![]() Non-kinetochore microtubules can be found as single microtubules, in parallel bundles, or in antiparallel bundles. Kinetochore microtubules end at the kinetochore and form parallel bundles known as kinetochore fibers or k-fibers. First, one can divide the microtubules with respect to whether they end at the kinetochore or not. To help understand how the spindle functions, a large amount of experimental data has been condensed into a simplified picture, in which the microtubules are divided into several categories (Fig. Spindles in mammalian cells contain hundreds of microtubules, which are connected in a complex fashion with the help of various microtubule-associated proteins including motor proteins and passive crosslinkers. The white line marks the cell outline time is given in minutes scale bar represents 5 µm Microtubules are shown in green and kinetochores in magenta, in a U2OS cell expressing CENP-A-GFP and mCherry-α-tubulin. Their connections have an impact on the force balance in the spindle and on chromosome movement during mitosis because the forces in interpolar bundles are transmitted to kinetochore fibers and hence to kinetochores through these connections. This review discusses the structure, function, and formation of kinetochore fibers and interpolar bundles, with an emphasis on how they interact. These interpolar bundles, termed bridging fibers, balance the forces acting at kinetochores and support the rounded shape of the spindle during metaphase. Thus, the spindle is made of modules consisting of a pair of sister kinetochore fibers and a bundle of interpolar microtubules that connects them. Most of the interpolar bundles are attached to a pair of sister kinetochore fibers and vice versa. Recent work in human cells has shown a close relationship between interpolar and kinetochore microtubules, where interpolar bundles are attached laterally to kinetochore fibers almost all along their length, acting as a bridge between sister k-fibers. Spindle microtubules can be divided into three major classes: kinetochore microtubules, which form k-fibers ending at the kinetochore interpolar microtubules, which extend from the opposite sides of the spindle and interact in the middle and astral microtubules, which extend towards the cell cortex. The attachment of microtubules to chromosomes is mediated by kinetochores, protein complexes on the chromosome. When a cell starts to divide, it forms a spindle, a micro-machine made of microtubules, which separates the duplicated chromosomes.
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