Cell Differentiation and Stem Cells

All cells in a multicellular organism contain the same DNA, yet they have very different structures and functions. How does a single fertilised egg give rise to the hundreds of different cell types in the human body? The answer lies in cell differentiation — the process by which cells become specialised. The Edexcel A-Level Biology specification (9BI0) requires an understanding of differentiation, stem cells and their potential applications.

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What Is Cell Differentiation?

Cell differentiation is the process by which a cell develops a more specialised structure and function by expressing certain genes while switching others off. During differentiation, the cell undergoes changes in:

• Shape and size
• Organelle content (e.g. more mitochondria in active cells, more RER in secretory cells)
• Proteins produced (determined by which genes are expressed)
• Function (e.g. contraction, secretion, photosynthesis, nerve impulse transmission)

How Does Differentiation Occur?

All cells in an organism contain the same genome (the complete set of DNA). Differentiation occurs not by changes to the DNA sequence, but by differential gene expression — different genes are switched on (expressed) or off (silenced) in different cell types.

The mechanisms controlling gene expression include: