The big idea: Every cell in your body carries the same complete set of genes — the same genome.
So how can a muscle cell, a nerve cell and a skin cell all look and behave so differently if they share the same instructions?
The answer is selective gene expression: each cell type switches on only the genes it needs and leaves the rest switched off.
Becoming specialized in this way is called differentiation.
- Differentiation
- The process by which an unspecialized cell becomes a specialized cell with a particular structure and function.
- Genome
- The complete set of genes in a cell. Every body cell of an organism has the same genome.
- Gene expression
- Switching a gene 'on' so that it is used to make its protein. An expressed gene is active; an unexpressed gene is silent.
- Selective gene expression
- Expressing only some of the genes in the genome — different genes in different cell types — so each cell makes only the proteins it needs.
- Specialized cell
- A cell adapted to do a particular job (for example a nerve cell or a red blood cell), as a result of differentiation.
Same genome, different genes used: Do not say differentiated cells have different genes — they have the same genes (the same genome).
What differs is which genes are switched on (expressed). A muscle cell and a skin cell carry identical instructions but read different parts of them.
If every cell has the same genome, something outside the cell must tell it which genes to express.
That 'something' is a chemical signal. As an organism develops, signalling molecules spread out from where they are made, forming a concentration gradient across the tissue.
- Chemical signal (signalling molecule)
- A molecule released by some cells that travels to other cells and influences how they behave.
- Concentration gradient
- A smooth change in the concentration of a substance from one place to another — here, high near the source of the signal and lower further away.
- Position information
- The idea that a cell's position in a gradient (the concentration it experiences) tells it where it is, and therefore which genes to express.
Signal gradients give a cell its position: A signalling chemical is made in one region and diffuses outward, so cells nearby meet a high concentration and cells further away meet a lower concentration.
The concentration a cell experiences depends on where it sits in the gradient.
That concentration switches on a particular set of genes, so cells in different positions differentiate into different cell types.
The cause-and-effect chain: Follow the chain from signal to specialized cell:
position in gradient → concentration of signal detected → specific genes switched on → specific proteins made → specialized structure and function.
Change the concentration a cell meets and you change which genes it expresses — and so the type of cell it becomes.
| Step | What happens | Result |
|---|---|---|
| 1. Position | A cell sits at a certain place in the developing organism | It meets a particular level of signalling chemical |
| 2. Signal | A chemical signal (in a gradient) reaches the cell | The signal is detected by the cell |
| 3. Selective expression | Specific genes are switched ON; others stay OFF | Only certain genes are expressed |
| 4. Proteins | The expressed genes are used to make particular proteins | The cell gains its specialized molecules |
| 5. Specialization | The cell takes on a specific structure and function | It has differentiated into a cell type |
What is the SAME in every cell
- The genome (full set of genes)
- The DNA sequence of every gene
- The instructions are complete in all cells
What is DIFFERENT after differentiation
- Which genes are expressed (switched on)
- The proteins the cell makes
- The cell's structure and function
| Gene switched ON (expressed) | Gene switched OFF (not expressed) | |
|---|---|---|
| Is the gene present? | Yes — it is in the genome | Yes — it is still in the genome |
| Is its protein made? | Yes — the gene is used to make its protein | No — the gene is not used |
| Effect on the cell | Gives the cell a specialized feature | That feature is absent in this cell |
| Example | Insulin gene ON in a pancreas β-cell | Insulin gene OFF in a skin cell |
A memory hook: Gradient → genes → proteins → cell type.
Where a cell sits sets the signal it meets; the signal sets which genes are expressed; the genes set the proteins; the proteins set the cell type.
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Where the marks are: On Paper 1A a one-mark question asks for the outcome when an unspecialized cell meets a gradient of signalling chemicals — the answer is that it differentiates (becomes a specialized cell).
Another one-mark item asks what makes specialized cells differ after differentiation — the answer is that different genes are expressed in each (not different genes present).
On Paper 1B / Paper 2 the same idea appears as a data question: a graph or table of signal concentration across a tissue, where you read the gradient to deduce which cells differentiate.
IB-style question — explain differentiation by gene expression
All the cells in a developing embryo contain the same genome, yet they differentiate into many different cell types. Explain how cells with the same genome can become different from one another. [3]
How to score all three marks
- Start from the shared genome. Every cell contains the same genome (the same complete set of genes), so the difference cannot come from having different genes.
- Bring in selective expression. In each cell type a different selection of genes is switched on (expressed) while the others stay switched off.
- Link to structure and function. The expressed genes are used to make different proteins, which give each cell its own specialized structure and function. (Mark 1: same genome / same genes in all cells. Mark 2: different genes expressed / switched on. Mark 3: so different proteins / different structure and function.)
Final answer
All cells have the same genome, but each cell type expresses a different selection of genes; the genes that are switched on are used to make different proteins, giving each cell its own specialized structure and function.
✓ Why this scores full marks: It nails the trap: the genes are the same, only expression differs.
Then it follows the chain all the way to the proteins and the specialized structure/function, so each of the three marking points is hit separately.
| Feature | All body cells of one organism | Why it matters |
|---|---|---|
| Genome (full set of genes) | Identical in every cell | Every cell carries the same complete instruction set |
| Genes switched ON (expressed) | DIFFERENT in each cell type | Each cell uses only the genes it needs |
| Proteins made | DIFFERENT in each cell type | Different proteins build different structures |
| Structure and function | DIFFERENT (specialized) | This is what makes a muscle cell unlike a nerve cell |