Within the cells of all animals and some primitive plants are clusters of tubes called centrioles. If you peered straight down the end of a centriole, you would see a ring-shaped structure formed by nine sets of three microtubules -- straight, hollow tubes made of protein. The function of the centriole in the cell varies depending on whether it occurs in a cell on its own or in pairs.
Going It Alone
When centrioles appear alone in a cell, their main purpose is to provide the power for movement. Specialized centrioles called basal bodies work as tiny motors to create the bending motion needed to waggle tail-like flagellum or wave hair-like cilia. These single centrioles have the basic ring structure of nine bundles of three microtubules, but also have another ring of nine sets of double microtubules.
Working In Pairs
When centrioles occur in pairs, their main job is to assist in the cell-division processes of mitosis and meiosis. Just prior to cell division, the pair of centrioles replicate so two pairs of centrioles are within the cell. In prophase, the initial phase of division, the pairs move apart and the mitotic spindle forms between the centrioles. This spindle will later provide the "ropes" to pull individual chromosomes apart from their sister chromosome before the cell splits. As the process of cell division continues, the centriole pairs migrate to the poles of the cell as the ends of the mitotic spindle attach to a chromosome. Each centriole pair then "reels in" the spindle fibers to pull the sister chromosomes apart.
Locating the Centriole
Centrioles can be hard to find when a cell is not actively dividing. The pairs are housed inside a structure called the centrosome, which is located near the nucleus. During the time between cell division, the centrosome shows up as a dark spot with little definition. However, just because the centrioles are at rest doesn't mean they aren't working. The microtubles of the centrioles serve to provide support to the cell, much like the framework of a building.
My Centriole is Missing
Higher plants such as ferns, flowers and trees don't have centrioles, but they still undergo cell division. Given that some plants, like beans, can grow several inches in a single day, you may wonder about the necessity of centrioles for cell division. According to the British Society for Cell Biology, researchers have shown that in animal cells that have had the centrioles removed, cell division still occurs but the mitotic spindle does not always form correctly and the separation of chromosomes is not reliable. In studies where the centrioles have been removed from fruit fly embryos, the embryos failed to develop. For now, it appears animals still need centrioles for mitosis and meiosis.
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