Centrosome And Centriole

Centrioles are minute cylindrical structures that usually occur in pairs within the centrosome and organise the spindle apparatus during cell division. In transverse section, the wall of a centriole shows nine evenly spaced peripheral fibrils, each a triplet of microtubules, with no microtubule in the centre, giving the characteristic 9+0 arrangement detailed in NCERT Class 11, Chapter 8 (Cell: The Unit of Life). The 9+2 pattern is incorrect for centrioles, because that doublet-plus-central-pair arrangement belongs to the axoneme of cilia and flagella, not to the centriolar wall. The option describing nine peripheral singlets around one central tubule does not match any standard cytological structure and confuses singlet with triplet organisation. The choice citing twelve peripheral triplets is simply wrong in number, since the count of peripheral fibrils is fixed at nine. Adjacent triplets are linked by protein bridges, and a central proteinaceous hub connects to each triplet through radial spokes, giving the cartwheel appearance that templates the ninefold symmetry. The same centriole later acts as a basal body that nucleates the axoneme of a cilium or flagellum, which is why both structures share a ninefold peripheral plan even though their central regions differ. A structural consistency check supports the answer: the hub-and-spoke cartwheel that nucleates a centriole is built on nine triplets and lacks central tubules, confirming the 9+0 configuration as correct.