Microtubules are hollow cylinders assembled from \(\alpha\)-\(\beta\) tubulin heterodimers that stack head-to-tail into protofilaments, usually thirteen of which close into a tube. Each dimer carries a GTP-binding site on \(\beta\)-tubulin; GTP hydrolysis after incorporation makes the polymer less stable, which is why microtubules are not rigid cables but dynamic structures that alternate between slow growth (rescue) and rapid shortening (catastrophe). This behavior is called dynamic instability, and it is the property the mitotic spindle depends on.
During mitosis the cell builds the mitotic spindle: two centrosomes nucleate microtubules that grow and shrink, probing space until some attach to kinetochores — protein complexes assembled on centromeric DNA of each sister chromatid. Correct attachment means each sister is captured by microtubules from opposite poles, so the pair is under tension and will be pulled in opposite directions at anaphase. Microtubule flux and depolymerization at the kinetochore then drive chromosome movement.
Two drug classes attack this machine from opposite directions. Vinca alkaloids (vincristine, vinblastine, vinorelbine) bind tubulin at the vinca domain and prevent dimers from polymerizing; at high concentration they depolymerize existing microtubules, and at the low concentrations achieved clinically they mainly suppress dynamic instability, so the spindle cannot remodel. Taxanes (paclitaxel, docetaxel) bind a distinct pocket on \(\beta\)-tubulin, stabilize the polymer, and also suppress the growth-and-shrink switching that spindle function requires. A stabilizer and a destabilizer therefore produce the same functional defect: a spindle that cannot achieve and maintain correct kinetochore attachments. That convergence is why both classes cause mitotic arrest even though one adds polymer and the other removes it.