Figure 3.

Assembly rates and dynamic balance. (A) “Antennae model” for microtubule length control, in which accumulation of kinesin-8 leads to length dependent microtubule depolymerization (Varga et al. 2006). (B) A dynamic balance model combining length-dependent disassembly (as in A) and a length-independent assembly leading to a single stably defined steady-state microtubule length. (C) Nuclear growth in early Xenopus embryos is limited by available importin α levels, which are reduced in smaller cells (see D). (D) For spherical cells of radius, R, surface area to volume ratio (SA/V) is inversely related to cell size. In small cells, increased SA/V results in greater sequestration of importin α to the membrane from the cytoplasm, resulting in a reduction in the concentration of free importin α. (E) “Timer model”: A structure that undergoes assembly faster (red) will end up larger than a structure that assembles more slowly (blue) in a given time interval.