The big idea: Almost every cell has all the genes, but at any time it expresses only some of them. A gene is expressed only if it is transcribed.
The points to remember
- Gene expression means making the product of a gene, usually a protein.
- Expression starts with transcription (DNA to mRNA), then translation (mRNA to polypeptide).
- Not all genes are expressed in a cell at any given time.
- A gene that is not transcribed makes no mRNA, so no protein.
Remember it as: No transcription, no expression.
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Transcription is the switch: Because transcription is the first stage of gene expression, it is the main place where a gene is switched on or off.
Gene switched on
- Transcribed into mRNA
- mRNA is translated
- The protein is made
Gene switched off
- Not transcribed
- No mRNA to translate
- No protein made
Real example: lactase. Most mammals switch off transcription of the lactase gene after weaning, so they can no longer digest milk sugar well. Many adult humans keep the gene switched on.
Switched off is not lost: A gene that is switched off is still in the DNA. It can be switched on again later.
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How it is asked: Why different cells make different proteins, or where gene expression is switched on and off.
IB-style questionExplain[2 marks]
A skin cell and a pancreas cell contain the same genes but make different proteins. Explain why.
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