The type of cell division in which number of chromosomes remains constant in the daughter generations is called mitosis. Mitosis is divided intc four phases: prophase, metaphase, anaphase, and telophase. However, thE process is continuous in a dividing cell.. Therefore, each phase smoothly enters into the next.
It is the first phase of mitosis. Following changes take place during prophase:
(i) The chromosomes become visible with the light microscope. They look threadlike structures.
(ii) The nucleoli and nuclear envelope disappear.
The two centrioles pairs move apart from each other. The centrioles pairs reached at the opposite poles of the cell at the end nf prophase. (iv)Microtubules arise from the centrioles. These micrAubules are called asters. These esters tie each centriole with the plasma membrane.
(v) The microtubules form a spindle between the centrioles. These fibers extend from pole to pole.
(vi)The asters, spindle, centrioles, and microtubules are collectively called the mitotic spindle (or mitotic apparatus).
(vii) Later, a second group of microtubules grows out from the kinetochore to the poles of the cell. These kinetochore microtubules connect each sister chromatid to the poles of the spindle.
The chromatids align in the center of the cell. They are attached at the equator of spindle. The centromeres divide at the end of metaphase. The two sister chromatids are detached from each other at the end of the metaphase. But the chromatids remain aligned at the equator. The centromere divide. Now the sister chromatids are considered complete chromosomes. Now they are called daughter chromosomes.
The microtubules in the mitotic spindle shorten. It pulls each daughter chromosome toward its respective pole. The daughter chromosomes moved to the poles of the cell and anaphase ends. Now each pole has a complete, identical set of chromosomes.
The daughter chromosomes arrive at the opposite poles of the cell and telophase begins. The mitotic spindle disassembles. A nuclear envelope reforms around each set of chromosomes. The daughter chromosomes begin to uncoil for gene expression. The nucleoli are resynthesized. The cell also begins to pinch in the middle. It ends the mitosis.
CYTOKINESIS: PARTITIONING THE CYTOPLASM
The final phase of cell division is cytokinesis. The cytoplasm divides during cytokinesis. Cytokinesis starts during late anaphase or early telophase. A contracting belt of microfilaments are present in the middle of the cell. These microfilaments are called the contractile ring. The ring contract and pinches the plasma membrane inward. It forms a cleavage furrow. The furrow deepens and joins each other. Thus two new, genetically identical daughter cells are formed.