Figure 17–24 The centrosome. (a) Electron micrograph of an s-phase mammalian cell in culture, showing a duplicated centrosome. Each centrosome contains a pair of centrioles; although the centrioles have duplicated, they remain together in a single complex, as shown in the drawing of the micrograph in (B). One centriole of each centriole pair has been cut in cross section, while the other is cut in longitudinal section, indicating that the two members of each pair are aligned at right angles to each other. The two halves of the replicated centrosome, each consisting of a centriole pair surrounded by pericentriolar matrix, will split and migrate apart to initiate the formation of the two poles of the mitotic spindle when the cell enters m phase. (a, from m. mcgill, d.p. Highfield, T.m. monahan, and B.R. Brinkley, J. Ultrastruct. Res. 57:43–53, 1976. With permission from academic press.) spindle microtubule dynein kinesin-4,10 kinesin-14 kinesin-5 dynein sister chromatids centrosome+ + + + + + + – – – – – – – plasma membrane Figure 17–25 Major motor proteins of the spindle. Four major classes of microtubule-dependent motor proteins (yellow boxes) contribute to spindle assembly and function (see text). The colored arrows indicate the direction of motor protein movement along a microtubule—blue toward the minus end and red toward the plus end.
proteins, also called chromokinesins, are plus-end directed motors that associate with chromosome arms and push the attached chromosome away from the pole (or the pole away from the chromosome). Finally, dyneins are minus-end directed motors that, together with associated proteins, organize microtubules at various locations in the cell. They link the plus ends of astral microtubules to components of the actin cytoskeleton at the cell cortex, for example; by moving toward the minus end of the microtubules, the dynein motors pull the spindle poles toward the cell cortex and away from each other.
multiple mechanisms Collaborate in the assembly of a Bipolar mitotic spindle
The mitotic spindle must have two poles if it is to pull the two sets of sister chromatids to opposite ends of the cell in anaphase. In most animal cells, several mechanisms ensure the bipolarity of the spindle. One depends on centrosomes. A typical animal cell enters mitosis with a pair of centrosomes, each of which nucleates a radial array of microtubules. The two centrosomes provide prefabricated spindle poles that greatly facilitate bipolar spindle assembly. The other mechanisms depend on the ability of mitotic chromosomes to nucleate and stabilize microtubules and on the ability of motor proteins to organize microtubules into a bipolar array. These “self-organization” mechanisms can produce a bipolar spindle even in cells lacking centrosomes.
We now describe the steps of spindle assembly, beginning with centrosomedependent assembly in early mitosis. We then consider the self-organization mechanisms that do not require centrosomes and become particularly important after nuclear-envelope breakdown.
Centrosome duplication Occurs Early in the Cell Cycle
Most animal cells contain a single centrosome that nucleates most of the cell’s cytoplasmic microtubules. The centrosome duplicates when the cell enters the cell cycle, so that by the time the cell reaches mitosis there are two centrosomes. Centrosome duplication begins at about the same time as the cell enters S phase. The G1/S-Cdk (a complex of cyclin E and Cdk2 in animal cells; see Table 17–1) that triggers cell-cycle entry also helps initiate centrosome duplication. The two centrioles in the centrosome separate, and each nucleates the formation of a single new centriole, resulting in two centriole pairs within an enlarged pericentriolar matrix (Figure 17–26). This centrosome pair remains together on one side of the nucleus until the cell enters mitosis.