Discussion
Assertion (A): A signalling pathway needs a mechanism for switching off as well as on.
Reason (R): A signal that could not be terminated would leave the cell permanently activated.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason states the consequence. A pathway with no off switch would respond once and never recover, so it could not report a change in conditions. Termination is built in at every level - the ligand is degraded or removed, the receptor is desensitised and internalised, GTP is hydrolysed on the G protein, phosphatases reverse the kinases, and second messengers are broken down.
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