The big idea: Substances from outside a cell can change which genes it expresses. Two examples: a hormone in human cells and lactose in bacteria.
The points to remember
- Oestradiol is a steroid hormone; it passes through the cell membrane and binds a receptor.
- The hormone-receptor complex acts as a transcription factor and switches particular genes on.
- Only cells with the receptor respond, so the hormone affects its target cells only.
- In E. coli, a repressor blocks the lactose genes when there is no lactose.
- Lactose binds the repressor, which leaves the DNA, so the genes are transcribed.
Remember it as: Hormone switches on; lactose lifts the block.
Real example: the uterus lining. Oestradiol rises in the first half of the menstrual cycle. In cells of the uterus lining it switches on genes that make the cells divide, so the lining thickens.
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Comparing the two examples: Both are external factors that change gene expression by acting on proteins that bind DNA.
| Oestradiol (hormone) | Lactose (biochemical) | |
|---|---|---|
| Organism | Humans and other mammals | The bacterium E. coli |
| Binds to | A receptor protein in the cell | The repressor protein |
| Effect on the DNA-binding protein | Complex becomes an active transcription factor | Repressor leaves the DNA |
| Result | Target genes switched on | Lactose genes transcribed |
Why it is useful: E. coli makes the lactose-digesting enzymes only when there is lactose to digest, so it does not waste energy making them.
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How it is asked: Written answers on how a steroid hormone or lactose changes gene expression, and 1A items on how each one works.
IB-style questionExplain[3 marks]
Explain how the presence of lactose changes gene expression in E. coli.
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See the answer broken into clear points, with guidance on how each point earns marks.