1. Biological clocks Clock periods Clock mechanisms 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
are perpendicular,
tidal excursion is
least

6. Neotropical bats exhibit lunar phobia

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. Isopod activity follows daily tidal flow
Isopods are usually covered with water at high tide. They retain
this activity even when kept in the lab with no tidal fluctuation.

9. Entrainment by environmental cycles
Temperature compensation
Clock cycles do not change with temperature
Environmental cues set cycle period
Species specific
Types of cues (zeitgebers)
Photoperiod
Light pulse
Food availability

10. Cricket calling entrains to dark
Constant light
for 12 days
12 h light/dark
for 12 days

11. Mouse activity entrains to light
12h light:12h dark
24 h dark
10 min light
10 mins of light per day are sufficient to reset the clock

12. Mole-rats lack daily cycles

13. Clock mechanisms Location of the clock Clock genes
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. Period
In hamsters, SCN lesion disrupts clock while SCN transplant restores clock

16. Isolated rat SCN cells 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. Distribution of circadian clocks in tissues and taxa

20. Mammalian clock pathways

21. period alleles exhibit altered circadian rhythyms
in Drosophila melanogaster

22. Genetic basis of the clock in flies
NOON: per and tim genes are turned on by CLOCK-CYCLE complex, which binds to promoter
SUNSET: PER and TIM transcription occurs
NIGHT: PER and TIM proteins build up inside the cell, and as a complex can enter nucleus
dbt codes for an enzyme that adds a phosphate to PER, which causes it to be destroyed & adds time delay
DAWN: Cryptochromes absorb blue light and activate cry gene expression. TIM protein is degraded by CRY protein.
PER is released from PER-TIM complex and broken down, so CLOCK-CYCLE can activate per and tim

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. 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. Melatonin causes entrainment
Short pulses of light entrain SCN and pineal cells
Photoreceptors occur in the pineal and eyes of birds
Photoreceptors occur in retina of mammals
Drosophila, honey bees, hamsters and humans share same genes - likely common ancestor was a flatworm that lived about 600 MYA

25. Clock neurons in fly brains
Hall, J.C Genetics and molecular biology of rhythms in Drosophila and other insects. Adv. Genet. 48: 1-286