block 10-circadian rhythms Flashcards
(46 cards)
Circadian clocks
-organised by an endogenous clock
temporal coordination
Temporal coordination refers to the ability of biological systems to time internal processes so they occur at the right moment — often in alignment with environmental cues like light, temperature, or time of day.
In plants, components of the photosynthetic machinery are synthesised at the time when they are needed – in the morning
- another reason why having a clock is important
What is temporal separation of incompatible processes?
-Temporal separation is a strategy where biological processes that cannot occur simultaneously (due to conflicting conditions) are separated in time rather than space. This ensures that each process can function optimally without interference.
-Cyanobacteria carry out both photosynthesis (which produces oxygen) and nitrogen fixation (which requires a low-oxygen environment).
However, oxygen is toxic to the enzyme that fixes nitrogen, so these two processes are incompatible.
To solve this, cyanobacteria use temporal separation — they do photosynthesis during the day, when light is available, and nitrogen fixation at night, when oxygen levels are lower.
control of metabolism
- metabolism is rhytmic
-Clock mutant mice (no circadian rhythm) have a tendency to gain weight due to abnormal regulation of appetite and fuel utilisation
-why having a clock is important
avodance of predators in normal vs mutant mice
A functional circadian clock helps animals stay active at appropriate times of day — often when it’s safer and predators are less active.
🧪 Study Example (US chipmunk experiment):
Normal chipmunks (with a working internal clock) stayed in burrows at night and were active during the day.
Mutant chipmunks (with a disrupted clock) emerged at night — and were heavily preyed on by nocturnal predators like weasels.
📌 Conclusion:
The circadian clock improves survival by syncing behavior (like feeding or hiding) to the safest time of day. Without it, animals may become active during dangerous periods, increasing their risk of predation.
orientation
changes of season due to clock
-breeding is seasonal
-some animals change coat colour in the winter
-dinal point on lecture slide to all the reason above why having a circadian clock is important
circadian variation in the expression of diseases
Many diseases and symptoms show daily (circadian) rhythms. For example, pain sensitivity is often higher in the morning, and asthma attacks are more common at night. This is because body functions like hormone levels, inflammation, and nerve activity change over a 24-hour cycle
-usefful for smart medicine= target critical times
situations where the body clock is out of synchrony with the environment
- After travelling through time zones (jet lag)
- When working rotating shifts
- When persistently having to re-set endogenous
timing to conform to social obligations (school,
work, socializing)
jet-lag and shift-work symptoms
Short term=
- disrupted sleep
- loss of appetite
- gastro-intestinal problems
- difficulty concentrating* feeling disorientated
- memory problems* clumsiness
- lack of energy
- irritability
long term= INCREASED RISK OF :▪ cancer▪ cardiovascular disease▪ depression
how can we study the circadian clocks in animals?
We start by identifying a measurable biological rhythm — such as hormone levels, body temperature, or activity patterns.
🧪 This often involves collecting biological samples (e.g., blood, urine, or buccal cells) and tracking physiological parameters over time.
📌 Note:
Best suited for larger animals.
Can be invasive, technically complex, and expensive, especially for long-term monitoring.
locomotor activity rhythm in small rodents and drosophila melanogaster
In small rodents or fruit flies (Drosophila), we can measure how much they move during light and dark periods.
These movement patterns form a clear rhythmic pattern (recorded in something called an actogram) — showing when they’re active or resting, both under normal light/dark cycles (LD) and even in constant darkness (DD
properties of the clock 1
Circadian clocks keep ticking even in constant conditions (e.g. constant darkness or constant light), a phenomenon known as free-running.
🔁 The internal rhythm has a period close to, but not exactly 24 hours.
💡 Key Points:
Each species has a characteristic range of periods, not a fixed value.
There’s natural variation in this period across individuals in both wild and lab populations.
normal patterns observed in actogram
An actogram is a visual representation of an animal’s activity over time.
🌀 Under normal light-dark (LD) cycles:
Animals show entrained rhythms, meaning they are active at the same time each day (e.g., nocturnal animals active during dark phase).
Activity starts and stops in sync with the external light cues.
🌑 Under constant conditions (e.g. constant darkness, DD):
The rhythm becomes free-running — it continues without light cues but shifts slightly each day, reflecting the animal’s internal clock (e.g., activity starts a little earlier or later each day).
✅ This shift helps demonstrate that the rhythm is endogenous (generated internally), not just a response to external cues.
free run patterns observed on the actogram
When light cues are removed and the animal is kept in constant darkness, it follows its internal circadian rhythm (usually not exactly 24 hours).
For example, if an animal has a 23.5-hour internal clock, its activity will start a bit earlier each day.
This appears in the actogram as a diagonal drift — each line of activity starts a little earlier than the one above.
properties of the clock 2
Entrainable. Through sensory modalities or even through
bespoke ‘clock molecules’ (for instance blue-light sensitive
proteins called cryptochromes) the clock is able to detect an
external rhythmic variable (light, temperature, vibrations,
food, social interactions, etc.). The clock aligns its period to it
and assumes a defined phase relationship towards it. Once
entrained, the clock tends to resist a change in the cycling
condition, before entraining to the new cycle. This proves
that entrainment is not simply synchronisation but
additionally a form of ‘memory’ deriving from two
oscillators resonating together. The external cycling stimulus
is called a Zeitgeber, which means time giver in German.
propertie of the clock 2 on the actogram-CHEAT SHEET I NECESSARY
📊 What You’d See on the Actogram:
1. Before Entrainment – Free-Running Rhythm
If the animal is in constant darkness (DD), you’ll see a diagonal drift in activity (not quite 24 hrs).
This shows its natural circadian rhythm is slightly off (e.g., 23.7 or 24.3 hrs).
- Entrainment Begins
Once the animal is exposed to a regular external cue (e.g. 12 hours light / 12 hours dark, or daily feeding time), its internal clock adjusts.
On the actogram, the diagonal drift stops.
Activity lines up vertically, showing that the rhythm is now stable and synced to the external cycle.
- Phase Relationship
The activity now begins at a consistent time relative to the Zeitgeber — for instance, just after lights-off.
This steady timing is the phase relationship between the internal clock and the external cue.
- After a Zeitgeber Shift – Transient Days
If the Zeitgeber (e.g. light timing) suddenly changes, the rhythm doesn’t shift instantly.
Instead, you’ll see a few days of gradual adjustment on the actogram — this is called a transient phase.
The clock “resists” change at first, showing it’s not just reacting immediately — it has a kind of memory.
properties of the clock 3
Temperature compensation. The circadian clock
measures time independently of temperature. The Q10
temperature coefficient is a measure of the rate of
change of a biochemical reaction as a consequence of
increasing the temperature by 10 °C. For most
biochemical reactions, Q10 = 2; for circadian period
length: Q10 ~ 1 (0.8 – 1.4). This is very important
especially for poikilotherms as the subjective perception
of time would be too variable to be useful.
-this shows circadian clock is barelt affected by temperature as the change in q1 is small
how would you observe property 3 on the circadian clock
On the actogram, you would see no shift in activity timing, even as the temperature fluctuates.
The activity bars will stay aligned in the same pattern each day, maintaining their regular timing.
If the temperature changes (e.g., a warm day followed by a cool night), there would be no observable shift in the time when activity begins — the rhythm remains consistent.
the genetic approcah
the study of utant genes which may have normal functions, research may better understand normal genes
Strengths
* unbiased
* does not require previous
knowledge
* starting point for
biochemical/molecular studies
Weaknesses
* blind to lethal genes
* blind to epistasis
* cannot solve non-
linearity
the genetic analysis of behavior
-Genetic analysis of behaviour began seriously in 1960’s, through selection experiments and strain comparisons. The underlying rationale was that behaviour is a quantitative trait, hence cannot be studied at the level of single genes
-drosphila is the perfect model system to study neurogenetics …
The godwin model
The Goodwin model is a theoretical framework that helps explain how circadian rhythms (or other oscillations in physiology) can be generated. Here’s what you need to know:
Oscillation via Negative Feedback: The model suggests that physiological oscillations (like the circadian cycle) are driven by negative feedback loops. This means that the accumulation of a protein over time will inhibit the gene that produces it, creating a feedback loop.
Example: Protein X (e.g., PER) is produced and increases in concentration, but eventually Protein X inhibits the gene that codes for Protein X. This negative feedback slows down or stops further production of Protein X.
Delay and Cooperativity:
Delay: There’s a time lag between the protein being produced and it inhibiting its own gene, which is a key feature of oscillations (a cycle).
Cooperativity: Multiple proteins or molecules may interact, amplifying the feedback loop and making the system more sensitive or robust.
In simpler terms, the model explains how an oscillation or rhythmic pattern can arise from feedback between proteins that regulate each other over time.
per as a pas protein
PER is also identified as a PAS protein. PAS proteins (Per-ARNT-Sim domain proteins) are a family of proteins that are involved in circadian rhythms, oxygen sensing, and other biological processes.
PAS proteins play a role in signal transduction and are involved in the regulation of gene expression in response to environmental changes, including the circadian rhythm cycle.
Negative feedback loop of PER
The PER (Period) protein, which is central to the circadian clock, has been shown to participate in a negative feedback loop that regulates its own expression over time. Here’s a summary of how this works:
PER accumulation: PER protein is produced and accumulates in the cell over a 24-hour cycle, particularly in the evening and night.
Transcriptional regulation: Once enough PER is present, it inhibits the transcription of the PER gene.
Nuclear localization: Late at night, PER moves into the nucleus, where it helps to block its own transcription — a process that’s essential for resetting the clock.
The feedback (inhibition) happens in a time-delayed manner, and the combination of these processes leads to the 24-hour rhythm.
