Mitosis is part of the cycle of cell division. The chromosomes of a cell are copied to make two identical sets of chromosomes, and the cell nucleus divides into two identical nuclei.

Before mitosis, the cell creates an identical set of its own genetic information – this is called replication. The genetic information is in the DNA of the chromosomes. At the beginning of mitosis the chromosomes wind up and become visible with a light microscope. The chromosomes are now two chromatids joined at the centromere. Since the two chromatids are identical to each other, they are called sister chromatids.