Microtubules are cylindrical tubes of 20-25 nm in diameter. They are composed of protofilaments which are in turn composed of alpha- and beta-tubulin polymers. Each microtubule is polarized, at one end alpha-subunits are exposed (-) and at the other beta-subunits are exposed (+). Microtubules act as a scaffold to determine cell shape, and provide a backbone for cell organelles and vesicles to move on, a process that requires motor proteins. The major microtubule motor proteins are kinesin, which generally moves towards the (+) end of the microtubule, and dynein, which generally moves towards the (-) end. Microtubules also form the spindle fibers for separating chromosomes during mitosis.