Stem cells: Stem cells from human embryos can be cloned and made to differentiate into most different types of human cells.

The idea

Embryonic stem cells are unspecialised cells taken from a very early human embryo, when it is a ball of cells. Because they have not yet switched on the genes that make a cell specialised, they are pluripotent: they can differentiate into almost any cell type, such as nerve, muscle, skin or blood cells. They can also be cloned, meaning the embryo is allowed to divide in the laboratory to produce many genetically identical stem cells, giving a large supply for research or treatment. Scientists can direct differentiation by controlling the conditions and chemical signals the cells receive. This makes embryonic stem cells medically valuable, but their use raises ethical issues because obtaining them involves destroying an early embryo.

What earns the marks

• Embryonic stem cells come from very early human embryos and are unspecialised, so they have not yet become a particular cell type.
• Cloning means the embryo's cells are allowed to divide in the laboratory to produce many genetically identical stem cells.
• Differentiation can be directed by changing the conditions or chemical signals the cells receive, so they become a chosen specialised cell type.
• The ability to form most cell types makes embryonic stem cells useful for research and for replacing damaged tissues, but their use raises ethical concerns.

Common mistakes and how to put them right

• Saying embryonic stem cells can become any cell type at all.

  Put it right by stating they can form most, not every, human cell type.

• Confusing cloning with differentiation.

  Put it right: Cloning produces more identical unspecialised cells, whereas differentiation is the process by which a cell becomes specialised.

• Claiming the embryo is grown into a baby during cloning.

  Put it right by explaining that the cells are grown in the laboratory as a cell supply, not as an embryo for development.

Exam tips

• Use the word pluripotent and immediately explain it as able to differentiate into most cell types, so the meaning is clear.
• Link cloning to the idea of producing many identical stem cells, which gives a large supply for research or treatment.
• If asked to evaluate use, give one benefit such as treating disease and one ethical concern such as destruction of an embryo, then reach a brief conclusion.