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Pronounced 'tubu-lin' not 'tub-ulin' this is the building block of microtubules (MT). These are hollow tubes 20 nm in diameter that stretch across the cell. If a large model were made, with one millimeter per nanometer, then a MT would be 2cm across. On this scale, a stalked particle is also 2 cm high and a bacterial cell is 2m. They transport organelles and vesicles around the cell -- or rather they are the railroad on which these are transported. The chromosomes are pulled apart by microtubules attached to the centriole (middle bit) during cell division.

The tubulin subunits are GTP hydrolases, and need to have guanidine cofactors to properly form MT. Bacteria have a homologous protein called FtsZ (not to be confused with pufferfish FtZ) also found in chloroplasts. It forms rings that tighten around the waist of two dividing cells -- like a belt. This helps to pinch them apart. It also uses GTP.