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. 2012 Dec 4;8:630. doi: 10.1038/msb.2012.62

Figure 6.

Figure 6

A design suitable for the cellular clocks with a fixed period. (A) A single-negative feedback loop with an additional fast positive feedback loop, with which activator (A) activates itself and degrades quickly. This structure has been identified in various biological oscillators like the cell cycle and pacemaker in the sino-atrial node (Tsai et al, 2008). (B) A single-negative feedback loop with an additional slow negative feedback loop, with which activator (A) represses itself and degrades slowly. Circadian clocks in mammals or Drosophila have been shown to have this structure (Supplementary Table 4) (Blau and Young, 1999; Benito et al, 2007; Liu et al, 2008). (C, D) The period of the NNF is nearly constant for the perturbations in transcription rates while the period of the PNF changes about two-fold. Parameters used are as in Supplementary Figure 5C. The period is plotted as a color where green refers to the period with the unperturbed parameters.