Role of spindle microtubules in the control of cell cycle timing.

Abstract

Sea urchin eggs were used to investigate the involvement of spindle microtubules in the mechanisms that control the timing of cell cycle events. Eggs were treated for 4 min with colcemid at prophase of the 1st mitosis. No microtubules were assembled for at least 3 h, and the eggs did not divide. These eggs showed repeated cycles of nuclear envelope breakdown (NEB) and nuclear envelope reformation (NER). Mitosis (NEB to NER) was twice as long in colcemid-treated eggs as in the untreated controls. Interphase (NER to NEB) was the same in both. Each cycle was prolonged entirely in mitosis. The chromosomes of treated eggs condensed and eventually split into separate chromatids which do not move apart. This c-anaphase splitting was substantially delayed relative to anaphase onset in the control eggs. Treated eggs were irradiated after NEB with 366-nm light to inactivate the colcemid. This allowed the eggs to assemble normal spindles and divide. Up to 14 min after NEB, delays in the start of microtubule assembly gave equal delays in anaphase onset, cleavage and the events of the following cell cycle. Regardless of the delay, anaphase followed irradiation by the normal prometaphase duration. The quantity of spindle microtubules also influences the timing of mitotic events. Short colcemid treatments administered in prophase of 2nd division caused eggs to assemble small spindles. One blastomere was irradiated after NEB to provide a control cell with a normal-sized spindle. Cells with diminished spindles always initiate anaphase later than their controls. Telophase events were correspondingly delayed. Spindle microtubules are involved in the mechanisms that control the time when the cell will initiate anaphase, finish mitosis and start the next cell cycle.