What makes a biological clock efficient ?

Biological clocks regulate the proper periodicity of several processes at the cellular and organismal level. The cell cycle and circadian rhythm are the best characterized among these but several other biological clocks function in cells at widely variable periodicity. The underlying molecular networks are controlled by delayed negative feedbacks, but the role of positive feedbacks and substrate-depletion has been also proposed to play crucial roles in the regulation of these processes. Here we will investigate which features of biological clocks might be important for their efficient timekeeping. Evolution of biological clocks The ability of organisms to temporally co-ordinate their physiology is evolutionarily advantageous, and is therefore ubiquitous in nature. Organisms have evolved numerous so-called biological clocks to optimise their fitness, by co-ordinating their physiology with the availability of resources. Simple experiments monitoring the growth of bacteria against varying nutrient availability illustrate an enormous flexibility in deciding how frequently cells choose to divide. Yet, the developmental programs that lead to the replication of entire organisms (mammals) are relatively inflexible. The cell cycle, which culminates in cell division, is controlled by regulatory networks that have numerous conserved features and components across the eukaryotic kingdom (Harashima et al., 2013). Circadian clocks allow organisms to co-ordinate their physiology with the external time of day, enabling anticipation of changes in temperature, light availability, predator activity, etc. In contrast to the cell cycle, circadian clocks have evolved multiple times and have many different features (though some shared components) across the eukaryotic kingdom (Dalchau and Webb, 2011). The overall structure of circadian networks involves input pathways, a core oscillator, and output pathways (Dunlap, 1999). The core oscillator comprises multiple feedback loops that sustain circadian rhythms with a period of approximately 24 h. Input pathways enable the