microtubules

Meaning: Microtubules are rigid, hollow cytoskeletal tubes built from α-tubulin and β-tubulin dimers. They are polar. The plus end grows and shrinks faster, and the minus end is usually anchored at a microtubule organizing center such as the centrosome. Tubulin bound to GTP adds to the tube, and microtubules switch between growing and shrinking (dynamic instability). Functions include the mitotic spindle, cell shape, vesicle and organelle transport, centrioles, and the 9 + 2 arrangement of doublets in cilia and flagella.

On the MCAT: Expect "which structure is responsible?" questions: chromosome separation in anaphase, axonal transport, or ciliary and flagellar movement. Motor proteins use ATP to move cargo along microtubules. Kinesin usually moves toward the plus end (in an axon, toward the terminal), and dynein moves toward the minus end (toward the cell body). A passage may add a drug such as colchicine, which blocks assembly, or paclitaxel, which stabilizes microtubules. Either one prevents normal spindle function, so predict cells arrested in mitosis.

Common confusion: Do not mix up the three cytoskeletal fibers. Microtubules are the thickest and carry kinesin and dynein. Microfilaments (actin) carry myosin and drive muscle contraction, the cytokinesis furrow and microvilli. Intermediate filaments give mechanical strength and are not tracks for these motors.