= Centriole
A <centriole> is a cylindrical <organelle> with a characteristic ninefold organization of <microtubules> and associated <proteins>. In many animal <cells>, two <centrioles> lie within the surrounding material of a <centrosome>. Much of the <microtubule> nucleation occurs in that surrounding material: a <centriole> and a <centrosome> are therefore different structural levels. A <centriole> can also become the <basal body> that organizes a <cilium>. <Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter 16, Microtubules and the Centrosome; Cilia and Flagella]{ref}
The geometry has a molecular basis. Structural and reconstitution experiments showed that <SAS-6> family <proteins> form elongated dimers whose head-to-head interactions can assemble a cartwheel-like hub with radiating spokes. Mutations interfering with oligomerization impaired <centriole> formation. This connects a repeated <protein> interaction to the symmetry of a larger cellular structure. <Structural basis of the 9-fold symmetry of centrioles by Daiju Kitagawa and others>[2011, summary: SAS-6 oligomerization, structural model and recombinant Bld12p assembly]{ref}
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