Centrosome

The centrosome is the main microtubule-organizing center in animal cells. It consists of a pair of cylindrical centrioles surrounded by a cloud of proteins, and it sets up the network of microtubules and the poles of the mitotic spindle.

Each centriole is a barrel of nine triplets of microtubules, about 0.2 micrometers wide and 0.5 micrometers long. The two are held at right angles to each other. They are embedded in the pericentriolar material, which contains ring-shaped complexes that nucleate new microtubules. Theodor Boveri described the centrosome in 1888 and later proposed that abnormal numbers of them could cause cancer.

The centrosome is duplicated once per cycle, in the S phase, in step with DNA replication. At the start of mitosis, the two centrosomes move to opposite ends of the nucleus and form the poles of the spindle, from which microtubules reach out to the centromeres of the chromosomes. Motor proteins such as kinesin and dynein push and pull the spindle into place. This is controlled as part of the cell cycle.

The same centrioles can act as the base of cilia and flagella. Flowering plants and most fungi have no centrioles, and they organize their spindles in other ways. Extra centrosomes are common in tumor cells and can lead to uneven chromosome separation, a cause of aneuploidy, and the defects are linked to developmental disease. The organelle is also involved in cytokinesis in some cells.

• Microtubule — the fibers it organizes
• Mitosis — the division it orchestrates
• Centromere — the chromosome site the spindle attaches to