Thick (24 nanometers in diameter) tubes made up of alpha-beta tubulin dimers that are in almost every kind of cell you can think of.

These filaments are polar. They have a + end which is more dynamic(grows and shrinks faster) than the - end. At the end of the + end there is usually an especially dynamic region made up of tubulin that has its alpha and beta subunits bound to a nucleotide-triphosphate (like ATP, but unlike microfilaments, it is usually bound to GTP). The middle and - end are, for the most part tubulin with a bound nucleotide-diphosphate (like ADP, but usually is actually GDP) on the beta subunit, and a nucleotide-triphosphate bound to the alpha subunit.

Along with the motor proteins that function on them (like dyneins and kinesins, microtubules are responsible for much of cell motility. Unlike the pseudopod, cell extension and crawling seen as a result of microfilaments, microtubules are seen in more dramatic situations like the separation of chromosomes in mitosis, and the flagella (tails) of sperm and other cells, and even the cilia (little hair-like projections which have highly coordinated beating paterns to move the cell and sometimes help it eat.

It may also be helpful to read about microtubule associated proteins (MAPs). See also intermediate filament, and microfilament.

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