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. 2021 Feb 1;32(3):218–222. doi: 10.1091/mbc.E20-05-0335

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© 2021 Tolic´ and Pavin. “ASCB®,” “The American Society for Cell Biology®,” and “Molecular Biology of the Cell®” are registered trademarks of The American Society for Cell Biology.

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FIGURE 1: — (A) Spindle in metaphase (left). Kinetochores are shown in red, microtubules and centrosomes in blue, and the rectangle marks the region around a pair of sister kinetochores that is enlarged on the right. A model for forces on kinetochores, F_elastic and F_MT (top right), predicts that upon laser cutting (bolt sign) of the centromere region, sister kinetochores will be pulled apart by kinetochore microtubules (bottom right). (B) The model from A also predicts that upon laser cutting of a kinetochore fiber, the kinetochores should get closer, but experiments showed that kinetochores can keep their distance and the kinetochore fiber stub rotates with its tip moving away from the spindle (top). In the bridging fiber model, tension on kinetochores, F_k, and compression at the pole, F₀, are balanced by the compression in the bridging fiber, F_b (bottom left). Also shown is a simplified revised picture of the spindle in which overlap bundles act as bridges between sister kinetochore fibers (bottom right); localization of the cross-linkers PRC1 (orange) and NuMa (gray) is indicated. (C) The bridging fiber model predicts that if the bridging fiber is thinner, sister kinetochores will be closer and kinetochore fibers will be straight, giving the spindle a diamond-like shape (left). Experiments revealed an additional unexpected effect that thinner bridging fibers also lead to misaligned kinetochores (right). (D) The bridging fiber model predicts that if rotational forces (bending, curved arrows) act at the spindle poles in the opposite directions, the structure attains a C-shape, but if the direction of the rotational force is reversed at one pole, the structure curves into an S-shape (top left). A model with rotational forces (bending and twisting) predicts helix-like shapes of microtubule bundles (top right). Experimental test of this prediction by looking at the spindle from pole to pole: bundles without twist extend radially from the pole, whereas in the case with twist the bundles turn around the pole (bottom left). Rotational forces act in the spindle, twisting microtubule bundles into spiral shapes (bottom right).