Periodic oscillations are the basis of time keeping. So begins an article in Nature .
Gore and van Oudenaarden 2009 The yin and yan of nature. Nature vol 457 pp271-2.
They describe how Gallileo’s observation on the pendulum altered time keeping. There are similar oscillatory networks in nature. Simplistically negative feed back with delay in between. Three genes may sequentially suppress each other. The three repressive interactions lead to net negative feed back with a delay in the biochemical processes involved. The delay must be a precise number of seconds. When conditions change then the system must retune. The key element of these oscillators is that they must be robust not transient.
The circadian pacemakers are based on a central pacemaker in the suprachiasmatic nucleus of the brain with subsidiary oscillators in cells. The gene circuitry are a set of transcriptional repressors ( CRY and PER) and activators ( BMAL1 and CLOCK).
Tigges et al describe such a system in mammalian cells. Negative feedback is provided by post-transcriptional repression of the gene encoding tTA by antisense RNA, and positive feedback because tTA enhances its own transcription.
The cell population must oscillate in a co-ordinated manner. Circadian oscillation is affected by sunlight and cell-cycle oscillations pause when there is a shortage of nutrients.
Tigges et al 2009 A tunable synthetic mammalian oscillator. Nature vol 457 pp 309-312.

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