Figure 2.

The centriole replication cycle. G₁ cells possess a single centrosome containing a pair of centrioles embedded in an amorphous pericentriolar material. In most somatic cells, centriole duplication occurs during S phase and is marked by the formation of procentrioles at the proximal end of each parental centriole. Procentrioles elongate until they reach the length of the parent centrioles in late G₂. The procentriole and daughter centrioles are held in an orthogonal configuration and the two pairs of centrioles remain connected, functioning a single microtubule-organizing center until late G₂. At the G₂-M transition centrosome maturation occurs and the centrosomes separate and form opposite poles of the bipolar microtubule spindle. During mitosis the centrosomes are equally divided such that each incipient daughter cell inherits one centrosome. At the end of mitosis, centrioles lose their orthogonal configuration in a process known as disengagement. It has been proposed that this step may be required to license an additional round of centriole duplication in the next cell cycle.²⁷