Centrioles are non-membranous organelles which are present in the cytoplasm near the nucleus of the cell. They are present in all animal cells and in flagellated motile cells of lower plants. They have ability to replicate and occur in pairs which are called diplosomes. In a diplosome, two centrioles lie at a right angle to each other. Diplosome is covered by cytoplasmic sheath which is known as centrosphere. The complex structure which is formed by centrospheres and centrioles is called centrosome.
Each centriole is cylindrical and about 500 nm long and 200 nm in diameter. Each centrole is composed of nine groups of microtubules arranged in a circular manner, i.e. 9 triplets+0 arrangement of microtubules. Each of nine groups of microtubules is a triplet which is composed of three microtubules at the periphery end and no microtubules at the centre. The adjacent triplets appear to be connected to each other by fibrils giving a cart-wheel structure of centrioles in transverse section (T.S.).
Centrioles perform different functions inside the cell. They help to form spindle fibres. During cell division, at the beginning of nuclear division, centrioles replicate, separate and migrate to opposite poles from where they give spindle fibres. Similarly, they also help to form basal body of cilia and flagella.
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