Biology Lecture No. 25: Biological Time-Keeping
Monday April 9 , 2012
-Biological time-keeping simply refers to how biological organisms manage to keep track of time. All life
has evolved on a constant rhythmic change in light and day. Such changes are known as diurnal changes.
-Any phenomena which change daily can be called diurnal phenomena. For any number of diurnal
phenomena, the period is defined as the amount of time required for diurnal phenomena to repeat. The
phasing of diurnal phenomena is subject to change and may be shifted from time to time.
-Examples of diurnal phenomena include: predator behaviour, foraging behaviour, sleep-wake cycles,
body temperature, hormone secretion, and photosynthesis. All of such patterns show diurnal
fluctuations and are not always constant.
The Value In Keeping Track Of Time:
-Is there value in knowing when the sun goes down? What benefit lies in anticipating environmental
changes, rather than just responding to them?
-Lhcb2 is a gene in plants that codes for a protein used in light harvesting. The very transcription of this
gene shows fluctuations as it is transcribed for the majority in the daytime. The amount of transcript
peaks during the night-time (right before dawn), demonstrating that the gene does not respond to
change, but rather anticipates it.
-It would be far more logical that the message for making protein would be available before sunlight, so
a photosystem could receive maximum sunlight during the day. Even under constant light, the increase
in amount of transcript shows that there is an internal method of keeping track of time and not merely
-The plant’s ability to keep track of time by itself can be explained by the innate ability that all organisms
possess an endogenous biological clock.
-A circadian rhythm is a biological rhythm with the period of around 24 hours or the time it takes for the
Earth to rotate once on its axis. Two major criteria are used to define a biological rhythm as circadian:
entrainment and free-running.
-Entrainment is used to denote a rhythm that can be set to the external environment. Free-running is
used to denote a rhythm that doesn’t require constant input and will continue in the absence of
external cue. The rhythm doesn’t necessarily have to be perfect forever, but for many days, it will often
retain the same rhythmic nature. Circadian Phenomena:
-Diurnal phenomena are not merely restricted to circadian phenomena. The most common examples of
circadian phenomena include sleep-wake cycles, body temperatures, and hormone levels.
-Although the genes are under circadian rhythmic control in photosynthesis, the process of fixing CO is 2
itself ultimately governed by the amount of light available from the environment. It is an example of
non-circadian control, the other form of diurnal phenomena.
-Many human genes are expressed under circadian and or circadian control.
The Nature Of The Clock:
-The biological clock of all organisms is designed to receive input in the form of entrainment in order to
achieve certain results or states as the output.
-All clocks oscillate. The biological clock simply repeats itself and oscillates between two states just like
any other working clock.
The Genetic Basis Of Circadian Rhythms:
-In a study, mutants in drosophila that contained a dysfunctional clock were isolated. In the wild type,
the developmental transition from being a larva to a fly shows an incredibly smooth and controlled
-The mutants however displayed a pupa to adult fly transition that was arrhythmic and abrupt. This
hinted the notion that the biological clock had a genetic basis.
The Biological Clock Of Neurospora:
-in every system, though the nuances may be different, the biological clock is an oscillation between
transcription and translation. In the fungus Neurospora, the gene frq is the basis for this clock. Frq
mRNA levels increase and then decrease as the mRNA periodically decays.
-The frq protein is delayed as it takes longer to make a protein. It is this frq protein that actually inhibits
the transcription factors WC-1 and WC-2. As the protein accumulates, it is its very transcription that
becomes more regulated. When the protein starts vanishing due to