A normal resting cell exists in a state called interphase in which the chromatin is undifferentiated in the heavily-stained nucleus, as illustrated above. Before the cell enters the mitosis phase, it first undergoes a synthesis or S phase where each chromosome is duplicated and consists of two sister chromatids joined together by a specific DNA sequence known as a centromere. Centromeres are crucial to segregation of the daughter chromatids during mitosis.