strand of RNA contains U, not T; and the stage affected is translation, not transcription.

6. (a) Any three from: benign grows slowly, malignant faster [1]; benign has a distinct border, malignant invades surrounding tissue [1]; benign does not spread, malignant forms secondary tumours by metastasis [1]. A further valid difference is that malignant cells are often less specialised, with abnormal nuclei. [3] (b) 10 days is 10 divisions [1]; 2¹⁰ = 1024 cells [1]. [2] Examiner insight: state both sides of each difference; for (b) count the divisions first and then use the power of two.

7. (a) The mutation changes the proto-oncogene into an oncogene [1]. Its product is made in excess or is permanently active [1], so the cell divides without the normal signals and uncontrolled division forms a tumour [1]. [3] (b) Hypermethylation of the promoter of the tumour suppressor gene [1] stops it being transcribed, so its protein is not made [1]. Division is no longer slowed and DNA damage is not repaired, so cells divide uncontrollably [1]. [3] Examiner insight: name what each altered gene normally does, and say what is lost or gained; “the gene stops working” alone is not enough.

8. (a) High: 54 / 600 × 100 = 9.0% [1]; low: 21 / 700 × 100 = 3.0% [1]. [2] (b) 9.0 / 3.0 = 3.0 times [1]. [1] (c) Oestrogen binds to receptors in breast cells and stimulates transcription of genes for cell division [1]. More cell divisions mean more DNA replication [1], so more chances of a mutation in an oncogene or tumour suppressor gene [1]. [3] (d) It is an association only [1]; other factors such as age, family history or diet might differ between the groups [1]. [2] Examiner insight: divide by the lower-exposure risk for a ratio; for (d) name a variable that could differ rather than writing “other factors”.

9. (a) Variant: 18 / 90 × 100 = 20% [1]; no variant: 120 / 2400 × 100 = 5.0% [1]. [2] (b) 20 / 5.0 = 4.0 [1]. [1] (c) 72 of the 90 carriers (80%) did not develop cancer, so the statement is false [1]. The variant raises risk four-fold, so it is linked to cancer [1], but other genes, environment and epigenetic changes must also be involved [1]. Only 90 carriers were studied, so 20% is a less precise estimate than 5.0% from 2400 people [1]. [4] Examiner insight: quote the numbers that contradict the claim, then comment on sample size; a judgement without data does not evaluate.

10. (a) Identical: 30 / 80 × 100 = 37.5% [1]; non-identical: 12 / 80 × 100 = 15% [1]. [2] (b) The rate is higher in identical twins, who share all their alleles (non-identical share about half), suggesting a genetic influence [1]. Because it is well below 100%, environment also affects risk [1]. [2] (c) Different environments or lifestyles [1]; different epigenetic changes or chance mutations in body cells [1]. [2] Examiner insight: compare the two figures in the same sentence and then explain what the gap from 100% means.

11. (a) Control 4.8 / 1.2 = 4.0 times [1]; siRNA 1.9 / 1.2 = 1.6 times [1]; (4.8 − 1.9) / 4.8 × 100 = 60% lower [1]. [3] (b) siRNA pairs with the oncogene mRNA by complementary bases [1], and the mRNA is broken down [1], so less of the protein that stimulates division is made, so cells divide less [1]. [3] (c) Fewer methyl groups on the promoter mean transcription factors and RNA polymerase can bind more easily [1], so more mRNA of the oncogene is made [1]. [2] (d) Add a control with siRNA that does not match the gene [1]; use replicate flasks or more time points [1]. [2] Examiner insight: in (d) say what the improvement would show; a control with a non-matching siRNA tests whether the effect is specific to this gene.

Where marks are usually lost

• Saying the hormone, rather than the hormone-receptor complex, enters the nucleus and binds to DNA.
• Writing that methylation or acetylation changes the base sequence.
• Getting the direction of acetylation wrong: less acetylation means more tightly bound DNA.
• Placing RNA interference at transcription, or writing T in an RNA strand.
• Dividing the change by the final figure when the starting figure should be the divisor.
• Stating “oncogenes cause cancer” without saying what the normal gene does.
• Treating a correlation as proof, or giving no evaluation of sample size.
• Describing a tumour as cancerous when it is benign.

Next steps

• Revision notes
• Study guide
• AQA A-level Biology hub
• Printable checklist
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Official syllabus

AQA AS and A-level Biology specification (7401 and 7402), for first teaching 2015, sections 3.8.2.2 and 3.8.2.3 (version 1.5), published by AQA.