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Biological clocks
• Clock periods
– Circannual
– Circalunidian
– Circadian
• Clock mechanisms
– Entrainment
– Neural location
– Genetic basis
Hibernation follows annual rhythm in
golden-mantled ground squirrels
Five animals were isolated at birth and kept in darkness at 3oC
Testes growth and
feather molt in
stonechats follows
annual cycles
Nestlings were removed from Kenya
and reared in Germany with constant
temperature and photoperiod and yet
retain annual molt and testes cycles.
Notice that the clock period drifted.
Horseshoe crabs mate on full moon
why?
Lunar position affects the tides
Spring tide
Sun and moon
align, tidal
excursion is
greatest
Neap tide
Sun and moon
are perpendicular,
tidal excursion is
least
Isopod activity follows tides
Isopods are usually covered with water at high tide. They retain
this activity even when kept in the lab with no tidal fluctuation.
Kangaroo rat
feeding shows
lunar cycles
K-rat activity at a
feeder is confined to
dark periods
occurred during period
of seed shortages
Entrainment by environmental cycles
• Environmental cues set cycle period
– Species specific
• Types of cues
– Photoperiod
– Light pulse
– Food availability
• Temperature compensation
– Clock cycles do not change with temperature
Cricket calling entrains to dark
Constant light
for 12 days
12 h light/dark
for 12 days
Mouse activity entrains to light
10 mins of light per day are sufficient to reset the clock
12h light:12h dark
24 h dark
10 min light
Seasonal testis
growth in white-
crowned sparrows
- light triggered?
H1: There is a photosensitive time window which follows a 24 h period.
H2: Early spring day length triggers testes growth.
Mole-rats lack daily cycles
Clock mechanisms
• Location of the clock
– Suprachiasmatic nucleus of the hypothalamus
– Pineal gland
• Clock genes
– Period
– Timeless
– Tau (doubletime)
Mammal and
bird clocks
reside in the
suprachiasmatic
nuclei (SCN),
which is in the
hypothalamus
In hamsters,
SCN lesion disrupts
clock while SCN
transplant restores
clock
Period
Isolated rat
SCN exhibit
clock activity
Isolated caudate (upper
brainstem) cells do not
cycle, but isolated SCN
cells do cycle
Isolated neurons from rat SCN
exhibit circadian rhythym
Neural firing is stopped with application of tetrodotoxin (TTX),
which blocks sodium channels, but clock kept ticking!
Pineal glands respond to light cycles
Melatonin release from chicken pineal glands cultured in vitro
Light cycles No light cycles
crem gene expression in rat pineal
glands is altered by light
crem product is produced in SCN and influences melatonin release
samples taken
Distribution of
circadian clocks in
tissues and taxa
Mammalian
clock
pathways
period alleles exhibit altered circadian rhythyms
in Drosophila melanogaster
Genetic basis of the clock in mammals
• per codes for a protein
(PER) that gradually builds
up over time
• tau codes for an enzyme
that breaks down PER
• tim codes for a protein
(TIM) that binds with PER
to cross the membrane and
suppress transcription of
PER
• Photoreceptor not yet
known
• Cycle repeats every 24 h
Genetic basis of the clock in flies
• per and tim genes are turned
on by clock and cycle
• PER and TIM proteins build
up inside the cell during dark
• dbt codes for an enzyme that
degrades PER & adds time
delay
• Cryptochromes absorb blue
light and activate cry gene
expression
• TIM protein is degraded by
CRY protein
Clock summary
• per/tim/tau(dbt) genes control pacemaker
• Pacemaker occurs in SCN in vertebrates, but is
distributed in brain cells in some insects
• SCN signals pineal to release melatonin
• Short pulses of light entrain SCN and pineal cells
• Drosophila, honey bees, hamsters and humans
share same genes - likely common ancestor was a
flatworm that lived about 600 MYA

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Biological clock.ppt

  • 1. Biological clocks • Clock periods – Circannual – Circalunidian – Circadian • Clock mechanisms – Entrainment – Neural location – Genetic basis
  • 2. Hibernation follows annual rhythm in golden-mantled ground squirrels Five animals were isolated at birth and kept in darkness at 3oC
  • 3. Testes growth and feather molt in stonechats follows annual cycles Nestlings were removed from Kenya and reared in Germany with constant temperature and photoperiod and yet retain annual molt and testes cycles. Notice that the clock period drifted.
  • 4. Horseshoe crabs mate on full moon why?
  • 5. Lunar position affects the tides Spring tide Sun and moon align, tidal excursion is greatest Neap tide Sun and moon are perpendicular, tidal excursion is least
  • 6. Isopod activity follows tides Isopods are usually covered with water at high tide. They retain this activity even when kept in the lab with no tidal fluctuation.
  • 7. Kangaroo rat feeding shows lunar cycles K-rat activity at a feeder is confined to dark periods occurred during period of seed shortages
  • 8. Entrainment by environmental cycles • Environmental cues set cycle period – Species specific • Types of cues – Photoperiod – Light pulse – Food availability • Temperature compensation – Clock cycles do not change with temperature
  • 9. Cricket calling entrains to dark Constant light for 12 days 12 h light/dark for 12 days
  • 10. Mouse activity entrains to light 10 mins of light per day are sufficient to reset the clock 12h light:12h dark 24 h dark 10 min light
  • 11. Seasonal testis growth in white- crowned sparrows - light triggered? H1: There is a photosensitive time window which follows a 24 h period. H2: Early spring day length triggers testes growth.
  • 13. Clock mechanisms • Location of the clock – Suprachiasmatic nucleus of the hypothalamus – Pineal gland • Clock genes – Period – Timeless – Tau (doubletime)
  • 14. Mammal and bird clocks reside in the suprachiasmatic nuclei (SCN), which is in the hypothalamus
  • 15. In hamsters, SCN lesion disrupts clock while SCN transplant restores clock Period
  • 16. Isolated rat SCN exhibit clock activity Isolated caudate (upper brainstem) cells do not cycle, but isolated SCN cells do cycle
  • 17. Isolated neurons from rat SCN exhibit circadian rhythym Neural firing is stopped with application of tetrodotoxin (TTX), which blocks sodium channels, but clock kept ticking!
  • 18. Pineal glands respond to light cycles Melatonin release from chicken pineal glands cultured in vitro Light cycles No light cycles
  • 19. crem gene expression in rat pineal glands is altered by light crem product is produced in SCN and influences melatonin release samples taken
  • 20. Distribution of circadian clocks in tissues and taxa
  • 22. period alleles exhibit altered circadian rhythyms in Drosophila melanogaster
  • 23. Genetic basis of the clock in mammals • per codes for a protein (PER) that gradually builds up over time • tau codes for an enzyme that breaks down PER • tim codes for a protein (TIM) that binds with PER to cross the membrane and suppress transcription of PER • Photoreceptor not yet known • Cycle repeats every 24 h
  • 24. Genetic basis of the clock in flies • per and tim genes are turned on by clock and cycle • PER and TIM proteins build up inside the cell during dark • dbt codes for an enzyme that degrades PER & adds time delay • Cryptochromes absorb blue light and activate cry gene expression • TIM protein is degraded by CRY protein
  • 25. Clock summary • per/tim/tau(dbt) genes control pacemaker • Pacemaker occurs in SCN in vertebrates, but is distributed in brain cells in some insects • SCN signals pineal to release melatonin • Short pulses of light entrain SCN and pineal cells • Drosophila, honey bees, hamsters and humans share same genes - likely common ancestor was a flatworm that lived about 600 MYA