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Centrioles function as a pair in most cells in animals but as a single centriole or basal body in cilia and flagella.

Centrioles in pairs
Cells entering mitosis have a centrosome containing two pairs of centrioles and associated pericentriolar material (PCM). During prophase the centrosome divides into two parts and a centriole pair migrates to each end or pole on the outside of the nuclear membrane or envelope. At this point microtubules are produced at the outer edge of the pericentriolar material and grow out in a radial form. The centriole pair and PCM is called an aster. Microtubules from the aster at one pole grow towards the aster at the opposite pole. These microtubules are called spindle fibres. Some of these will become attached by centromeres to chromosomes lined up on the 'equator' of the dividing cell. Others, though not attached to chromatids/chromosomes by centromeres, will assist in pushing apart the two parts of the dividing cell.

A single centriole or basal body.
At the base of each cilium or flagellum there is a single centriole. This structure and associated pericentriolar material, construct microtubules in a linear direction. These microtubules form most of the inside of cilia and flagella and are largely responsible, using protein motors, for the mechanical aspects of their movement. The centriole at the base of each one also appears to exert some degree of direction and control over the movement of the cilia and flagella.

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