(j) the features and differentiation of stem cells

The idea

Stem cells are unspecialised cells that can divide repeatedly and can differentiate into other cell types. Their key features are the ability to self-renew by mitosis, so the stem cell population is maintained, and potency, the range of cell types they can become. Totipotent cells can form any cell type including extra-embryonic tissue; pluripotent cells can form most body cells but not extra-embryonic tissue; multipotent cells are restricted to a related family of cells, such as blood stem cells forming red and white blood cells; unipotent cells form one cell type. Differentiation occurs when a cell expresses particular genes, switching on some and off others, so it develops the structure and proteins suited to a specialised function. In multiple-choice questions, match the potency term to the described capability and link differentiation to selective gene expression.

What earns the marks

• Stem cells are unspecialised cells that can divide by mitosis to self-renew and can differentiate into specialised cells.
• Potency describes the range of cell types a stem cell can become: totipotent, pluripotent, multipotent and unipotent.
• Totipotent cells can form all cell types including extra-embryonic tissue; pluripotent cells form most body cells but not extra-embryonic tissue.
• Multipotent cells are limited to a related family of cells, for example blood stem cells producing red and white blood cells; unipotent cells form a single cell type.
• Differentiation involves selective gene expression: some genes are switched on and others off, giving the cell its specialised structure and function.
• Self-renewal maintains the stem cell population while some daughter cells differentiate.

Common mistakes and how to put them right

• Treating pluripotent and totipotent as the same.

  Put it right: Totipotent cells can also form extra-embryonic tissue, whereas pluripotent cells cannot.

• Thinking differentiation involves loss of genes.

  Put it right: The genes remain but are selectively expressed, with some switched on and others off.

• Assuming all stem cells can become any cell type.

  Put it right: Multipotent and unipotent cells are restricted in the cell types they can form.

Exam tips

• Match the potency word in the stem to its precise definition before choosing an option.
• Look for 'selective gene expression' or 'genes switched on and off' as the correct description of differentiation.
• Reject options that say differentiation removes genes or that stem cells cannot self-renew.