What is Dynamic instability?

Microtubules do something stranger than treadmilling. A growing plus end carries a GTP cap of freshly added dimers; behind it the GTP is hydrolysed, and GDP-tubulin, which prefers a curved conformation, is held straight in the lattice under strain. If the cap is lost — the end pauses, or growth slows — the strained protofilaments peel outward and the microtubule shortens at $300\,\mathrm{nm}/\mathrm{s}$, twenty times its growth rate: a catastrophe, which ends when a GTP dimer lands and a new cap forms (a rescue). In one population, therefore, some microtubules grow while neighbours collapse: this is dynamic instability. It lets a cell explore its own volume with plus ends every few minutes and rebuild the whole array when the centrosome moves or the cell divides. The cell tunes it through proteins that track plus ends, stabilise (tau, MAP2), sever (katanin) or depolymerise (kinesin-13) microtubules; the drugs colchicine and the vinca alkaloids block assembly and taxol blocks disassembly, all of them poisons of the mitotic spindle used against cancer.