Figure 1. Cartoon of a centrosome isolated from vertebrate cells. The centrosome contains a mother centriole, possessing two sets of distally associated appendages, and a daughter centriole, which has assembled in the preceding cell cycle by orthogonal budding from the mother centriole. Schematic localization of -tubulin, a marker of microtubule nucleation, and of centrin, an early marker of centrosome duplication, is indicated. Centrosomal-associated centrin is confined within the distal lumen of each centriole, where a conspicuous luminal material is present, and -tubulin is distributed in the pericentriolar matrix but is often closely associated with centrioles.

Figure 2. The duplication cycle of the centrosome in animal cells. (a) The centrosome as observed in living cells stably expressing a centrin-green fluorescent protein (GFP) fusion protein, which concentrates in the lumen of the two centrioles and which is an early marker of duplication (see Figure 1). The centrosome is observed as a centriole pair in G₁. The initiation of procentriole budding at the G₁-S border, the separation of the daughter centrosomes in late G₂ and the two centriole pairs at the mitotic poles can be monitored. (b) Corresponding cartoon. The duplication of the centriole pair involves the orthogonal budding of a procentriole on each centriole in late G₁, the eventual maturation of the former daughter centriole in late G₂, the semiconservative segregation of the two centriole pairs at the G₂-M transition. The disorientation of the centrioles from their orthogonal configuration takes place long before the end of cell division (not represented). The maturation of the centrosomal matrix involves a progressive accumulation from early G₁, where it is mainly associated with the mother centriole, to M phase, and several structural and biochemical modifications in late G₂, as demonstrated by the disappearance of the appendages and the appearance of a reactivity with antiphosphoepitope antibodies (this is represented by a darkening of the pericentriolar matrix). Finally, an abrupt dispersion or degradation of the matrix takes place at the metaphase to anaphase transition. The doubling of the centrosome–microtubule system parallels the doubling of chromosomes and accordingly could be involved in the maintenance of the cell mass and ploidy ratio. (c) Major kinase activities that are necessary, respectively, for the initiation of centrosome duplication, the duplication process and the separation of the duplicated centrosomes at the onset of mitosis. Ca, calcium; CaM, calmodulin; Cdk, cyclin-dependent kinase.