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Stem cells: what a cell can still become, and what fixes it questions

What a stem cell is and what self-renewal means, the four levels of potency from totipotent to unipotent with a real example of each, the four sources stem cells are obtained from and the potency each one gives, induced pluripotent cells and what they show about the genome, why differentiation switches genes off rather than removing them, the treatments already in use against the ones still proposed, and the positions people hold on the use of embryos.

8 original questions · 26 marks · the stem cells: what a cell can still become, and what fixes it notes · Cell cycles, reproduction and development

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• 6 marks · Evaluate

  Evaluate the use of embryonic stem cells as a source of cells for treating human disease.

  Mark scheme

  1. B1 for: embryonic stem cells are pluripotent, so a far wider range of tissues can be produced from them than from adult or cord blood cells
  2. B1 for: they divide in culture apparently without limit, so one cell line can supply large numbers of cells, and they offer treatment for conditions with no present cure
  3. B1 against: obtaining them ends the development of a blastocyst, which some hold to be a human individual from fertilisation, so that no benefit to a patient can justify it
  4. B1 against: the cells come from another individual and may be rejected, and undifferentiated cells remaining in a transplant may form tumours
  5. B1 alternatives: adult and umbilical cord blood cells raise no such objection but are multipotent, and induced pluripotent cells offer pluripotency from the patient's own tissue without using an embryo
  6. A1 a judgement is stated and supported by the points made above it; credit either conclusion provided the reasoning leads to it

• 4 marks · Describe

  Describe what is meant by totipotent, pluripotent, multipotent and unipotent, giving one example of a cell at each level.

  Mark scheme

  1. B1 totipotent: can become every cell type of the organism and the extra-embryonic tissues, for example a zygote or a cell of the first few divisions
  2. B1 pluripotent: can become every cell type of the body but not extra-embryonic tissue, for example a cell of the inner cell mass of a blastocyst
  3. B1 multipotent: can become a limited number of related types, for example a haematopoietic stem cell in bone marrow, which forms blood cells
  4. B1 unipotent: can become one type only, for example the cells that give rise to new cardiomyocytes in heart muscle

• 4 marks · Discuss

  Induced pluripotent stem cells are made by treating an adult body cell with four transcription factors. Discuss the advantages and the limitations of using these cells rather than embryonic stem cells.

  Mark scheme

  1. B1 advantage: no embryo is used, so the ethical objection to ending the development of a blastocyst does not apply
  2. B1 advantage: the cells can be made from the patient's own tissue, so they carry the patient's own antigens and the risk of rejection is much lower
  3. B1 limitation: reprogramming succeeds in only a small proportion of the cells treated, and methods that insert the genes using a virus increase the risk of tumours
  4. B1 limitation: induced pluripotent cells have not been shown to be equivalent to embryonic cells, which are still used as the standard for comparison

• 3 marks · Explain

  A neurone contains the gene coding for insulin but never produces insulin. Explain how this is possible.

• 3 marks · Compare

  Compare stem cells obtained from adult bone marrow with stem cells obtained from an early embryo.

• 3 marks · Suggest

  A patient is treated with nerve cells that have been grown from embryonic stem cells. Suggest three difficulties that this treatment may present.

• 2 marks · State

  State the two properties a cell must have if it is to be described as a stem cell.

• 1 mark · Name

  A cell is removed from the inner cell mass of a five-day-old human blastocyst. Name the level of potency this cell shows.

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