Biological Clock

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.

12. Mole-rats lack daily cycles

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

21. Mammalian
clock
pathways

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