Growing a tail poses a challenge for cells. How do you get the building blocks of tails from the body of the cell, where they are made, to the tip of the tail where they are needed? Large molecules cannot move freely along cilia and f lagella, so some kind of active transport is essential. The answer evolution came up with was a kind of train that travels up and down, using one of the microtubules that make up the tail as a rail.
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