Figure 1.

Basic characteristics of circadian rhythms. Circadian rhythms have a period of ∼24 h. A: the amplitude of various biological processes, such as eating, locomotion, gene expression, etc., vary considerably across organisms and physiological events, as does the phase of the rhythm. B: actograms are used to depict the rhythm of an organism over the 24-h cycle and typically consist of digitized activity values that are presented as a double plot, where a line contains data for that day as well as the proceeding day. In this example, the red arrow denotes the activity of a mouse in a 12-h light/12-h dark cycle, while the blue arrow denotes the activity of the animal as a result of a switch to constant darkness (DD). Such activity in DD is referred to as “free running.” C: the circadian clock depends in part on negative transcriptional/translational feedback mechanisms in which proteins participate in the production of their own negative-feedback regulators. D: the positive and negative regulators in the core clock system. Positive factors TOC (Timing of Cab Expression1), WC-1 (White Collar-1) and WC-2 (White Collar-2), CLK (CLOCK), CYC (Cycle), CLK, BMAL1 (Brain and Muscle Arnt-like Protein-1) regulate the transcription of their own negative regulators, CCA1 (Circadian Clock-Associated1), LHY (Late Elongated Hypocotyl), FRQ (Frequency), PER (Period), TIM (Timeless) and PER, CRY (Cryptochrome). The bacterial system does not rely on a negative transcriptional feedback mechanism but rather relies on three Kai proteins. The rhythmicity governing this system relies on rhythmic phosphorylation and can be recapitulated in vitro by the combination of the three Kai proteins and ATP (164).