Mitotic Phase

Mitotic Phase

The mitotic phase (M phase) is composed of two tightly coupled processes: mitosis and cytokinesis. During mitosis the chromosomes in the cell nucleus separate into two identical sets in two nuclei. This is followed by cytokinesis in which the cytoplasm, organelles and cell membrane split into two cells containing roughly equal shares of these cellular components. We will now describe what takes place during the stages of M-phase, which includes the four broad phases of mitosis (prophase, metaphase, anaphase, telophase) and the fifth phase of cytokinesis:

1. Prophase

During prophase, the chromatin material shortens and thickens into individual chromosomes which are visible under the light microscope. Each chromosome consist of two strands or chromatids joined by a centromere (see figure below).

As prophase progresses, the nuclear membrane and nucleolus disintegrates. In animal cells the centrioles separate and move to opposite poles. The centrioles give rise to the spindle fibres which form between the poles. In plant cells there are no centrioles to move to the poles, so spindle fibres form in the cytoplasm.

Schematic Diagram: Animal cell	Micrograph: Plant cell

2. Metaphase

During metaphase, chromosomes line up on the equator of the cell. The chromosomes appear in a straight line across the middle of the cell. Each chromosome is attached to the spindle fibres by its centromere.

Schematic Diagram: Animal cell	Micrograph: Plant cell

3. Anaphase

During anaphase the chromatids are pulled to opposite poles of the cell by the shortening of the spindle fibres. The chromatids are now called daughter chromosomes.

Schematic Diagram: Animal cell	Micrograph: Plant cell

4. Telophase

During telophase, a nuclear membrane reforms around the daughter chromosomes that have gathered at each of the poles. The daughter chromosomes uncoil to form chromatin once again. The nuclear membrane reforms.

Schematic Diagram: Animal cell	Micrograph: Plant cell

5. Cytokinesis

The cytoplasm then divides during a process called cytokinesis. Cytokinesis is not a stage of mitosis but the process of the cytoplasm splitting into two. In an animal cell the cell membrane constricts. This invagination or in-folding of the cytoplasm divides the cell in two. In a plant cell a cross wall is formed by the cell plate dividing the cytoplasm in two.

Schematic Diagram: Animal Cell		Micrograph

There are now two genetically identical daughter cells which are identical to the parent cell and to each other.

Summary Video: Mitosis

The Amoeba Sisters video below walks you through the reason for mitosis with mnemonics for prophase, metaphase, anaphase, and telophase.