Gene expression at Higher Level: The same statement as SL. At HL it leads into how transcription is started at a promoter and how gene expression is regulated.
Practise this as you read
- Outline the role of transcription in gene expression.
- Explain why cells with the same genes make different proteins.
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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: living at altitude. When oxygen in the blood is low, as high in the mountains, kidney cells switch on transcription of the gene for the hormone EPO, which tells the bone marrow to make more red blood cells.
Switched off is not lost: A gene that is switched off is still in the DNA. It can be switched on again later.
How it is asked: Why different cells make different proteins, or where gene expression is switched on and off.
IB-style questionOutline[2 marks]
Outline the role of transcription in the expression of genes.
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