COMMENTARY Chance encounters and precision in mitosis

The flawless distribution of chromosomes to the daughter cells in mitosis depends in part on unlikely sources microtubule instability and chance events. Equal chromosome distribution results from the regular arrangement of microtubules shown in Fig. 1. The result is attachment of the partner chromosomes to opposite spindle poles and their movement to opposite poles in anaphase, so that each daughter cell will contain one copy of each chromosome. Attachment probably arises from the capture of spindle microtubules by kinetochores. Capture is facilitated by a novel form of instability called 'dynamic instability', which is driven by GTP hydrolysis or at least leads to GTP hydrolysis. Thus, energy may be used to purchase lability rather than stable bonds (for reviews see Kirschner & Mitchison, 1986; Cassimeris et al. 1987). The key feature of dynamic instability is a remarkable state in which some microtubules shorten rapidly,