Gene control
Introduced| English | Meaning |
|---|---|
| lac operon/læk ˈɒpərən/ | The E. coli gene switch that turns lactose-digesting enzymes on only when lactose is present. |
| structural gene/ˈstrʌktʃərəl dʒiːn/ | A gene whose product performs a cellular function rather than primarily regulating the expression of other genes. |
| regulatory gene/ˌreɡjʊˈleɪtəri dʒiːn/ | A gene whose product controls expression of other genes. |
| inducible enzyme/ɪnˈdjuːsɪbl ˈenzaɪm/ | An enzyme whose production increases in response to a particular substance or signal. |
| repressor/rɪˈpresə/ | A regulatory protein that reduces gene expression, often by binding DNA and blocking transcription. |
| transcription factor/trænˈskrɪpʃn ˈfæktə/ | A protein that binds specific DNA sequences or interacts with transcription machinery to regulate gene expression. |
| gene expression/dʒiːn ekˈspreʃn/ | The use of information in a gene to produce a functional RNA molecule or protein. |
Switching genes on and off
- Not all genes are switched on all the time.
- Cells control which proteins they make, and when.
- A famous bacterial example is the lac operon 乳糖操纵子.
Types of gene and enzyme
- structural genes 结构基因 code for useful proteins such as enzymes.
- regulatory genes 调节基因 control whether other genes are switched on.
- an inducible enzyme 可诱导酶 is made only when needed; a repressible enzyme is normally made but can be switched off.
A regulatory gene:
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Structural genes code for proteins like enzymes; regulatory genes control whether other genes are switched on.
An enzyme that is made only when it is needed (like the lac enzymes) is described as ______.
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Inducible enzymes are produced only when required, which avoids wasting resources.
The lac operon
- In a bacterium, the lac operon controls the digestion of lactose:
- no lactose: a repressor 阻遏物 protein binds the operon and blocks transcription — no enzymes made.
- lactose present: lactose binds the repressor and pulls it off, so transcription happens and the enzymes are made.
- These enzymes are therefore inducible.
The lac operon switches its genes on only when lactose is present
The lac operon
Step through the switch. With no lactose the genes are blocked; lactose pulls the repressor off and switches them on.
In the lac operon, when there is NO lactose:
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Without lactose, the repressor sits on the operator and blocks transcription of the lac genes.
When lactose IS present in the lac operon:
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Lactose removes the repressor, allowing transcription — so the lac enzymes are inducible.
Put the lac operon switching ON in order.
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Lactose acts as the signal: it removes the repressor, so the genes are only on when their enzymes are needed.
In the lac operon, lactose switches the genes ON by removing the repressor from the operator.
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Lactose binds and distorts the repressor so it lets go — transcription can then proceed.
Control in eukaryotes
- Transcription factors 转录因子 are proteins that bind DNA and control gene expression 基因表达 (raising or lowering transcription).
- Gibberellin works this way: it causes the breakdown of DELLA repressors, which normally block the factors that switch genes on.
In eukaryotes, transcription factors:
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Transcription factors bind DNA to raise or lower transcription; gibberellin acts by removing DELLA repressors.
You've got it
- structural genes (proteins/enzymes) vs regulatory genes (control); inducible (made when needed) vs repressible (switched off)
- lac operon: no lactose → repressor blocks transcription; lactose → pulls repressor off → enzymes made (inducible)
- in eukaryotes, transcription factors control gene expression; gibberellin removes DELLA repressors