Stem cells: In therapeutic cloning an embryo is produced with the same genes as the patient. Stem cells from the embryo are not rejected by the patient’s body so they may be used for medical treatment.

The idea

Therapeutic cloning begins by taking the diploid nucleus from one of the patient's body cells and placing it into an egg cell whose own nucleus has been removed. The resulting cell divides to form an early embryo that is genetically identical to the patient, so its stem cells carry the patient's own antigens. Because the immune system recognises these cells as 'self', it should not attack them, avoiding the rejection that can occur with donor tissue. The stem cells can then be cultured and stimulated to differentiate into the needed cell type, such as pancreatic cells for diabetes or nerve cells for paralysis. The technique is still experimental and raises ethical questions about creating and destroying embryos.

What earns the marks

• Therapeutic cloning transfers the patient's nucleus into an egg cell with its nucleus removed, producing an embryo genetically identical to the patient.
• Because the embryo's genes match the patient's, the stem cells carry the same antigens and are not recognised as foreign.
• Avoiding immune rejection means the patient may not need immunosuppressant drugs, which have side effects.
• The stem cells can be stimulated to differentiate into the cell type needed for treatment, such as pancreatic or nerve cells.
• Therapeutic cloning is a possible medical treatment but remains experimental and raises ethical concerns about embryo use.

Common mistakes and how to put them right

• Saying the embryo is produced from the patient's sperm and egg.

  Put it right by describing the transfer of the patient's body-cell nucleus into an enucleated egg cell.

• Claiming rejection is impossible.

  Put it right by saying the cells are much less likely to be rejected because they are genetically matched, not that rejection can never occur.

• Confusing therapeutic cloning with reproductive cloning.

  Put it right by stating that in therapeutic cloning the embryo is used as a source of stem cells for treatment, not implanted to produce a baby.

Exam tips

• Sequence the method clearly: remove the egg cell nucleus, insert the patient's nucleus, allow an embryo to form, then extract stem cells.
• Explain the link between identical genes, matching antigens and reduced rejection rather than just stating 'no rejection'.
• Mention one ethical concern, such as destruction of the embryo, to show awareness of issues surrounding the technique.