filament

Meaning: A filament is a long, thin chain of protein subunits. In the cytoskeleton, microfilaments are actin polymers that resist tension, drive cell movement, form the contractile ring in cytokinesis, and support microvilli. Intermediate filaments (keratin, lamins) are rope-like, give mechanical strength, and hold the nuclear lamina and cell junction anchors in place. Microtubules are hollow tubes of tubulin, so they are usually listed separately. In muscle, thin filaments are actin and thick filaments are myosin.

On the MCAT: Expect matching questions: cell shape and cytokinesis point to actin, nuclear structure to lamins, and tracks for kinesin and dynein to microtubules. Muscle passages use the sliding filament model. Myosin heads bind actin, perform the power stroke, and pull thin filaments toward the sarcomere center. During contraction the sarcomere, I band, and H zone shorten, while the A band stays the same length.

Common confusion: Students think the filaments themselves shorten during contraction. They keep their length and slide, so the overlap increases. Another mix-up is treating intermediate filaments as motor tracks. Myosin moves on actin, while kinesin and dynein move on microtubules.