mitotic spindle
/ my-TOT-ik SPIN-dul /
Imagine you need to sort a heap of objects into two equal piles, but you have no hands — only a set of ropes you can extend, attach, and reel in. To split its chromosomes fairly, a cell builds exactly this kind of rope-and-pulley machine out of protein cables. The mitotic spindle is the temporary, football-shaped scaffold of fibers that a cell assembles to capture its chromosomes and drag the two copies to opposite ends before it divides.
The spindle is built from microtubules — stiff, hollow protein tubes made of tubulin that can grow and shrink rapidly. They radiate out from two organizing centers (in animal cells, the centrosomes) at opposite poles of the cell. Some microtubules reach in and grab a chromosome at its kinetochore; others overlap in the middle and push the poles apart; still others anchor the poles to the cell's edge. During anaphase the fibers gripping the chromosomes shorten, pulling the two sets of DNA toward the poles, while the spindle as a whole elongates the cell.
The spindle matters because accurate chromosome sorting depends entirely on it: errors here give daughter cells the wrong number of chromosomes. This is also why the spindle is a major drug target — chemotherapy agents like taxol and vinca alkaloids work by jamming the spindle's microtubules, freezing dividing cancer cells so they cannot complete division. A common misconception is that the spindle is a permanent structure; it is built fresh for each division and disassembled the moment its job is done.
The cancer drug taxol grips spindle microtubules so tightly that they can no longer shorten — dividing tumor cells get frozen partway through, with their chromosomes stuck and unable to be pulled apart.
Chemotherapy drugs like taxol work by jamming the spindle.
Plant cells build a perfectly functional spindle without centrosomes or centrioles, which proves those structures help organize the spindle but are not strictly required to make one.