A third force acting on chromosomes is the polar ejection force, or polar wind. Plus-end directed kinesin-4 and 10 motors on chromosome arms interact with interpolar microtubules and transport the chromosomes away from the spindle poles (see Figure 17–25). This force is particularly important in prometaphase and metaphase, when it helps push chromosome arms out from the spindle. This force might also help align the sister-chromatid pairs at the metaphase plate (Figure 17–36).
One of the most striking aspects of mitosis in vertebrate cells is the continuous oscillatory movement of the chromosomes in prometaphase and metaphase. When studied by video microscopy in newt lung cells, the movements are seen to switch between two states—a poleward state, when the chromosomes are pulled toward the pole, and an away-from-the-pole, or neutral, state, when the poleward forces are turned off and the polar ejection force pushes the chromosomes away
Figure 17–35 Microtubule flux in the metaphase spindle. (a) To observe microtubule flux, a very small amount of fluorescent tubulin is injected into living cells so that individual microtubules form with a very small proportion of fluorescent tubulin. such microtubules have a speckled appearance when viewed by fluorescence microscopy. (B) Fluorescence micrograph of a mitotic spindle in a living newt lung epithelial cell. The chromosomes are colored brown, and the tubulin speckles are red. (C) The movement of individual speckles can be followed by time-lapse video microscopy. Images of the thin vertical boxed region (arrow) in (B), taken at sequential times, show that individual speckles move toward the poles at a rate of about 0.75 μm/min, indicating that the microtubules are moving poleward. (d) microtubule length in the metaphase spindle does not change significantly because new tubulin subunits are added at the microtubule plus end at the same rate as tubulin subunits are removed from the minus end. (B and C, from T.J. mitchison and E.d. salmon, Nat. Cell Biol. 3:E17–21, 2001. With permission from macmillan publishers Ltd.) interpolar plus-end-directed or astral kinesin-4,10 microtubule motor proteins from the pole. The switch between the two states may depend on the degree of tension in the kinetochore. It has been proposed, for example, that, as chromosomes move toward a spindle pole, an increasing polar ejection force generates tension in the kinetochore nearest the pole, triggering a switch to the away-fromthe-pole state and gradually resulting in the accumulation of chromosomes at the equator of the spindle.
The apC/C Triggers sister-Chromatid separation and the Completion of mitosis
After M-Cdk has triggered the complex processes leading up to metaphase, the cell cycle reaches its climax with the separation of the sister chromatids at the metaphase-to-anaphase transition (Figure 17–37). Although M-Cdk activity sets the stage for this event, the anaphase-promoting complex (APC/C) discussed earlier throws the switch that initiates sister-chromatid separation by ubiquitylating several mitotic regulatory proteins and thereby triggering their destruction (see Figure 17–15A).
During metaphase, cohesins holding the sister chromatids together resist the poleward forces that pull the sister chromatids apart. Anaphase begins with the sudden loss of sister-chromatid cohesion, which allows the sisters to separate and move to opposite poles of the spindle. The APC/C initiates the process by targeting the inhibitory protein securin for destruction. Before anaphase, securin