How do changes in the expression of just a few genes turn a normal cell into a tumour cell?
The role of the increased and decreased expression of genes in the development of tumours. The roles of tumour suppressor genes and oncogenes in the development of tumours. The role of abnormal methylation of tumour suppressor genes and oncogenes in the development of tumours. The increased exposure to oestrogen can increase the chances of developing some breast cancers. Benign and malignant tumours can be distinguished by their characteristics.
An exam-focused answer to the AQA A-Level Biology 3.8 dot point on cancer. Explains how oncogenes and tumour suppressor genes control the cell cycle, how mutation and abnormal methylation lead to tumours, the difference between benign and malignant tumours, and the role of oestrogen in some breast cancers.
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What this dot point is asking
AQA wants you to explain how cancer results from changes in gene expression that disturb the cell cycle. You need the normal and mutated roles of oncogenes and tumour suppressor genes, the contribution of abnormal methylation, the difference between benign and malignant tumours, and the link between oestrogen and some breast cancers.
Cancer as uncontrolled cell division
Cancer arises when cells divide uncontrollably to form a tumour. The cell cycle is normally tightly controlled by genes. Mutations or abnormal expression of these genes upset the balance between cell division and cell death, so cells divide too often. Two groups of genes are central.
Oncogenes
When a proto-oncogene mutates into an oncogene, it stimulates cell division continuously, even without a signal. The cell may behave as if a growth factor is permanently present, so it divides uncontrollably. Oncogenes therefore work by increased gene expression (the gene is too active).
Tumour suppressor genes
A tumour suppressor gene acts as a brake. If it is inactivated by mutation, the brake is lost: faulty cells are not stopped or destroyed, so they continue to divide. Tumour suppressor genes therefore cause cancer through decreased gene expression (the gene is switched off).
Abnormal methylation
Cancer can also be driven by epigenetic change, especially abnormal methylation:
- Hypermethylation (increased methylation) of the promoter of a tumour suppressor gene stops transcription factors binding, silencing the gene. The control protein is not made, so cell division is uncontrolled.
- Hypomethylation (decreased methylation) of an oncogene can switch it on or increase its expression, stimulating cell division.
Because these are epigenetic changes, the DNA base sequence is unchanged, so abnormal methylation is potentially reversible, which is why it is a target for cancer drugs.
Benign versus malignant tumours
| Feature | Benign | Malignant |
|---|---|---|
| Growth | Slower | Faster |
| Spread | Stays in one place (localised) | Invades and spreads to other tissues (metastasis) |
| Boundary | Often surrounded by a capsule | No capsule; invades surrounding tissue |
| Cells | More like normal cells, specialised | Less specialised (de-differentiated) |
| Danger | Usually less dangerous unless pressing on an organ | Life-threatening; forms secondary tumours |
Oestrogen and breast cancer
Oestrogen can act as a transcription factor that stimulates cell division in breast tissue. Increased exposure to oestrogen can therefore increase the risk of some breast cancers:
- Many breast tumour cells have many oestrogen receptors. Oestrogen binds, enters the nucleus and switches on genes that drive cell division, so the tumour grows faster.
- A controlling cell could already carry a mutation; the extra stimulation by oestrogen makes the faulty cells divide more, increasing the chance of a tumour developing.
- Increased lifetime exposure to oestrogen (for example earlier puberty, later menopause) is linked to higher risk, and some treatments work by blocking the oestrogen receptor.
Common mistakes
Try this
Q1. Explain why a single mutation is often not enough to cause cancer. [2 marks]
- Cue. The cell cycle is controlled by several genes; usually mutations in more than one gene (for example an oncogene and a tumour suppressor gene) are needed before division becomes fully uncontrolled.
Q2. A breast tumour is described as oestrogen-receptor positive. Suggest why a drug that blocks oestrogen receptors could slow its growth. [2 marks]
- Cue. Blocking the receptor prevents oestrogen forming the active transcription factor complex, so the genes that stimulate cell division are not switched on, slowing tumour growth.
Q3. Distinguish between a benign and a malignant tumour. [2 marks]
- Cue. A benign tumour grows slowly and stays localised; a malignant tumour grows faster, invades surrounding tissue and spreads (metastasises) to form secondary tumours.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 AQA Paper 24 marksExplain how a mutation in a proto-oncogene and a mutation in a tumour suppressor gene can both lead to the development of a tumour.Show worked answer →
A 4-mark answer needs the normal role and the mutated effect of each gene.
Proto-oncogene / oncogene.
- A proto-oncogene normally stimulates cell division (e.g. by coding for a growth factor or its receptor).
- A mutation turns it into an oncogene that is permanently active or over-expressed, so cell division is stimulated continuously and uncontrollably.
Tumour suppressor gene.
- A tumour suppressor gene normally slows the cell cycle or causes cell death (apoptosis) in faulty cells.
- A mutation inactivates it, so this control is lost and faulty cells continue to divide.
In both cases the rate of cell division increases and a tumour forms. Markers reward the contrast: oncogenes are over-active, tumour suppressor genes are inactivated.
2021 AQA Paper 23 marksExplain how increased methylation of a tumour suppressor gene can lead to the formation of a tumour.Show worked answer →
A 3-mark answer should link methylation to loss of the gene's control function.
- Increased methylation of the tumour suppressor gene's promoter prevents transcription factors / RNA polymerase binding.
- The gene is not transcribed, so its protein (which normally slows the cell cycle or triggers apoptosis) is not made.
- Cell division is no longer controlled, so cells divide uncontrollably and a tumour forms.
Markers reward the chain: methylation, gene silenced, loss of control protein, uncontrolled division.
Related dot points
- Gene mutations involve a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include addition, deletion, substitution, inversion, duplication and translocation of bases. The degenerate nature of the genetic code means that some substitutions do not change the amino acid coded for. Some gene mutations change only one triplet code; the position of a deletion or addition mutation within a gene is important. Mutagenic agents increase the rate of mutation. Stem cells are unspecialised cells capable of dividing and differentiating, and are described as totipotent, pluripotent, multipotent or unipotent.
An exam-focused answer to the AQA A-Level Biology 3.8 dot point on gene mutations and cell specialisation. Covers substitution, deletion, addition, inversion, duplication and translocation, the role of the degenerate code, mutagenic agents, and totipotent, pluripotent, multipotent and unipotent stem cells with their uses.
- The control of transcription by specific transcription factors which move from the cytoplasm to the nucleus. In eukaryotes, transcription of target genes can be stimulated or inhibited when specific transcription factors bind to DNA. The effect of oestrogen on gene transcription. The control of translation of mRNA by RNA interference using small interfering RNA (siRNA), which can lead to the breakdown of mRNA or block its translation.
An exam-focused answer to the AQA A-Level Biology 3.8 dot point on regulating gene expression. Explains how specific transcription factors control transcription, how oestrogen acts as a transcription factor complex, and how siRNA in RNA interference breaks down or blocks mRNA to control translation.
- Epigenetic control of gene expression in eukaryotes. Epigenetics involves heritable changes in gene function, without changes to the base sequence of DNA. These changes are caused by changes in the environment that inhibit transcription by increased methylation of DNA or decreased acetylation of associated histones. The increased methylation of DNA and decreased acetylation of histones can inhibit transcription. Epigenetic changes can be inherited and have a role in the development of disease.
An exam-focused answer to the AQA A-Level Biology 3.8 dot point on epigenetics. Explains how increased DNA methylation and decreased histone acetylation inhibit transcription without changing the base sequence, how these heritable changes respond to the environment, and their role in disease such as cancer.
- Recombinant DNA technology involves transferring fragments of DNA from one organism, or species, to another. Because the genetic code is universal, the transferred DNA can be translated in the recipient. Fragments of DNA can be produced by conversion of mRNA to complementary DNA using reverse transcriptase, by using restriction endonucleases to cut a fragment containing the desired gene, and by creating the gene in a gene machine. DNA fragments can be amplified using in vivo techniques involving vectors and the use of the polymerase chain reaction (PCR) in vitro. The use of recombinant DNA technology to produce transformed organisms that benefit humans, and the use of gene therapy.
An exam-focused answer to the AQA A-Level Biology 3.8 dot point on recombinant DNA technology. Covers isolating a gene with reverse transcriptase, restriction endonucleases and the gene machine, amplification by in vivo cloning with vectors and in vitro PCR, transformation and marker genes, and the principles of gene therapy.
- The use of labelled DNA probes that can be used to locate specific genes by complementary base pairing (DNA hybridisation). The use of these techniques in medical diagnosis. The principles of DNA sequencing and the development of high-throughput sequencing. Genetic fingerprinting and its use in determining genetic relationships and the genetic variability within a population, based on variable number tandem repeats (VNTRs), separated by size using gel electrophoresis.
An exam-focused answer to the AQA A-Level Biology 3.8 dot point on gene probes, sequencing and genetic fingerprinting. Explains labelled DNA probes and hybridisation, their use in medical diagnosis, the principles of DNA sequencing, and how VNTRs and gel electrophoresis produce a genetic fingerprint for forensics and relationship testing.