The big idea: Every cell in your body has the same genes. Cells differ because each one uses a different set of them.
The points to remember
- Differentiation: an unspecialized cell becomes a specialized cell, with a structure suited to one job.
- All the cells of an organism have the same genome: the same genes.
- Each kind of cell expresses a different set of genes: some are switched on, others off.
- Different genes expressed means different proteins made, so the cells develop differently.
Remember it as: Same genes, different genes switched on.
You can see it in your own blood:
Real example: haemoglobin: Every cell in your body carries the genes for haemoglobin, but only the cells that become red blood cells switch them on. A skin cell has the genes and never uses them.
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The big idea: Which genes a cell switches on is often triggered by a change in its environment.
The points to remember
- A cell's environment includes signal chemicals from other cells, its position, and outside conditions.
- In an embryo, a signal chemical spreads out to form a gradient: cells at different places get different amounts.
- The amount of signal decides which genes are expressed, so cells in different places become different tissues.
The trigger
- A signal chemical, or light, reaches the cell
- The amount differs from place to place
The response
- Some genes are switched on, others off
- The cell makes new proteins and specializes
Real example: a seedling in the dark: A bean seedling grown in the dark stays pale. Once light reaches it, the leaf cells switch on the genes for chloroplasts and turn green: the environment triggered the change.
Keep the two ideas apart:
The genes do not change: Differentiation does not add or remove genes. The DNA stays the same; only which genes are used changes.
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How it is tested: Paper 1A names a cause, such as gradients of signalling chemicals, and asks what results: differentiation.
IB-style questionOutline[2 marks]
Muscle cells and nerve cells develop from identical cells in a human embryo. Outline the basis of this cell differentiation.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Use the word: Write 'gene expression', not just 'genes': the genes are the same, their expression differs.