During mitosis, the four centrioles appear visibly and move to the ends of the nucleus, one pair at each end. They produce a series of threads that attach to the chromosomes. During cell division, the threads split the chromosomes and draw them into the centrioles.
Prophase is the period when centrioles move to the ends of the nucleus and begin producing threads. The centrioles begin pulling the chromosomes in during anaphase. Telophase marks the splitting of the cell in two; at this point, the chromosomes unravel, the new envelopes for the nuclei start appearing, and the centrioles begin to disappear again, having completed their function.