Gene expression and cancerAQA A-Level Biology: Revision notes
Section 1
Benign and malignant tumours
A tumour is a mass of cells formed by uncontrolled division.
Benign tumours: usually grow slowly, are often surrounded by a capsule, do not spread to other tissues, cells resemble normal cells and they are usually not life-threatening.
Malignant tumours: grow rapidly, have no capsule, invade nearby tissue and metastasise (cells spread in blood or lymph to form secondary tumours), cells have large, dark nuclei and irregular shapes, and are life-threatening.
Section 2
Oncogenes and tumour suppressor genes
Cell division is controlled by two types of gene.
Proto-oncogenes code for proteins that stimulate division. A mutation can turn one into an oncogene, whose protein is active all the time or made in excess, so cells divide continually.
Tumour suppressor genes code for proteins that slow division, repair DNA or cause damaged cells to die. A mutation that inactivates the gene means cell division is not controlled.
Oncogene = accelerator stuck on. Tumour suppressor gene lost = brakes not working.
Section 3
Abnormal methylation
Abnormal methylation can contribute to tumour development. Increased methylation of a tumour suppressor gene stops it being transcribed, so its protein is not made. Decreased methylation of an oncogene can lead to the gene being transcribed more, so too much protein is made. These are epigenetic changes: the base sequence is not altered.
Section 4
Oestrogen and breast cancer
Oestrogen binds to receptors in breast cells and stimulates transcription of genes for cell division. Increased oestrogen concentrations mean more cell division, so more DNA replication and a greater chance of mutations. This contributes to the development of some breast cancers, whose cells have oestrogen receptors.
Section 5
Genetic and environmental factors
Evidence for cancer risk comes from correlations. Compare the figures, for example the risk in carriers of a mutated allele with that in non-carriers. Genetic factors include inherited alleles of tumour suppressor genes such as BRCA1. Environmental factors include smoking, alcohol, diet and radiation. A correlation does not prove cause: look for other variables, sample size and whether the study was controlled.
Do not say a correlation proves one factor caused the other.
Section 6
Prevention, treatment and cure
Understanding oncogenes and tumour suppressor genes allows:
- screening for mutated alleles, so at-risk people can change lifestyle, be monitored or have preventative surgery
- drugs that block oncogene proteins or oestrogen receptors
- gene therapy to insert a functional tumour suppressor allele
- tumour DNA tests to choose the most effective treatment
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Gene expression and cancer
- A surgeon removes lumps from two different patients. Lump A grew slowly, was surrounded by a capsule and contained cells that looked similar to normal cells. Lump B grew rapidly, had no capsule and contained cells with large dark nuclei and very different sizes. Cells identical to those in lump B were later found in nearby lymph nodes.Explain why lump B is more life-threatening than lump A.2 marks
- Cell division in a tissue is controlled by two genes. The protein from gene X stimulates cells to divide. The protein from gene Y slows cell division and can cause a damaged cell to die. In tumour 1 the gene X protein was active all the time. In tumour 2 the promoter region of gene Y was heavily methylated.Explain how the changes in tumour 1 and tumour 2 could each lead to uncontrolled cell division.2 marks
- Some breast cancers grow faster when the concentration of oestrogen is high. The cells of these tumours have oestrogen receptors. Researchers have designed a drug with a shape similar to oestrogen that binds to the receptor but does not activate it.Explain how an increased concentration of oestrogen can contribute to the development of some breast cancers.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).