Density fractionations of yeast stationary phase cells have shown that after day 2, the cultures contain a mixture of quiescent cells, generated as daughter cells in the last cell division after the diauxic shift, and non-quisecent cells, which ultimately lose the ability to reproduce and become necrotic or apoptotic. The cell population after 3 days is thus heterogenous, which should be kept in mind when interpreting the data. Microtubule dynamics allow cells to rapidly assemble, remodel, or disassemble polarized arrays of microtubules. Pure tubulin in vitro shows intrinsic dynamic instability, whereby microtubules spontaneously nucleate, grow steadily, and then spontaneously and rapidly depolymerise. Dynamic instability is well described by four parameters: growth rate, shrinkage rate, catastrophe frequency and rescue frequency. In cells, these parameters are all heavily regulated. Amongst the regulators are members of the kinesin-13, -14 and -8 families, which modulate the catastrophe frequency. MCAK and related kinesins-13 can diffuse along the microtubule lattice to the growing microtubule tip and drive tubulin subunits to dissociate, either alone or in complex with the MCAK, driven by an ATPase cycle that is distinct from that of translocating kinesins. Kar3, a kinesin-14, is a minus end directed translocase that heterodimerises with Cik1, targets the plus ends of taxol-stabilised microtubules, and depolymerizes them at a rate dependent on the taxol concentration. The kinesins-8 are plus end-directed translocases, at least one of which, S. cerevisiae Kip3, can step processively along the lattice of GMPCPP microtubules to their plus ends, where it enhances the Rapamycin 53123-88-9 off-rate of GMPCPP tubulin heterodimers. The tail of Kip3 contains a microtubule and tubulin-heterodimer binding site that enhances its processivity and microtubule end binding. Although, like MCAK, Kip3 ATPase is stimulated by tubulin heterodimers the mechanism of depolymerisation is different. In that Kip3- tubulin complexes are displaced from the microtubule tip by the arrival of another Kip3. The cell biology of Kip3 is consistent with this type of length-dependent catastrophe mechanism operating in vivo. The velocity of Kip3 in vivo is 47–73 nm s21, similar to its single molecule velocity of 50 nm s21 on brain microtubules in vitro, and is sufficient to account for its accumulation at the ends of microtubules growing at 23 nm s21 in vivo. Deletion of kip3 leads to unusually long spindles, longer cytoplasmic microtubules, effects on chromosome congression and a decrease in microtubule catastrophe frequency consistent with microtubule depolymerase activity. However in vivo Kip3 also increases microtubule growth rate, rescue frequency and pause duration whilst decreasing shrinkage rate, suggesting Kip3 has a wide range of effects on dynamic microtubules. The tail of Kip3 is required in vivo for the increase in microtubule rescue frequency and reduction in microtubule shrinkage rate. These effects may result directly from binding of the Kip3 tail to microtubules since in vitro the tail reduces the shrinkage rate of GDP microtubules. The cell biology of other kinesins-8 is also only partially consistent with their having solely a microtubule depolymerase activity. Some observations are consistent with depolymerase activity.