Constant Change: Microtubule Dynamic Instability

If some microtubules are lengthening while others are shortening at the same time and concentration, then the system is in a state of microtubule dynamic instability.

If GTP-bound tubulin dimers are added to the end faster than the GTP is hydrolyzed, then a protective cap is maintained; if this cap is present, then the structure remains stable.

If a microtubule loses its GTP cap and exposes GDP-bound tubulin to the exterior, then it will undergo a rapid disassembly known as a catastrophe.

If the rate of GTP hydrolysis exceeds the rate of new GTP-tubulin addition, then the GTP cap is lost. If the cap is lost, then the microtubule becomes unstable and shrinks.

If GTP hydrolysis happens faster or if the temperature drops low enough to destabilize the polymer, then the frequency of catastrophe increases, promoting instability.

If a shrinking microtubule can regain a GTP cap by adding new GTP-tubulin dimers, then it can transition back to a growth phase; therefore, the process is reversible.

If the depolymerization phase is halted by the addition of a new cluster of GTP-bound tubulin, then the microtubule has been "rescued" and returns to the assembly phase.

If the minus end is typically anchored and stabilized by the centrosome, then the plus end is left free. If the plus end is free, then it is the dynamic site where assembly and disassembly occur.

If microtubules can grow and shrink rapidly, then they can probe the cytoplasm to find kinetochores. If they find them and then shrink, then they generate the tension needed to move chromosomes.

If tubulin is a GTPase that binds specifically to guanosine triphosphate, then it uses GTP, not ATP, to regulate its assembly and disassembly cycle.

If the concentration of free dimers is at a specific threshold, then the rate of addition equals the rate of loss. If these rates are equal, then the system maintains a steady-state balance.

If a microtubule is a heterodimer, then it must be made of two distinct parts; in this case, the two parts are alpha and beta tubulin.

If a cell needs to change its shape or move its organelles quickly, then it needs a flexible skeleton. If microtubules can be destroyed and rebuilt in minutes, then the cell can reorganize efficiently.

If mitosis requires microtubules to be dynamic to move chromosomes, then a drug that freezes them in place will prevent the spindle from working. If the spindle fails, then the cell cannot divide.

If the bond between the second and third phosphate in GTP is broken by water, then the process is defined as hydrolysis, which changes the tubulin's shape.

If the GTP cap is lost and GDP-tubulin is exposed, then the protofilaments lose their affinity for each other and peel outward, causing the structure to fall apart.

If there are plenty of GTP-tubulin dimers available, then the rate of addition will almost always exceed the rate of hydrolysis. If addition is faster, then the GTP cap is rarely lost.

If the centrosome acts as a microtubule-organizing center (MTOC), then it caps the minus ends. If the minus ends are capped, then dynamic instability is limited to the plus ends.

If the plus end specifically alternates between lengthening and shortening, then the process is defined as dynamic instability rather than treadmilling.

If GTP is hydrolyzed to GDP, then the tubulin dimer changes shape slightly. If this new shape is curved, then it puts mechanical strain on the tube, making it prone to "exploding" if the cap is lost.