The centrioles are two cylindrical, microtubular structures found near the nucleus. When a centriole supports a flagellum or cilium, it is called thebasal body.
The centrioles occur in most of the animal cells, algal cells and some fern cells. They are absent in prokaryotes, red algae, yeast cells and flowering plants and some non -flagellated or non-ciliated protozoans.
The centrioles range in size from 0.15-0.25 �m in diameter. They are usually 0.3-0.7 �m in length.
Each centriole and basal body is formed of nine triplet microtubules equally spaced around a perimeter. Each microtubule has a diameter of 200-260 � in diameter.
The microtubules are made up of a structural protein, tubulin, along with lipid molecules.
It was initially considered that new centrioles arise by the division of existing centrioles. This idea is no longer accepted. It appears that new cen-trioles are produced de novo or are synthesized using an existing centriole as a template.
In most of the animal cells the centrioles are the focal point for the centrosome. The centrosome organizes cytoplasmic microtubules duringinterphase in mitosis. It provides the two poles of the mitotic spindle.
The centrioles form the basal body and the cilia. In spermatozoon one centriole gives rise to the tail fibre or flagellum. The centrioles are also in-volved in ciliary and flagellar activity.