Biological clocks Clock periods –Circannual –Circalunidian –Circadian Clock mechanisms...
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Transcript of Biological clocks Clock periods –Circannual –Circalunidian –Circadian Clock mechanisms...
Biological clocks
• Clock periods– Circannual– Circalunidian– Circadian
• Clock mechanisms– Entrainment– Neural location– Genetic basis
Hibernation follows annual rhythm ingolden-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.
QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.
Lunar position affects the tides
Spring tide
Sun and moonalign, tidalexcursion isgreatest
Neap tide
Sun and moonare perpendicular,tidal excursion isleast
Kangaroo rat feeding shows lunar cycles
K-rat activity at a feeder is confined to dark periods
QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.
occurred during periodof seed shortages
Isopod activity follows daily tidal flow
Isopods are usually covered with water at high tide. They retainthis activity even when kept in the lab with no tidal fluctuation.
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
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
Clock mechanisms
• Location of the clock– Suprachiasmatic nucleus of the hypothalamus– Pineal gland
• Clock genes– Period– Timeless– Tau (doubletime)
Isolated rat SCN cells
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 cyclesMelatonin release from chicken pineal glands cultured in vitro
Light cycles No light cycles
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
http://www.nimh.nih.gov/publicat/bioclock.cfm
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
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