Lecture 29: Chronobiology I Flashcards
Why are clocks important
Strongly influence behavior, physiology, health & metabolism on timescales (days to years)
Will and always affect everything
What are the biological rhythms that match rhythmic environments and mirror geophysical rhythms
Earth rotation on N/S axis - day/night - circadian rhythms
Earth rotating at 23.5° - seasons - circannual
Moon cycling around the earth (lunar month) - Spring/Neap tidal cycles - circalunar
Internal clock hypothesis
Timing in field is controlled by internal biological clocks to some extent which continue to function when there are no external time cues
What did the experiment on adult flies kept in light/dark cycles and constant darkness show?
Light/dark cycles : active during the day
Constant darkness : endogenous clock still wakes the fly but earlier each day. Endogenous clock ticks at a time lesser than 24hrs, slightly faster
Internal (endogenous) circadian clock
- continue without external time cues
- clock period in these conditions approximates the period of geophysical cycle
- endogenous clock period = ‘free-running period’ (τ)
If τ is shorter than 24h, wake up earlier each day.
If τ is longer than 24h, wake up later each day.
Measurements
- human behavioral rhythms (example)
- human physiological rhythms (example) -> show circadian rhythm
Done using different instruments and show what occurs in different rhythms
- show during Antarctic summer, person sleeps and wakes up same time each day
- core body temperature (high during day, low during night), blood cortisol levels (high in day, low in night)
How is the endogenous clock entrained?
In humans?
Why is having an inaccurate clock better than an accurate one?
Entrained clock hypothesis
Light
Morning light
Adjusted on a daily basis -> be adaptable to light/dark cycle and make sure that phase relationship is right.
Environment information matches internal clock so that period = that of environment and phase relationship is correct
How does light affect the clock at different times
By phase delays and phase advances
Shows it is harder to adapt to earlier time zone than later ones
Central circadian clock in the brain
Suprachiasmatic nuclei (SCN)
What did experiments show when the SCN was removed and implanted in the wild type and τ mutant hamsters?
SCN lesion to wildtype hamster and τ mutant hamster (faster circadian rhythm) . SCN from opposite hamsters were implanted in them
Behaviour came back but from the donor tissue -> transfer of circadian rhythm form donor to host.
What are the genes that drive the transcription - translation feedback loop and how do they do it?
The PERIOD gene, the TIMELESS gene
Period gene and timeless gene transcript mRNA -> proteins produce -> dimerise -> go back to nucleus to switch off the genes
Time taken for biochemical loop to complete determines directly the behavioural day.
Describe the translation -transcription feedback loop and its importance
Period gene in nucleus -> transcribe to period RNA -> in cytoplasm, RNA translated into protein -> protein enter nucleus -> turn off own genes transcription
Drives daily rhythms. Time taken to complete loop = behavioural period
What did the experiments in the 90s show
What did single base substitutions of the same a functional gene show?
PERIOD protein levels oscillation = behavioural τ
PER protein oscillations lag 6h behind mRNA.
PER enters nucleus
Per^s G to A (SER to ASN): shorter behavioural period (19h)
Per^L T to A (VAL to ASP): longer behavioural period (28h)
Per^0 C to T (STOP codon introduced - shorter mRNA): no rhythm
Circadian clocks adaptive advantage
Synchronise activities
Anticipate rhythmic event occurance
Sélective advantage
- Bacteria experiment : biological clock with endogenous innate period close to LD cycle -> survive and populate
- chipmunk experiment : active at night -> more nocturnal activity -> more detectable by predators
