These are about 8 nm in diameter and, being the thinnest of the cytoskeletal filaments, are also called microfilaments . There are four major types of intermediate filament, each constructed from one or more proteins characteristic of it. For example, the nucleus is held within the cell by a basketlike network of intermediate filaments made of the protein keratin . However, both processes always occur more rapidly at one end, called the plus end . The other, less active, end is the minus end .
Microtubules participate in a wide variety of cell activities.
Most involve motion. The motors are kinesins. The migration of chromosomes in mitosis and meiosis takes place on microtubules that make up the spindle . Both kinesins and dyneins are used as motors as we shall see. The MTOC in animal cells is the centrosome .