Biological Clocks Flashcards
Circadian Rhythm
(nocturnal vs diurnal)
Examples
Circadian rhythm: 24-hour rhythmic patterns of behaviors and physiological processes
Nocturnal animals (hamsters and mice): active during night Diurnal animals (humans): activity is largely restricted to the daytime
Examples:
• Animal migration patterns (butterflies, whales)
• Breeding cycles of reproductive activity
• Hibernation cycles in squirrels
• Transformation rhythm in carpet beetles
Biological clock
& Chronobiology
internal mechanism in organisms that controls periodicity of various functions and activities, such as metabolic changes, sleep cycles, or photosynthesis
Chronobiology: scientific study of biological rhythms
Circadian Rhythm of Metabolic Rate (O2 consumption) and Motor Activity for a Chaffinch Study
To test that daily rhythm is an endogenous circadian rhythm:
1) The bird was initially kept under normal light–dark cycle, which entrained the rhythm.
o Metabolic rate high during day, low during night
o Hopping activity increase during early day (morning), and then decrease
2) Later, bird was kept in constant dim light ->free-running rhythm started earlier in each successive 24-h period
When environment cues are absent, free running rhythm (circadian rhythm not exactly 24 hrs.)
Aschoff (1952) Study kept mice under natural day/night conditions and monitored their locomotor activities for several days
Under 12 hrs light, locomotor activity peaked in early night hours and repeated itself every 24h
Then kept mice in constant darkness (DD) for 9 days –still observed rhythmic cycling of locomotor activity BUT peak shifted to earlier hrs of the day = free running period was shorter than 24 hrs cycle (normally range of 23-26h)
FRP: length of time it takes an organism to complete its daily activity
- not accurate compared to mechanical clocks
- may change length of its period over a course of days/months/years
Concluded biological period was different from physical period
Phenomena of circadian rhythms provides evidence of existence of biological clocks
***Aschoff’s Rules
set of statements used to describe and predict the FRP of different animals housed under different conditions (constant lighting conditions)
***The 4 Aschoff’s Rules
Aschoff’s first rule: FRP of nocturnal organisms is typically less than 24 hrs
Aschoff’s second rule: FRP of diurnal organisms is typically greater than 24 hrs
Aschoff’s third rule: States that increasing intensity of light in LL will typically increase length of a FRP in nocturnal organisms (become less active)
Aschoff’s forth rule: States that increasing intensity of light in LL will typically decrease the FRP in diurnal organisms (become more active)
Temp Compensation of Free-Running Period
When temp rises by 10 °C, rate of most biochemical reactions increases
If biological clocks were affected by changes in temp in a similar way=problem regarding biological clock reliability as timekeepers
Pittendrich found rhythmic occurrence of pupal–adult hatching persists in constant darkness, and independent of variations in temp
The results show that biological clocks have ability to compensate for variability caused by temp
***Biological Clocks Characteristics (4)
1) Continue to run under constant conditions and in absence of environmental cues producing rhythmic patterns
2) Are temp compensated (not sensitive to temp)
3) Under genetic control: alterations in certain clock genes affect period length of the clock so that it runs shorter or longer than 24 hrs
4) Adjustable: biological clock can be reset by external environmental cues
These stimuli include light (photoperiod), noise, food availability and social cues
Benzer et al. (1970s) identified the clock gene in the fruit fly
Suggested that mutations (from X-ray, UV light, or chemical) of the clock gene could lead to:
- Alternation in timing of pupa-adult ecolsion in Drosophila
- Affect rhythmicity of behavioral patterns of the adult fly (locomotors activity and courtship singing)
This gene termed period (per)
Mutagenesis of this gene=abnormal behavior particular pronounced in males, indicating that the clock gene might be located on a sex chromosome
Second gene identified = timeless (tim)
Per/Tim process
When fly kept under L/D 12:12 cycle, levels of RNA of per and tim are low early morning and increase over the day and reach peak level in cytoplasm at night
Per protein phosphorylated in cytoplasm by action of gene called doubletime (dbt)
Per unstable until it binds to tim
The 2 proteins form heterodimer (per/tim) complex –protects per from degradation & promotes per/tim to translocate to nucleus later in the night
Transcription of tim/per and other clock-controlled genes (ccgs) are activated by binding of heterodimeric complex formed by protein cycle (cyc) and protein clock (dclk)
Activity of cyc/dclk complex is inhibited by per/tim complex
In early morning, when sun comes up, cryptochromes protein in nucleus absorb light and become active=degradation of tim/per complex and disappear=leads to activity of cyc/dclk and beginning of next cycle
Anatomical Localization of the Biological Clock in Mammals
principle site of biological clock is a tiny pair of more densely packed neuron clusters in hypothalamus that serves as a biological clock =>suprachiasmatic nuclei (SCN)
Retinal ganglion cells send direct projections to SCN=>retino-hypothalamic tract
SCN as biological clock (Evidence)
When SCN is stim electrically, circadian rhythms can be shifted in a predictable way
Removal: eliminates circadian rhythmicity of physical activity, sleeping and waking, and feeding and drinking
Cortisol & melatonin
They have inverse relationship: when one is high, the other is low
The cycle of these 2 hormones helps to determine your sleep patterns –when you sleep/wake & quality of sleep
Adults can get the deepest sleep generally b/w 2:00-4:00 am, when cortisol is lowest and melatonin is highest
If cortisol is not low enough, or melatonin is not high enough, your sleep will be restless and you may wake frequently
Central vs Peripheral Clock
Central clock: residing in suprachiasmatic nucleus (SCN) of the brain
Peripheral clocks: present in many tissue and organ systems (liver, intestines and heart)—able to introduce rhythmic circadian output in absence of the SCN
Shift Work –Coping w Biological Clock
Biological circadian clock adapts slowly, so rapid transitions b/w diff shift schedules leads to misalignment of rhythmic physiological activities, such as sleep, alertness, performance, metabolism, etc.
Shift work can create a mismatch b/w circadian rhythm and natural light/darkness cycles
Consequences: sleep deprivation, poor performance and increased risk of major disease (heart disease and cancer)
