450
Fig. 9.1 N:C Ratio Model. (a) The N:C ratio model argues for a maternally provided inhibitor (I)
of an activity (A), for example cell cycle checkpoints or zygotic transcription, and that the inhibitor
is titrated by a nuclear component (N). As the number of nuclei increases, the equation shifts to the
right, freeing the activator. (b) A variation on this model is that the inhibitor competes with the
activator for a limited number of binding sites. As the number of binding sites (for example DNA)
increases with each cell cycle, the inhibitor (for example histones) becomes limiting and is no
longer sufficient to impede the activator (for example a transcription factor). (c) The exponential
increase in the number of nuclei (in blue) leads to a sharp increase in the nuclear factor and a sharp
decline in the inhibitor as embryos approach the MBT. The inhibitor falls below a threshold and
events at MBT commence. The scale for the inhibitor is arbitrary
M. Zhang et al.