Mitosis Mitosis is the mechanism that allows the nuclei of cells to split and provide each daughter cell with a complete set of chromosomes during cellular division. Mitosis is the mechanism that allows the nuclei of cells to split and provide each daughter cell with a complete set of chromosomes during cellular division. A normal resting cell exists in a state called interphase in which the chromatin is undifferentiated in the heavily-stained nucleus. 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.
The first phase of mitosis is known as the prophase , where the nuclear chromatin starts to become organized and condenses into thick strands that eventually become chromosomes. Late prophase, or prometaphase begins with the disruption of the nuclear envelope, which is broken down into small membrane vesicles that closely resemble the endoplasmic reticulum and tend to remain visible around the mitotic spindle. During this period the chromosomes continue to condense and gradually shorten and thicken until they have completely formed the units that will undergo mitosis. The nucleolus also disappears during this period. The mitotic spindle microtubules are now free to enter the nuclear region, and formation of specialized protein complexes called kinetochores begins on each centromere.