How Common Are They?
They are common in bacteria, plants, and animals. Without biological clocks, neither life nor pleasure would exist. When we sing a musical note that we just heard we are able to reproduce the original frequency with high accuracy. We also know from everyday experience that humans are able to keep the beat to within a few percent for a long time. When doing sport or when dancing, we are able to keep the timing to high accuracy. (For shorter or longer times, the internal clocks are not so precise.
Where Are They Located?
All these clocks are located in the brain. Brains process information. Also, computers do this, and like computers, all brains need a clock to work well. Every clock is made up of the same components. It needs an oscillator to determine the rhythm and a mechanism to feed the oscillator with energy. In addition, every clock needs an oscillation counter, i.e., a mechanism that reads out the clock signal, and a means of signal distribution throughout the system is required, synchronizing the processes attached to it. Finally, a clock needs a reset mechanism.
How Does the Idea of Living Clocks Work?
If the clock has to cover many time scales, it needs several oscillators with different oscillation frequencies and a way to reset their relative phases. Even though physicists know fairly well how to build good clocks, we still do not know many aspects of biological clocks. Most biological oscillators are chemical systems; some, Ref. 22 like the heart muscle or the timers in the brain, are electrical systems. The general elucidation of chemical oscillators is due to Ilya Prigogine; it has earned him a Nobel Prize for chemistry in 1977. But not all the chemical oscillators in the human body are known yet, not to speak of the counter mechanisms.
What Processes Are Governed By Living Clocks?
For example, a 24-minute cycle inside each human cell has been discovered only in 2003, and the oscillation mechanism is not yet fully clear. (It is known that a cell fed with heavy water ticks with 27–minute instead of Ref. 23 24–minute rhythm.)
It might be that the daily rhythm, the circadian clock, is made up of or reset by 60 of these 24–minute cycles, triggered by some master cells in the human body. The clock reset mechanism for the circadian clock is also known to be triggered by daylight; the cells in the eye who perform this resetting action have been pinpointed only in 2002. The light signal from these cells is processed by the suprachiasmatic nuclei, two dedicated structures in the brain’s hypothalamus. The various cells in the human body act differently depending on the phase of this clock. The clocks with the longest cycle in the human body control aging. One of the more famous aging clocks limits the number of divisions that a cell can undergo.
Indeed, the number of cell divisions is finite for most cell types of the human body and typically lies between 50 and 200. (An exception is reproductory cells – we would not exist if they would not be able to divide endlessly.)
The cell division counter has been identified; it is embodied in the telomeres, special structures of DNA and proteins found at both ends of each chromosome. These structures are reduced by a small amount during each cell division. When the structures are too short, cell division stops. The purely theoretical prediction of this mechanism by Alexei Olovnikov in 1971 was later proven by a number of researchers. (Only the latter received the Nobel Prize in medicine, in 2009, for this confirmation.)
Research into the mechanisms and the exceptions to this process, such as cancer and sexual cells, is ongoing. Not all clocks in human bodies have been identified, and not all mechanisms are known. For example, the basis of the monthly period in women is interesting, complex, and unclear. Other fascinating clocks are those on the basis of conscious time. Of these, the brain’s stopwatch or interval timer has been most intensely studied. Only recently was its mechanism uncovered by combining data on human illnesses, human lesions, magnetic resonance studies, and the effects of specific drugs.
The basic interval timing mechanism takes place in the striatum in the basal ganglia of the brain. The striatum contains thousands of timer cells with different periods. They can be triggered by a ‘start’ signal. Due to their large number, for small times of the order of one second, every time interval has a different pattern across these cells. The brain can read these patterns and learn them. In this way, we can time music or specific tasks to be performed, for example, one second after a signal. Even though not all the clock mechanisms in humans are known, biological clocks share a property with all human-built and non-living clocks: they are limited by quantum mechanics. Even the simple pendulum is limited by quantum theory.