Editing Microtubule (section)

===Dynamic instability===
[[File:MicrotubuleDynamicInstability.ogv|thumb|Animation of the microtubule dynamic instability.  Tubulin dimers bound to GTP (red) bind to the growing end of a microtubule and subsequently hydrolyze GTP into GDP (blue).]]

Dynamic instability refers to the coexistence of assembly and disassembly at the ends of a microtubule. The microtubule can dynamically switch between growing and shrinking phases in this region.<ref>{{Cite book |url=https://archive.org/details/cellmolecularbio04edkarp/page/355 |title=Cell and Molecular Biology: Concepts and Experiments |vauthors=Karp G |publisher=John Wiley & Sons |year=2005 |isbn=978-0-471-46580-5 |location=USA |page=[https://archive.org/details/cellmolecularbio04edkarp/page/355 355]}}</ref> Tubulin dimers can bind two molecules of GTP, one of which can be hydrolyzed subsequent to assembly.  During polymerization, the tubulin dimers are in the [[Guanosine triphosphate|GTP]]-bound state.<ref name="weisenberg" />  The GTP bound to α-tubulin is stable and it plays a structural function in this bound state. However, the GTP bound to β-tubulin may be [[hydrolysis|hydrolyzed]] to [[Guanosine diphosphate|GDP]] shortly after assembly. The assembly properties of GDP-tubulin are different from those of GTP-tubulin, as GDP-tubulin is more prone to depolymerization.<ref>{{Cite journal |vauthors=Weisenberg RC, Deery WJ, Dickinson PJ |date=September 1976 |title=Tubulin-nucleotide interactions during the polymerization and depolymerization of microtubules |journal=Biochemistry |volume=15 |issue=19 |pages=4248–54 |doi=10.1021/bi00664a018 |pmid=963034}}</ref> A GDP-bound tubulin subunit at the tip of a microtubule will tend to fall off, although a GDP-bound tubulin in the middle of a microtubule cannot spontaneously pop out of the polymer.  Since tubulin adds onto the end of the microtubule in the GTP-bound state, a cap of GTP-bound tubulin is proposed to exist at the tip of the microtubule, protecting it from disassembly.  When hydrolysis catches up to the tip of the microtubule, it begins a rapid depolymerization and shrinkage. This switch from growth to shrinking is called a catastrophe.  GTP-bound tubulin can begin adding to the tip of the microtubule again, providing a new cap and protecting the microtubule from shrinking. This is referred to as "rescue".<ref name="pmid6504138">{{Cite journal |vauthors=Mitchison T, Kirschner M |year=1984 |title=Dynamic instability of microtubule growth |journal=Nature |volume=312 |issue=5991 |pages=237–42 |bibcode=1984Natur.312..237M |doi=10.1038/312237a0 |pmid=6504138 |s2cid=30079133}}</ref>