Stem cells: A stem cell is an undifferentiated cell of an organism which is capable of giving rise to many more cells of the same type, and from which certain other cells can arise from differentiation.

The idea

A stem cell is unspecialised: it has not yet switched on the particular genes that give a cell its job, so it has no special shape or chemistry for one task. Two abilities define it. First, it can divide by mitosis to make many more cells of the same unspecialised type, so a stem cell population can be maintained or expanded. Second, its daughter cells can differentiate, meaning they switch on specific genes and develop the structure and proteins needed for a particular role, such as a red blood cell making haemoglobin or a neurone growing a long axon. For example, a cell in bone marrow can divide repeatedly and some daughters become white blood cells. Both clauses matter: self-renewal alone is not enough, and differentiation alone is not enough.

What earns the marks

• States that a stem cell is undifferentiated, meaning it has not become specialised for a particular function.
• Explains that a stem cell can divide by mitosis to produce many more cells of the same unspecialised type, maintaining the stem cell population.
• Explains that some daughter cells can differentiate, switching on specific genes to become specialised cells with particular structures and functions.
• Uses a correct example, such as a bone marrow stem cell producing many stem cells and also producing red blood cells or white blood cells.
• Distinguishes stem cells from specialised cells, which have a fixed role and generally cannot divide to produce other cell types.
• Links differentiation to gene activity, for example genes for haemoglobin being switched on in a developing red blood cell.

Common mistakes and how to put them right

• Saying a stem cell is simply 'a cell that divides'.

  Put it right: All body cells can divide in some contexts, so the key idea is that it is undifferentiated and its daughters can become other cell types.

• Claiming a stem cell can become any cell in any organism.

  Put it right: Potency varies; embryonic stem cells are more versatile than adult stem cells, and plant meristem cells give rise to plant tissues.

• Writing that differentiation changes the genes a cell contains.

  Put it right: The genes stay the same; differentiation changes which genes are switched on and therefore which proteins are made.

Exam tips

• Define the term first, then give one example of self-renewal and one example of differentiation so both clauses of the definition are covered.
• Use the words undifferentiated, mitosis and differentiate accurately; vague phrases such as 'can turn into anything' lose credit.
• If asked to compare, set out embryonic, adult and meristem stem cells in separate sentences rather than mixing their features together.