Where the marks are: Expect one-mark items on Paper 1A and data questions on Paper 1B / Paper 2.
What you may be asked
- Paper 1A: the outcome when an unspecialized cell meets a gradient of signalling chemicals. Answer: it differentiates (becomes a specialized cell).
- Paper 1A: what makes specialized cells differ after differentiation. Answer: different genes are expressed in each (not different genes present).
- Paper 1B / Paper 2: the same idea 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 |
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