The centrosome in animal cells contains centrioles surrounded by a mass of protein called the pericentriolar material or PCM. These proteins are important for microtubule nucleation and anchoring. The centrosomes are important as the microtubules interact with the chromosomes and build the spindle that aids in mitotic division. The centrosome is copied once per cell cycle. The daughter cell inherits one centrosome. This in turn has two centrioles. Centrioles contain a ring of nine groups of microtubules. There are three fused microtubules in each group. The centrioles are aligned perpendicular to eachother. In the cell cycle, the centrosome replicates during the S phase. During prophase, the centrosomes move to opposite poles in the cell and the spindle forms between the two centrosomes. After mitosis, each daughter cell receives one centrosome and the cell cycle repeats.