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 Figure 17–29 The centrosome cycle. In 
early G1, the centrosome consists of two 
centrioles linked by a protein tether, as 
well as associated pericentriolar material 
for nucleation of microtubules (light green). 
One centriole, the mature parent, carries 
protein appendages required for certain 
centrosome functions. Upon entry into 
the cell cycle, G1/S-Cdk and Plk4 initiate 
centriole duplication, whereby a new 
centriole (procentriole) is assembled at 
a single site on the side of each parent 
centriole (yellow dots). The elongation 
of the procentrioles is usually completed 
in G2. The two centriole pairs remain 
close together in a single centrosomal 
complex until entry into mitosis, when the 
protein kinases M-Cdk, Plk1, and other 
regulators trigger numerous changes: the 
tether between centrosomes is removed, 
the immature parent centriole acquires 
appendages (centriole maturation), the 
pericentriolar material expands to enable 
more microtubule nucleation (centrosome 
maturation), and the centrosomes 
separate, forming a new spindle between 
them. After the completion of mitosis, the 
two centrioles in each centrosome detach 
(centriole disengagement), and the new 
centriole acquires pericentriolar material 
(centriole-to-centrosome conversion). 
These two processes are required for 
centriole duplication in the subsequent cell 
cycle; thus, progression through mitosis is 
required for duplication, helping to ensure 
that centrioles (and centrosomes) duplicate 
only once per cell cycle