The two halves of a duplicated chromosome are called chromatids, and they are formed during the interphase of mitosis. Mitosis is the process of cell division that results in two daughter cells. Chromatids stay together during interphase but separate shortly thereafter, during anaphase.
In anaphase, the chromatids split apart, moving to opposite sides of the cell. Once separated, the chromatids become actual chromosomes, each containing one piece of DNA.
A human cell forms 92 chromatids during interphase. Once the chromatids separate during anaphase, two separate groups of 46 chromosomes are formed. The final phase of mitosis is telephase, which involves nuclear membranes forming around the chromosomes.