Most of a cell’s DNA is not translated (A-level only)
AQA
A-Level
A stem cell is an undifferentiated cell that can keep dividing by mitosis and can differentiate into other cell types. Totipotent cells are the most versatile of all: they can divide and produce every type of body cell and, in mammals, the extra-embryonic tissues such as the placenta, so a single totipotent cell can give rise to a whole organism. The fertilised egg is totipotent, and so are the cells produced by the first few divisions after fertilisation. Every one of these cells contains exactly the same DNA as every other cell in the body; what makes a specialised cell different is not which genes it has but which genes it expresses. As development proceeds, a totipotent cell transcribes only part of its DNA and translates only the mRNA produced from those genes, and the proteins produced determine the structure and function it takes on.
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Objectives
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Exam Tips
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Pitfalls
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Key Terms
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Mark Points
Subtopics in this area
Totipotent cells can divide and produce any type of body cell.
During development, totipotent cells translate only part of their DNA, resulting in cell specialisation.
Totipotent cells occur only for a limited time in early mammalian embryos.
Pluripotent cells are found in embryos; multipotent and unipotent cells are found in mature mammals and can divide to form a limited number of different cell types.
Pluripotent stem cells can divide in unlimited numbers and can be used in treating human disorders.
Unipotent cells, exemplified by the formation of cardiomyocytes.
Induced pluripotent stem cells (iPS cells) can be produced from adult somatic cells using appropriate protein transcription factors.
Students should be able to evaluate the use of stem cells in treating human disorders.
Most of a cell’s DNA is not translated (A-level only) Revision Guide
Learning Objectives
What you need to know and understand
- Define a totipotent cell and state what it can produce that a pluripotent cell cannot.
- Explain why all the body cells of one organism contain the same DNA yet have different structures.
- Give one example of a totipotent cell in a mammal, and explain why being able to divide, as well as to differentiate, is part of the definition.
- Explain how two cells with identical DNA can have different structures and functions.
- Describe the sequence from a gene being switched on to a protein determining a cell's specialisation.
- Explain why a cell's potency decreases as development proceeds due to gene silencing.
- State at which stages of mammalian development totipotent cells are present.
- Explain why the cells of a blastocyst's inner cell mass are pluripotent rather than totipotent.
- Explain, in terms of gene expression, why potency decreases as an embryo develops.
- Define pluripotent, multipotent and unipotent cells and identify where they are found.
- Explain how multipotent and unipotent cells differ in the number of cell types they can form.
- Describe how the restriction of cell potency relates to gene expression and epigenetic silencing.
- State the two properties of pluripotent stem cells that make them useful in treatment.
- Explain how supplying stem cells to damaged cardiac muscle could raise ventricular pressure.
- Interpret data from a stem cell treatment experiment, using the control to justify a conclusion about differentiation.
- Define unipotent cells and describe their restricted differentiation potential.
- State that the formation of cardiomyocytes is an example of unipotent cell activity.
- Compare unipotent cells with other types of stem cells based on their potency.
- Describe how an adult somatic cell is converted into an induced pluripotent stem cell.
- Explain how a protein transcription factor switches a gene on, referring to the promoter region and RNA polymerase.
- Give two advantages of using iPS cells rather than embryonic stem cells in treatment.
- Evaluate the use of stem cells for a named disorder, giving at least two points for and two against.
- Explain why using a patient's own adult stem cells or iPS cells reduces the risk of rejection, referring to antigens.
- Identify two limitations of evidence from an animal stem cell trial when it is applied to human patients.
Marking Points
Key points examiners look for in your answers
- one mark for totipotent cells being able to divide and produce any type of body cell
- one mark for stating that they can also form extra-embryonic or placental tissue, and therefore a whole organism
- one mark for stating that all body cells of an organism contain the same genes or DNA
- one mark for specialisation arising because only some genes are expressed, not because genes are lost
- one mark for a correct example, such as the zygote or the cells of the earliest embryo
- Totipotent cells can divide and differentiate into any cell type; they contain the full genome but translate only part of their DNA.
- Transcription factors bind to promoter regions, allowing transcription of only some genes into mRNA.
- Translation of this specific mRNA produces only certain proteins, which determine the structure and function of the specialised cell.
- Unexpressed genes are not lost or destroyed; they remain present but are switched off, for example by epigenetic methylation of the promoter region.
- As development proceeds, cell potency decreases because more genes become silenced.
- The zygote and the cells of the first few divisions in early mammalian embryos are totipotent.
- Totipotency is lost early in development, for example by the blastocyst stage.
- The inner cell mass of the blastocyst is pluripotent because it cannot form extra-embryonic or placental tissue.
- Stem cells in a mature mammal are multipotent or unipotent rather than totipotent.
- The loss of potency is explained by genes being switched off and no longer transcribed.
- Pluripotent cells are found in embryos and can differentiate into almost any type of body cell, but not extra-embryonic tissue such as the placenta.
- Multipotent cells are found in mature mammals and can divide to form a limited number of different cell types, such as bone marrow stem cells forming various blood cells.
- Unipotent cells are found in mature mammals and can divide to form only one specific cell type.
- The restriction of potency is explained by fewer genes remaining available for expression due to epigenetic silencing, rather than the loss of genetic material.
- A comparison of two potency types should state both what each can form and what it cannot.
- Pluripotent cells divide in unlimited numbers by mitosis (self-renewal).
- Pluripotent cells can differentiate into any type of body cell.
- A named lost cell type is replaced, e.g. cardiomyocytes replacing cells lost as scar tissue after a heart attack.
- The replaced cells are linked to restored function, e.g. stronger ventricular contraction and higher ventricular pressure.
- Differentiation is controlled by a signalling molecule binding to a receptor and switching on specific genes.
- Data are interpreted by comparing treated and control groups and linking any difference to differentiation and restored function.
- Unipotent cells are stem cells that can divide by mitosis and differentiate into only one specific type of cell.
- Cardiomyocytes (heart muscle cells) are the specific specification example of cells formed from unipotent cells.
- Unipotent cells differ from multipotent cells because they have a more restricted differentiation potential (one cell type versus multiple).
- Like all stem cells, unipotent cells retain the ability to self-renew alongside their capacity to differentiate.
- iPS cells are produced from adult somatic or body cells.
- Specific protein transcription factors are introduced or expressed, often using a viral vector.
- Transcription factors bind to the promoter region so that RNA polymerase can transcribe the gene into mRNA.
- The reprogrammed cell then behaves like an embryonic pluripotent cell, dividing indefinitely and able to form any body cell type.
- An advantage, such as the cells carrying the patient's own antigens so there is no immune rejection, or no embryo being destroyed.
- At least one point supporting the use of stem cells, with a named disorder such as macular degeneration.
- At least one point against, such as destruction of an embryo or the risk of tumour formation.
- A point about immune rejection, either as an advantage of the patient's own cells (adult or iPS) or a disadvantage of donated cells.
- A valid limitation of the evidence, such as work done on mice rather than humans, a small sample, or unknown long-term effects.
- A justified conclusion that refers back to the data or arguments given.
Examiner Tips
Expert advice for maximising your marks
- 💡Learn the four potency terms as a ladder, totipotent, pluripotent, multipotent, unipotent, with what each can and cannot form.
- 💡Answer how does a cell become specialised with gene expression, never with gene loss.
- 💡If asked what distinguishes totipotent from pluripotent, the placenta is the deciding point.
- 💡Detail the full pathway: gene, transcription, mRNA, translation, protein, cell structure and function.
- 💡When explaining why two cells from the same organism differ, always state first that they contain the exact same genes.
- 💡Name the developmental stage whenever you claim a cell is totipotent, because the mark often depends on the stage.
- 💡Link any statement about lost potency to genes no longer being transcribed.
- 💡When comparing potency types, explicitly state what each can form and what it cannot (e.g., pluripotent cells cannot form placenta).
- 💡Use the specification's exact wording 'mature mammals' when describing where multipotent and unipotent cells are found.
- 💡In data questions, compare the treated group with the control and state what the difference shows about differentiation.
- 💡Finish with function: cells replaced, tissue works, measurable change in the data.
- 💡Quote figures from the graph or table when justifying that the treatment had an effect.
- 💡Always use the specific term 'cardiomyocytes' when asked for an example of unipotent cell differentiation, as this is explicitly required by the specification.
- 💡Be prepared to compare the properties of unipotent cells with totipotent, pluripotent, and multipotent stem cells in extended response questions.
- 💡Say what a transcription factor does mechanically before saying what it achieves; the mechanism carries the mark.
- 💡When comparing iPS cells with embryonic stem cells, make one point about immune rejection and one about ethics.
- 💡Use somatic cell or body cell rather than normal cell.
- 💡Split the answer into supporting and against, and check you have at least one point under each before you run out of time.
- 💡Use the source material: quote a named finding or a figure from the stem rather than relying on general knowledge alone.
- 💡Finish with one sentence saying which way the evidence points and why.
Common Mistakes
Pitfalls to avoid in your exam answers
- writing that specialised cells lose or destroy the genes they no longer need
- using totipotent loosely for any stem cell, including bone marrow cells
- saying totipotent cells can become any cell without distinguishing them from pluripotent cells, which cannot form placental tissue
- describing stem cells as cells that have not been used yet, rather than undifferentiated cells able to divide and differentiate
- confusing differentiation with cell division
- Stating that unused DNA is removed or destroyed during specialisation, rather than being switched off. Correction: the genes remain present but are not expressed.
- Confusing transcription with translation, such as describing mRNA being made at the ribosome. Correction: transcription makes mRNA in the nucleus; translation makes protein at the ribosome.
- Claiming the cell 'chooses' which genes to use, omitting the mechanistic role of transcription factors and promoter regions. Correction: transcription factors binding to promoters control which genes are transcribed.
- Describing embryonic stem cells taken from a blastocyst as totipotent when they are pluripotent. Correction: state that inner cell mass cells are pluripotent.
- Claiming adults retain totipotent cells in their bone marrow. Correction: adult bone marrow contains multipotent stem cells.
- Explaining the loss of potency as cells using up or losing their DNA. Correction: genes remain present but are switched off and not transcribed.
- Using pluripotent and totipotent interchangeably. Correction: Totipotent cells can form a whole organism including extra-embryonic tissues; pluripotent cells cannot.
- Claiming multipotent bone marrow cells can form any body cell. Correction: They form a limited range of related blood cell types, not neurones or muscle.
- Stating that unipotent cells cannot divide. Correction: Unipotent cells can still divide by mitosis, but they differentiate into only one specific cell type.
- Believing genes are permanently deleted as cells differentiate. Correction: Genes are epigenetically silenced (switched off) rather than permanently removed from the genome.
- Writing that stem cells repair damaged cells directly. Correction: they divide and differentiate to replace lost cells.
- Treating movement of stem cells to the damaged site as the whole explanation. Correction: the differentiation step into the required cell type must also be stated.
- Claiming pluripotent cells can form a whole organism including placental tissue. Correction: they cannot form extra-embryonic tissues; that is totipotency.
- Presenting stem-cell therapy for heart attack as established. Correction: it remains experimental, so conclusions should be cautious and data-based.
- Reading partial recovery in data as proof of complete repair. Correction: state the extent of recovery shown and avoid overclaiming.
- Confusing unipotent cells with multipotent cells. Correction: Multipotent cells can differentiate into a limited number of different cell types (e.g. bone marrow forming various blood cells), whereas unipotent cells differentiate into only one type.
- Stating that unipotent cells cannot divide. Correction: Unipotent cells are stem cells, meaning they retain the capacity to divide by mitosis to self-renew, as well as differentiate.
- Assuming unipotent cells can form any tissue. Correction: Unipotent cells have the most restricted differentiation potential and are limited to a single lineage, such as forming cardiomyocytes.
- stating that iPS cells are taken from embryos
- describing transcription factors as enzymes that build mRNA, when RNA polymerase does that and the factor allows it to bind
- calling iPS cells totipotent
- claiming iPS treatment is risk free, when incomplete reprogramming and tumour formation are genuine concerns
- confusing transcription factors with growth factors or with hormones
- Writing only the benefits and omitting the opposing side, which weakens an evaluation. Correction: always include points for and against.
- Writing it is unethical without stating what the ethical objection actually is. Correction: specify that using embryonic stem cells destroys an embryo.
- Claiming all stem cell treatments destroy embryos, when adult and iPS sources do not. Correction: distinguish between embryonic, adult, and iPS cells.
- Stating that results obtained in mice apply directly to humans. Correction: acknowledge that animal models may not perfectly predict human responses.
- Ending without a judgement when the command word is evaluate. Correction: provide a concluding sentence weighing the evidence.