Chromatids separate into two sister chromosomes during the anaphase of cell division. Mitosis is a process of nuclear division in which two daughter cells are created from a parent cell.
During the anaphase of cell division, enzymes cause the bonds between sister chromatids to breakdown. This allows for the creation of individual chromosomes, which form new nuclear membranes around themselves before cytokinesis. During cytokinesis, the parent cell is forced to split apart, transferring a portion of the nuclear membrane around the new chromosomes. The cell membrane reacts to this process and pinches itself together until it splits apart and reforms around the genetically identical daughter cells.