a drawing of a metaphase chromosome showing its two sister chromatids attached to the plus ends of kinetochore microtubules. Each kinetochore forms a plaque on the surface of the centromere.
Electron micrograph of an anaphase chromatid with microtubules attached to its kinetochore. While most kinetochores have a trilaminar structure, the one shown here (from a green alga) has an unusually complex structure with additional layers.  (a, courtesy of B.R. Brinkley; C, from
J.d. pickett-Heaps and L.C. Fowke, Aust. J. Biol. Sci. 23:71–92, 1970. With permission from CsIRO.)
Figure 17–31 Microtubule attachment sites in the kinetochore. (a) In this electron micrograph of a mammalian kinetochore, the chromosome is on the right, and the plus ends of multiple microtubules are embedded in the outer kinetochore on the left. (B) Electron tomography (discussed in Chapter 9) was used to construct a low-resolution three-dimensional image of the outer kinetochore in (a). several microtubules (in multiple colors) are embedded in fibrous material of the kinetochore, which is thought to be composed of the ndc80 complex and other proteins. (C) Each microtubule is attached to the kinetochore by interactions with multiple copies of the ndc80 complex (blue). This complex binds to the sides of the microtubule near its plus end, allowing polymerization and depolymerization to occur while the microtubule remains attached to the kinetochore.  (a and B, from Y. dong et al., Nature Cell Biol. 9:516–522, 2007. With permission from macmillan publishers Ltd.) + early prometaphase: lateral kinetochore mid prometaphase: metaphase:late prophase attachments, chromosome arms pushed outwards end-on attachment bi-orientation
Figure 17-32 Chromosome attachment to the mitotic spindle in animal cells. (a) In late prophase of most animal cells, the mitotic spindle poles have moved to opposite sides of the nuclear envelope, with an array of overlapping microtubules between them. (B) Following nuclear envelope breakdown, the sister-chromatid pairs are exposed to the large number of dynamic plus ends of microtubules radiating from the spindle poles. In most cases, the kinetochores are first attached to the sides of these microtubules, while at the same time the arms of the chromosomes are pushed outward from the spindle interior, preventing the arms from blocking microtubule access to the kinetochores. (C) Eventually, the laterally-attached sister chromatids are arranged in a ring around the outside of the spindle. most of the microtubules are concentrated in this ring, so that the spindle is relatively hollow inside. (d) dynamic microtubule plus ends eventually encounter the kinetochores in an end-on orientation and are captured and stabilized. (E) stable end-on attachment to both poles results in bi-orientation. additional microtubules are attached to the kinetochore, resulting in a kinetochore fiber containing 10–40 microtubules.
they separate in anaphase. How is this mode of attachment, called bi-orientation, achieved? What prevents the attachment of both kinetochores to the same spindle pole or the attachment of one kinetochore to both spindle poles? Part of the answer is that sister kinetochores are constructed in a back-to-back orientation that reduces the likelihood that both kinetochores can face the same spindle pole. Nevertheless, incorrect attachments do occur, and elegant regulatory mechanisms have evolved to correct them.