Centrioles are microtubule-based, conserved eukaryotic
cell structures measuring 200 500 nm in size that
have a distinctive radial nine-fold symmetry. Typically,
an animal cell requires two cell cycles to build a fully
functional centriole, which is then required to form an
independent centrosome and a cilium (Figure 1). In the
first cell cycle, the building of a centriole starts at S phase
with the formation of a procentriole (Figure 1A and B). At
its core, the procentriole harbors a central tubule that
radiates nine spokes, which together resemble a cartwheel
(Figure 1B). The cartwheel spokes are connected
to nine triplet microtubules that form the centriole wall.
In G2, M and G1 phases, the centriole elongates to
500 nm and is referred to as an immature centriole.
During this time, the immature centriole appears to be
non-functional.