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M phase is the most dramatic period of the cell cycle, involving a major reorganization of virtually all cell components. During mitosis (nuclear division), the chromosomes condense, the nuclear envelope of most cells breaks down, the cytoskeleton reorganizes to form the mitotic spindle, and the chromosomes move to opposite poles.
Summary. Cell division occurs during M phase, which consists of nuclear division (mitosis) followed by cytoplasmic division (cytokinesis). The DNA is replicated in the preceding S phase; the two copies of each replicated chromosome (called sister chromatids) remain glued together by cohesins.
The M phase is the most spectacular phase—a complex phase of cell cycle in which the replicated genome splits during mitosis, in other words, the nuclear contents are segregated into the daughter cells.
The eukaryotic cell cycle is divided into four phases: gap 1 (G1), synthesis (S), gap 2 (G2), and mitotic phase (M). A cell may reach a dormant state called gap 0 phase (G0) between M and G1, during which it is neither dividing nor prepared to divide.
M phase is itself composed of two tightly coupled processes: mitosis, in which the cell's nucleus divides, and cytokinesis, in which the cell's cytoplasm and cell membrane divides forming two daughter cells. Activation of each phase is dependent on the proper progression and completion of the previous one.
The mitotic phase is a multistep process during which the duplicated chromosomes are aligned, separated, and move into two new, identical daughter cells. The first portion of the mitotic phase is called karyokinesis or nuclear division.
In eukaryotic cells, the cell cycle includes four conventional phases: Gap phase 1 (G1); DNA synthesis phase (S); Gap phase 2 (G2), during which the cell prepares itself for division; and mitosis phase (M), during which the chromosomes separate and the cell divides.
