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. Author manuscript; available in PMC: 2010 Sep 7.
Published in final edited form as: J Theor Biol. 2009 May 31;260(1):110–120. doi: 10.1016/j.jtbi.2009.05.018

Figure 1. The Novák-Tyson model of the cell cycle engine in frog egg extracts.

Figure 1

Positive feedback between Cdk1 and Cdc25 (Gautier et al., 1991; Izumi and Maller, 1993; Kumagai and Dunphy, 1991; Kumagai and Dunphy, 1992) and double-negative feedback between Cdk1 and Wee1 (Mueller et al., 1995a; Smythe and Newport, 1992) create the abrupt activation of Cdk1 at G2/M. Inactivation of Cdk1 results from negative feedback in which Cdk1 indirectly targets cyclin B for degradation through the Anaphase Promoting Complex (APC) (Lorca et al., 1998). Synthesis of cyclin is represented by an asterisk (*). Computer simulations of the model reproduce oscillations of Cdk1 (MPF) activity as they are observed in cell-free egg extracts (Novak and Tyson, 1993). Novák and Tyson proposed that unreplicated DNA blocks entry into mitosis by activating the phosphatases that oppose MPF's action on Cdc25 and Wee1.