text: Microtubules are polymers of tubulin subunits arranged in cylindrical tubes. The subunit is made up of alpha and beta tubulin. GTP binds to alpha tubulin irreversibly. Beta tubulin binds GTP and hydrolyzes to GDP.  It is the GDP bound to beta-tubulin that regulates the growth or disassembly of the microtubule. However, this GDP can be displaced by GTP. Beta-tubulin bounded to GTP are described as having a GTP-cap that enables stable growth.
  Microtubules exist in either a stable or unstable state. The unstable form of a microtubule is often found in cells that are undergoing rapid changes such as mitosis. The unstable form exists in a state of dynamic instability where the filaments grow and shrink seemingly randomly. A mechanistic understanding of what causes microtubules to shrink is still being developed.