Figure 1. Common mechanisms regulate circadian rhythms in eukaryotes.

Circadian oscillators function to generate a free-running period that is close to 24 h when the organism is kept in constant environmental conditions. The oscillators can be reset by environmental signals, such as light and temperature, through input pathways. The oscillators send phase information through the output pathways to control the expression of clock-controlled genes (ccgs) and rhythmic processes. Eukaryotic circadian oscillators are composed of positive and negative elements that form autoregulatory feedback loops. In these networks, positive elements of the loop activate transcription of the genes that encode negative elements. As the concentrations of the negative elements rise they repress the activity of the positive elements. Phosphorylation-induced decay of the negative elements reduces their concentrations, leading to reactivation of the positive elements allowing the cycle to start again. Squiggly lines indicate rhythmicity; arrows represent activation; blocked lines indicate repression.