Fig. 1.

Summary of the processes in a 2-stage cascade of two-state systems. (A) A diffusible ligand (solid circle) is reversibly bound by cell surface receptors. A diffusible intracellular molecule (star) shuttles in and out of the nuclei. (B) Transitions between mobile and immobile states for a particle in the extracellular stage of the cascade. (C) Immobile particles in the first stage initiate the production of mobile particles in the second stage. In this case, a ligand-receptor complex is an enzyme acting on a pool of inactive intarcellular molecules (present in excess; denoted by an empty square). (D) A mobile intracellular molecule is reversibly trapped by immobile traps. In this case, the traps are nuclei distributed in a shared cytoplasm of the early embryo.