Q.42 Given below are two statements:
Statement I: A circadian rhythm cannot be entrained.
Statement II: Hormone melatonin regulates circadian rhythms.
In the light of the above statements, choose the
correct answer from the options given below:
- Both Statement I and Statement II are true
- Both Statement I and Statement II are false
- Statement I is true but Statement II is false
- Statement I is false but Statement II is true
Circadian rhythms are entrainable by zeitgebers like light, and melatonin plays a key regulatory role in their synchronization.
Statement I is false as circadian rhythms must entrain to external cues, while Statement II correctly identifies melatonin’s regulatory function.
Question Breakdown
Statement I claims a circadian rhythm cannot be entrained, which contradicts the definition—circadian clocks (~24h period) require daily resetting via zeitgebers (light, melatonin, feeding) to match the 24h solar day. Free-running periods (24.2-25h in humans) drift without entrainment. Statement II is true: melatonin from pineal gland signals darkness, phase-shifting the SCN master clock.
Option Analysis
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Both true: Incorrect. I fundamentally wrong.
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Both false: Wrong. II accurate.
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Statement I true, II false: False. Both reversed.
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Statement I false, Statement II true: Correct answer. Core chronobiology principle.
Correct Answer
Statement I is false but Statement II is true.
Circadian Clock Fundamentals
Circadian rhythms are entrainable biological ~24h cycles driven by SCN molecular loops (CLOCK/BMAL1 → PER/CRY feedback). Without entrainment, free-running periods desynchronize from solar time, causing jet lag/shift work disorder. Hormone melatonin regulates circadian rhythms via MT1/MT2 receptors, peaking 2-4h before bedtime.
Statement I: Entrainment Essential
“A circadian rhythm cannot be entrained” violates basic definition. Light (primary zeitgeber) via ipRGCs → SCN resets PER/CRY degradation. Phase response curve: morning light advances, evening delays clock. Melatonin supplements entrain delayed sleep phase.
Statement II: Melatonin Role
Pineal melatonin synthesis (tryptophan → serotonin → N-acetylserotonin → melatonin) dark-induced via norepinephrine → cAMP. Suppresses SCN firing, advances phase at physiological doses (<0.3mg). Light suppresses melatonin (phase marker).
Zeitgeber Effect Timing Light Primary entrainer Dawn advance, dusk delay Melatonin Phase advance Evening (2h pre-bed) Exercise/Meals Secondary Variable Exam Strategy
NEET/GATE hallmarks: Entrainment = zeitgeber synchronization; melatonin = darkness hormone. Statement I tests definition; II tests pineal function. Mnemonic: “Cir-CAN-train” (entrainable).
Perfect chronobiology question for your molecular biology exam prep.
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