Looping and capture of kinetochore fiber minus ends (arrow) contributes to forming a bipolar spindle.
In the established model of spindle formation in vertebrate cells, spindle microtubule minus ends are focused at spindle poles in association with centrosome-nucleated microtubules. Kinetochore fibers, which pull chromosomes to opposite poles, form when the plus ends of centrosome-associated microtubules capture kinetochores. A second mechanism must also form kinetochore fibers, however, because functional spindles form in the absence of centrosomes.
In their new article, the authors got a close look at this second mechanism by focusing on unobstructed kinetochores. They avoided the obtrusive mass of non-kinetochore microtubules that usually blocks imaging by inducing monopolar spindle formation and examining kinetochores that faced away from the centrosome. Fully formed bundles...
