Stem cells from human embryos can be cloned and made to differentiate into…

The idea

Human embryonic stem cells are pluripotent, so they can be grown in the laboratory and stimulated to differentiate into most human cell types, such as nerve cells, muscle cells or insulin-producing cells. Cloning here means producing many genetically identical copies of these cells by mitosis in culture, giving a large supply for research or potential medical use. Scientists control differentiation by changing the culture conditions, for example adding specific growth factors or chemicals. This makes embryonic stem cells valuable for treating conditions such as diabetes or paralysis, but their use raises ethical issues because obtaining them involves destroying early embryos. You should be able to explain both the scientific potential and the ethical concerns, and to describe how cloning and differentiation are achieved in principle.

What earns the marks

• States that human embryonic stem cells are pluripotent and can differentiate into most human cell types.
• Explains that cloning produces many genetically identical stem cells by mitosis in culture.
• Describes how differentiation is controlled by culture conditions such as growth factors or chemicals.
• Gives a named potential use, such as making insulin-producing cells or nerve cells for treatment.
• Discusses ethical issues arising from the use of human embryos as a source of stem cells.

Common mistakes and how to put them right

• Saying embryonic stem cells can become every cell type without exception.

  Put it right by stating that they can become most, not all, cell types.

• Confusing cloning with sexual reproduction.

  Put it right by stating that cloning here means producing identical cells by mitosis.

• Ignoring ethical issues; correct this by including a balanced comment on the source of embryonic stem cells.

Exam tips

• Use the term pluripotent accurately and contrast it with adult stem cells.
• When discussing ethics, give both a benefit and a concern rather than only one side.
• Link differentiation to a specific control method such as growth factors in the culture medium.