daytrippper
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I've looked at the mark scheme for this question and it doesn't make sense? The question:

"DDT is a pesticide which is used to kill mosquitoes. Mosquitoes have a gene called KDR. Some have an allele of this gene, KDR minus, giving them resistance to DDT. KDR plus is another allele which is not resistant to DDT. The KDR plus allele codes for Na+ channels found in neurons. When DDT binds to a Na+ channel the channel remains open all the time. Use this information to suggest how DDT kills insects"

The answer:
1) Neurons remain depolarised
2) No action potentials occur

**if the neuron remains depolarised, doesn't that increase the likelihood of an action potential?? The only way that no action potential can occur is to decrease the membrane potential?? ie. make it more negative?
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Old_Microbials
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The insect is allowed to essentially send a final impulse, before permanent depolarisation occurs.

When the neuron depolarises the first time to send the impulse, it is unable to repolarise.
This means that it will never be able to send another impulse.

Hence the long term outcome is that because the neurons remain depolarised, they cannot produce another action potential, and so the insect will die as its neurons will not function.
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Jpw1097
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(Original post by daytrippper)
I've looked at the mark scheme for this question and it doesn't make sense? The question:

"DDT is a pesticide which is used to kill mosquitoes. Mosquitoes have a gene called KDR. Some have an allele of this gene, KDR minus, giving them resistance to DDT. KDR plus is another allele which is not resistant to DDT. The KDR plus allele codes for Na+ channels found in neurons. When DDT binds to a Na+ channel the channel remains open all the time. Use this information to suggest how DDT kills insects"

The answer:
1) Neurons remain depolarised
2) No action potentials occur

**if the neuron remains depolarised, doesn't that increase the likelihood of an action potential?? The only way that no action potential can occur is to decrease the membrane potential?? ie. make it more negative?
Not quite. Once an action potential has been fired, sodium channels close and become inactivated, such that they will not open again until the membrane has repolarised. Therefore, if you keep a neurone depolarised, it will fire an action potential initially, but unless it repolarises, the sodium channels will remain inactivated and it will be impossible to fire another action potential. This is the same principle as depolarising neuromuscular blockers (e.g. suxamethonium) which are used during surgery to prevent movement.
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Old_Microbials
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(Original post by Jpw1097)
Not quite. Once an action potential has been fired, sodium channels close and become inactivated, such that they will not open again until the membrane has repolarised. Therefore, if you keep a neurone depolarised, it will fire an action potential initially, but unless it repolarises, the sodium channels will remain inactivated and it will be impossible to fire another action potential. This is the same principle as depolarising neuromuscular blockers (e.g. suxamethonium) which are used during surgery to prevent movement.
While this too would inhibit an action potential.
The question states that the channels are permanently open. meaning that repolarisation doesn't occur because the Na channels don't close. Preventing the potassium from repolarise the cells.
Last edited by Old_Microbials; 1 year ago
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daytrippper
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Thank you!!😂 it finally makes sense!
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