Introductory analysis of the GTP-cap phase-change kinetics at the end of a microtubule.

An introductory analysis is provided for the two-phase macroscopic kinetic model of the end of a microtubule. Some general relations are derived for one end of a very long microtubule in solution but the main results refer to the steady-state properties of microtubules grown on nucleated sites, as in the experiments of Mitchison and Kirschner [Mitchison, T. & Kirschner, M. W. (1984) Nature (London), in press]. The two-phase model makes it possible to understand qualitatively how long microtubules can grow well below the critical concentration and also how grown microtubules can rapidly disappear from a nucleated site by shortening following a phase change.