Spindle structure and chromosome behaviour. The mitotic spindle in vertebrate cells comprises two overlapping arrays of polar microtubules (MTs), some of which have been released from the centrosome, and all of which are oriented with their ‘plus’ (+) ends distal and their minus-ends proximal to their poles. This diagram illustrates the typical sequence of events that chromosomes in vertebrate cells experience during mitosis. Initially, one kinetochore on the chromosome becomes attached to, and glides rapidly poleward (long arrow) along the surface of a single MT (a). During this poleward motion, the attaching kinetochore on the now mono-oriented chromosome acquires additional MTs that terminate in its outer plate (b). It then begins to oscillate between poleward and away from the pole states of motion (short double arrows) around a position between the pole and the spindle equator. When the unattached kinetochore on this chromosome encounters a MT growing from the distal pole, the chromosome moves to the spindle equator in a process known as congression (c). As a result of congression, the chromosome adopts an average position near the spindle equator around which it oscillates, and, over time, the sister kinetochores acquire similar numbers of MTs (d). During anaphase, the chromatids separate (e), and, although the single kinetochore on each still exhibits a modified form of directionally unstable behaviour, there is net movement of each towards their respective spindle poles. Unattached kinetochores (red label) stain strongly for proteins such as CENP-E and cytoplasmic dynein/dynactin, which are involved in attachment and movement, and Mad2, Bub1 and the 3F3/2 epitope, which are involved in the checkpoint controlling the metaphase–anaphase transition. Modified, with permission, from Ref. .