Sleep, Circadian Rhythms and the Clock in the Brain · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Your brain keeps a daily clock

Science · The Human Body · ages 21-22
Name ______________________   Date ____________

  1. What do accumulating PER and CRY proteins do in the core clock feedback loop?

    • They inhibit transcription driven by CLOCK and BMAL1
    • They stimulate translation of Per and Cry RNA
    • They activate transcription of Per and Cry genes

  2. Where is the mammalian SCN, which coordinates many daily rhythms?

    • In retinal layers at the back of the eye
    • In the hypothalamus, above the optic chiasm
    • In the motor nerves controlling the eye muscles

  3. Light must pass through the image-forming visual cortex before it can reset the SCN.

    Circle one:   True   False

  4. About how many hours does one full cycle of the core circadian feedback loop take?

    Answer: ______________

  5. For a person on a usual day-active schedule, light in the late biological night or early biological morning usually shifts circadian timing in which direction?

    • It delays the clock to a later phase
    • It leaves the clock's phase unchanged
    • It advances the clock to an earlier phase

  6. A report links large, slow delta waves during sleep with better fact recall. Which stage and interpretation fit this result?

    • N2; the result allows contributions from other sleep stages
    • N3; the result allows contributions from other sleep stages
    • N3; the result rules out contributions from other sleep stages

  7. A hamster's original free-running period is 24 hours. After SCN lesion and a graft from a 20-hour donor, activity rhythms return with a 20-hour period. What does this support?

    • The graft restores the response to daily light cues
    • The donor SCN influences the restored activity period
    • The host retains its original free-running activity period

  8. In a hypothetical case, two people lack image vision. Independent tests establish intact SCN rhythm generation in both, but light shifts circadian phase in only one. Which difference could explain this?

    • Retinal input to the SCN is functional in only one
    • The SCN generates an internal rhythm in only one
    • The two clocks require image-forming vision to respond to daylight
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Your brain keeps a daily clock W1-mt_cXx4SaLJUM-s1

  1. They inhibit transcription driven by CLOCK and BMAL1 · PER and CRY act as repressors in the loop. Their accumulation reduces the transcription that produces their RNA.
  2. In the hypothalamus, above the optic chiasm · The SCN is a pair of nuclei in the hypothalamus above the optic chiasm. It is not in the retina or eye muscles.
  3. False · Retinal ganglion cells project directly to the SCN through the retinohypothalamic tract. Image-forming visual cortex is not a required relay.
  4. 24 · Production, repression and breakdown give a period of about 24 hours. Individual free-running periods need not be exactly 24 hours.
  5. It advances the clock to an earlier phase · Light at this biological phase usually advances timing. An advance means the same internal event occurs earlier.
  6. N3; the result allows contributions from other sleep stages · The waves identify N3. A link with fact recall does not rule out contributions from other stages or establish an exclusive role for N3.
  7. The donor SCN influences the restored activity period · The restored period matches the donor's period. This links the grafted SCN to activity-rhythm generation, but does not establish restored light entrainment.
  8. Retinal input to the SCN is functional in only one · The case independently establishes SCN rhythm generation in both people. Different retinal input could explain their different light responses. Image vision is not required for this input.
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