Alternating phases of DNA synthesis and mitosis, during the first 12 cell divisions of frog embryos, are driven by autonomous cytoplasmic oscillations of M-phase promoting factor (MPF). Cell-free extracts of frog eggs provide a convenient preparation for studying the molecular machinery that generates MPF oscillations and the surveillance mechanism that normally prevents entry into mitosis until chromosomal DNA is fully replicated. Early experiments suggested that unreplicated DNA blocks MPF activity by inducing phosphorylation of a crucial tyrosine residue, but recent evidence implicates a stoichiometric inhibitor (an MPF binding protein) as the 'braking' agent. Using a realistic mathematical model of the mitotic control system in frog egg extracts, we suggest that both tyrosine phosphorylation and a stoichiometric inhibitors are involved in the block of MPF by unreplicated DNA. Both pathways operate by raising the cyclin threshold for MPF activation. As a bonus, in the process of analyzing these experiments, we obtain more direct and reliable estimates of the rate constants in the model.

No abstract

A systematic study of the potential response of platinum electrode to various redox species in ClO 2 -based chemical oscillators proves that it is the Cl(+4)/Cl(+3) redox pair that determines the potential in these systems. According to the results reported here, the electrode potential can be predicted from known concentrations of the Cl(+4) and Cl(+3) species, or inversely, these concentrations can be estimated from the measured potential. Thus, at constant ClO 2 levels the platinum behaves as a "chlorite selective" electrode.To demonstrate the feasibility of this approach, potentiometric oscillations were recorded in two different ClO 2 -based oscillating systems, and the experimental results were compared with potentiometric traces calculated by two alternative models.