The centrosome duplication cycle. Adapted from [3, 14, 107]. Centrosome duplication starts at the G1/S transition with the separation of the paired centrioles. Procentrioles form near the proximal ends of each pre-existing centriole. Procentriole formation is completed during S phase. During mitosis the two centrosomes are present and form the mitotic spindle poles. Each cell inherits one centrosome after the completion of cytokinesis.