When two fibers from opposite poles of the cell meet, they are bound together by a special protein. While the addition and subtraction of dimers is one of the main ways that spindle fibers help carry chromosomes about the cell, there are two other primary methods. It is also the main way they are separated during anaphase of mitosis or meiosis. This is one of the main ways that the chromosomes get aligned on the metaphase plate, a hypothetical middle of the cell. Here, there are also proteins which can shorten and extend the spindle fibers. Special proteins within the centromere of the chromosome can attach to the microtubule. As the microtubule travels, it eventually reaches a chromosome. At first, the MTOCs must add many of these dimers to the microtubule, to extend it across the cell. The main feature of microtubules, and therefore of larger fibers, is that the proteins which control them can extend or contract the microtubule by adding or removing tubulin dimers. Functions of Spindle Fibers Shrinkage and Growth Microtubules are formed by small proteins, and together many microtubules form a spindle fiber. A single microtubule can be seen in the graphic below. Many individual microtubules together are called spindle fibers. Together, these two small proteins form the structure of a microtubule. To do this, they must add subunits of alpha-tubulin and beta-tubulin. These proteins respond to signals from the cell, and when it is time for cell division, they begin lengthening the spindle fibers. On the surface of the MTOC are small proteins, responsible for lengthening or shortening the microtubules. The centrosome, or MTOC, always has some microtubules preassembled. But how does this complex process take place? Structure of Spindle Fibers They carefully assemble and divide the chromosomes, and have been doing so for billions of years. The spindle fibers act like small machines during cell division. Each spindle fiber is formed from several microtubules. The spindle fibers can be seen extending in all directions from the centrosomes in step 6. The process can be seen in the image above. With each fiber comes the chromosome it is attached to, which separates the chromosomes into each daughter cell. Once attached, the spindle fiber is pulled back.
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