Microtubules
The 3rd type of cytoskeletal filaments, the microtubules, find out the position of membrane-bound organelles and directs their intracellular transport. For instance, during mitosis the mitotic spindle involved in separating the replicated chromosome is an assembly of microtubules. Microtubules are like hollow cylindrical structures containing an outer diameter of 25 nm that are built from the protein tubulin. Rigid wall of a microtubule is made up of a helical array of alternating α- and β-tubulin subunits, each 50 kDa in size. Through a microtubule a cross- section reveals that there are 13 tubulin subunits per turn of the filament. In cells Microtubules are formed by the addition of α- and β- tubulin molecules to nucleation centers or pre-existing filaments. Usually One end of the microtubule is linked to a microtubule-organizing center called a centrosome. The drugs vinblastine and colchicine inhibit the polymerization of microtubules, so blocking cell processes such as cell division that based on functioning microtubules. Another compound, taxol, stabilizes tubulin in promotes polymerization and microtubules. Some of these compounds, like taxol and vinblastine, are being used as anticancer drugs as they block the proliferation of quickly dividing cells by interfering with the mitotic spindle.