When do chromosomes duplicate during a cell life cycle?

Chromosomes duplicate during the S phase of interphase. This is part of the cell life cycle in which the cell is not actually dividing. In other words, the interphase is not part of mitosis. The S phase is a critical point during interphase during which the cell duplicates its chromosomes, resulting in two identical copies of each DNA. Each chromosome replicates to make a paired chromatid. At this stage, the cell contains double its normal number of chromosomes and is ready to move into the first phase of mitosis or cell division.