Microtubules

During the S and G2 phases of the cell cycle, along with the growth and DNA synthesis, part of the cell’s preparation for M-phase includes the duplication of centrosomes. Centrosomes are the microtubule organizing centers in cells and create the spindle poles during M-phase. Microtubules are protein fibers made of tubulin and along with actin and intermediate filaments, make up the cytoskeleton. Not all microtubules are the same and the differences give polarity and directionality to the fibers. They are made of subunits of alpha and beta tubulin. The microtubules extend from the spindle poles of the centrosomes on either side of the cell during M-phase and overlap in the center where other proteins hold them the overlapping units together, thereby stabilizing the spindle system. This utilizes kinetochore microtubules and interpolar microtubules. Meanwhile, the third type of microtubules, aster microtubules, connect to the cell cortex with other proteins.