8.2The Control of Gene Expression (A Level only)
Regulation of Gene Expression (A Level only) — AQA A Level Biology 7402 Revision Notes & Topical Questions
15 min read 22 parts Lesson 6 of 34 Unit quiz
Stem Cells and Potency — AQA A LEVEL BIOLOGY 7402 notes: Define the levels of potency of stem cells and evaluate the uses of stem cells in treating disease.A stem cell is a cell that can divide by mitosis an unlimited number of times.
- Advantages of adult stem cells
- Worked example 1
- Worked example 2
- Worked example 3
- Totipotent
- Pluripotent
Section 1 of 22Stem cells and differentiation
A stem cell is a cell that can divide by mitosis an unlimited number of times.
- such as a blood cell or a muscle cell, by differentiation.
Section 2 of 22Levels of potency
| Type | Description |
|---|---|
| Totipotent | Early embryonic cells that can differentiate into any cell type, including the extra-embryonic cells that make up the placenta |
| Pluripotent | Embryonic stem cells that can differentiate into any cell type found in an embryo (but not extra-embryonic cells); can divide an unlimited number of times and keep replacing themselves; can be used in treating human disorders |
| Multipotent | Adult stem cells that can differentiate into a limited range of cell types (e.g. bone marrow stem cells into blood cells) |
| Unipotent | Adult cells that can differentiate only into their own lineage, one specific cell type, but retain the ability to self-renew; found in specific organs or tissues |
Learn the levels of potency and the range of cell types each can differentiate into.
Section 3 of 22Uses and ethics of stem cells
Because of their ability to differentiate into multiple cell types, stem cells have huge potential in treating disease and producing transplant tissues and organs. The embryos used for research are specifically donated for this purpose by couples who have undergone IVF, and many countries impose strict regulations on the use of embryonic stem cells.
Advantages of adult stem cells
- no embryo destruction, as they are harvested from consenting adults, so there are fewer ethical issues;
- a lower risk of immune rejection if from the same patient.
- Many people donate bone marrow to help treat leukaemia patients, but donor and recipient need to be a close match in blood type and other body antigens.
Section 4 of 22Choosing a source of stem cells
Compare sources by asking three questions
- how many cell types can they form, how easy are they to collect, and what are the ethical and immune risks?
- Embryonic stem cells give wide potency but raise ethical objections;
- adult stem cells have fewer ethical problems but are less versatile and harder to isolate in large numbers;
- iPS cells combine wide potency with the patient's own genome but are costly and less efficient to make.
Section 5 of 22Worked examples
Distinguish between totipotent and pluripotent cells. [2]2 marks
Totipotent cells can form any cell type including placenta cells; pluripotent cells form any cell type of the embryo but not extra-embryonic cells.
Give two advantages of using adult stem cells rather than embryonic stem cells. [2]2 marks
No embryo is destroyed, so fewer ethical objections; lower risk of immune rejection if taken from the same patient.
Explain why bone marrow donors must be a close match to the patient. [2]2 marks
Surface antigens differ between people; a close match reduces the chance that the immune system of the recipient rejects the transplanted cells.
Section 6 of 22Induced pluripotent stem (iPS) cells
iPS cells are made by converting adult somatic cells (e.g. skin cells) into pluripotent cells using specific transcription factors. Advantages: they are tailored to the individual (personalised medicine), with a lower risk of immune rejection, and no embryos are destroyed. Disadvantages: low efficiency and high cost, since only a small fraction of treated cells become pluripotent and there are high costs from specialised transcription factors and careful cell culture conditions.
Section 7 of 22Producing tissue cultures from cauliflower explants
Cauliflower is used because it is comprised mostly of actively dividing cells and can withstand being handled.
- which ensures that only cauliflower cells are present;
- remove the explant using sterilised forceps and add it to a container of agar growth medium.
Section 8 of 22Regulation of transcription
Eukaryotes use transcription factors, proteins that help control transcription, to regulate gene expression. They either activate or repress the transcription of a particular gene: they bind to a specific DNA region near the gene (promoter) and either help RNA polymerase to bind or block it.
Exampleoestrogen is involved in controlling the female fertility cycle and also stimulates sperm production in males.
- forming a hormone-receptor complex ; (3) the complex enters the nucleus and binds to specific DNA sequences; (4) it stimulates (or inhibits) transcription of the target genes,
- (a transcription factor)
- so more (or less) of the protein is made.
Section 9 of 22Evaluating data about gene expression
Gene expression is the process by which genetic information is used to synthesise proteins or functional RNA. Investigations often measure levels of mRNA or protein under different conditions to determine how genes are regulated; higher mRNA or protein suggests increased expression.
Section 10 of 22Worked examples
Evaluate the use of iPS cells in treating disease. [4]4 marks
Advantages: personalised, low risk of rejection, no embryos used. Disadvantages: inefficient conversion and high cost.
Explain why the cauliflower explant is sterilised. [2]2 marks
To ensure that only cauliflower cells grow, with no bacteria or fungi competing for the nutrients.
Describe how oestrogen affects gene expression. [4]4 marks
It diffuses through the membrane; binds to a receptor; the complex enters the nucleus and binds to DNA; it activates transcription of the gene.
Section 11 of 22Separating genetic and environmental influences
Several kinds of study help to separate the effects of genes and environment on phenotype:
When evaluating data, consider the strength and direction of any correlations between variables, and whether the results suggest causation or merely association.
Section 12 of 22Epigenetics
Epigenetics involves changes in gene function without changes to the base sequence of DNA.
- which coil tightly around each other to form the chromosome structure.
| Modification | What happens | Effect on transcription |
|---|---|---|
| Acetylation of histones | Acetyl groups (COCH3) are added to lysine amino acids on histone proteins, so the DNA wraps less tightly around the histones | Chromatin is open; transcription factors and RNA polymerase can reach the gene, so transcription increases |
| DNA methylation | Methyl groups (CH3) are added to carbon atoms of cytosine bases in sequences containing many cytosine and guanine bases | Methylated bases attract proteins that bind to the DNA and inhibit transcription, so the gene is suppressed |
Remember that epigenetics involves heritable changes in gene function without changing the base sequence of DNA.
Section 13 of 22Epigenetics and disease
Epigenetic therapies aim to reverse epigenetic changes to restore normal gene expression.
- which inhibit DNA methylation so silenced genes can be expressed again; and histone deacetylase (HDAC) inhibitors,
- which prevent histone deacetylation,
- so acetylation is maintained and chromatin remains open,
- giving increased expression of beneficial genes.
- (for example those that induce apoptosis)
Section 14 of 22RNA interference (RNAi)
RNAi is a form of post-transcriptional gene regulation that occurs in the cytoplasm. It is sequence-specific silencing of gene expression and can therefore be very precise. In eukaryotes and some prokaryotes, translation of the mRNA produced from target genes can be inhibited by RNAi.
Section 15 of 22Worked examples
Describe how histone acetylation affects gene expression. [3]3 marks
Acetyl groups added to histones make DNA wrap less tightly; transcription factors and RNA polymerase can bind; transcription increases.
Explain how DNA methylation can silence a gene. [3]3 marks
Methyl groups added to cytosine bases attract proteins that bind to the DNA and prevent transcription factors and RNA polymerase from binding.
Explain what monozygotic twin studies show. [2]2 marks
They share identical genotypes, so differences in phenotype must be due to environmental influences or epigenetic factors.
Explain why epigenetic therapies are attractive for treating disease. [2]2 marks
Epigenetic changes are reversible, unlike mutations, so drugs such as azacitidine or HDAC inhibitors can restore normal expression.
Section 16 of 22How siRNA works
Short interfering RNA (siRNA) targets mRNA by complementary base pairing.
Uses
- siRNAs created against viral genetic material signal for its degradation and stop the virus using the host's cellular machinery to replicate;
- siRNAs can be used in cancer treatment by targeting oncogenes that have been expressed or upregulated.
Section 17 of 22Two types of tumour
Tumours are groups of abnormal cells that form lumps or growths. They can start in any of the cells in the body, and their characteristics depend on whether they are cancerous (malignant) or non-cancerous (benign).
| Feature | Benign | Malignant |
|---|---|---|
| Growth | Slow | Rapid |
| Boundaries | Enclosed (often in a capsule); stay in one place | Invade surrounding tissue; no clear boundary |
| Spread | Do not spread | Spread by metastasis, forming secondary tumours |
| Cells | Differentiated, close to normal | Abnormal, often undifferentiated, with large nuclei |
| Harm | Can cause blockages or pressure on the organ or surrounding organs; usually do not grow back when removed | Life-threatening |
Benign tumours include polyps found in the nose, colon and ovaries.
Section 18 of 22Tumour development
Cancer arises from uncontrolled mitosis due to mutations in genes regulating the cell cycle. Cancerous cells divide uncontrollably, forming a tumour. Several processes must occur for metastatic cancer to develop, as cells break away from the tumour, travel in blood or lymph and form secondary tumours elsewhere.
- A mutation such as an inversion or translocation can convert one into an oncogene, whose protein production cannot be switched off,
- so cells keep dividing.
- If they are mutated or silenced, division is not restrained.
- If proteins that induce apoptosis are also affected, tumours can form.
- because transcription factors are unable to bind;
- decreased methylation of proto-oncogenes can increase their expression.
Section 19 of 22Exam application
Apply knowledge of oncogenes, tumour suppressor genes and epigenetic and post-transcriptional mechanisms to explain cancer development, prevention and treatment.
Section 20 of 22Worked examples
Describe how siRNA prevents the production of a protein. [4]4 marks
siRNA binds to complementary base sequences on mRNA; the mRNA is hydrolysed in a protein complex; fragments cannot be translated, so the protein is not made.
Compare benign and malignant tumours. [3]3 marks
Benign tumours grow slowly, stay localised and do not metastasise; malignant ones grow rapidly, invade and spread to form secondary tumours.
Explain how hypermethylation of a tumour suppressor gene can lead to cancer. [3]3 marks
Methylation prevents transcription factors binding, silencing the gene; there is no protein to slow cell division or trigger apoptosis, so cells divide uncontrollably.
Section 21 of 22Key terms
| Term | Meaning |
|---|---|
| Stem cell | A cell that can divide unlimited times and differentiate. |
| Potency | Ability of stem cells to differentiate into other cell types. |
| Totipotent | Able to become any cell type, including extra-embryonic cells. |
| Pluripotent | Able to become any cell type of the embryo. |
| Multipotent | Able to become a limited range of cell types. |
| Unipotent | Able to become only one cell type. |
| Differentiation | Development of an unspecialised cell into a specialised one. |
| Term | Meaning |
|---|---|
| iPS cell | Induced pluripotent stem cell made from an adult somatic cell. |
| Explant | A piece of tissue used to start a tissue culture. |
| Transcription factor | Protein that activates or represses transcription. |
| Gene expression | Use of genetic information to make proteins or functional RNA. |
| Steroid hormone | Lipid-soluble hormone that diffuses into cells. |
| Term | Meaning |
|---|---|
| Epigenetics | Heritable changes in gene function without a change in DNA base sequence. |
| Epigenome | All chemical modifications of histones and DNA other than base changes. |
| Acetylation | Addition of acetyl groups to histones, loosening DNA and increasing transcription. |
| Methylation | Addition of methyl groups to cytosine in DNA, suppressing transcription. |
| HDAC inhibitor | Drug that stops histone deacetylation. |
| RNA interference | Sequence-specific silencing of gene expression after transcription. |
| Term | Meaning |
|---|---|
| siRNA | Short interfering RNA that binds to complementary mRNA and causes its breakdown. |
| Benign tumour | A non-cancerous tumour that does not spread. |
| Malignant tumour | A cancerous tumour that invades and spreads. |
| Metastasis | Spread of cancer cells to form secondary tumours. |
| Oncogene | Mutated proto-oncogene that stimulates uncontrolled cell division. |
| Tumour suppressor gene | Gene that slows cell division or triggers apoptosis. |
Section 22 of 22Exam technique
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