What are the main stages of Mitosis?

Mitosis is a type of equational cell division in which a single parent cell divides to produce two genetically identical daughter cells. It usually occurs in diploid cells and maintains the same number of chromosomes (2n) in daughter cells as present in the parent cell. Mitosis is commonly found in somatic cells and also in diploid germ cells like spermatogonia and oogonia, where it helps in producing more cells before meiosis begins. It is also seen during growth, repair, regeneration and asexual reproduction in some lower organisms. This division helps in tissue formation, wound healing, and replacement of old or damaged cells. It is a part of the cell cycle and occurs during the M phase. Mitosis has two main stages. The first stage is Karyokinesis which means division of the nucleus, and the second stage is Cytokinesis which means division of the cytoplasm.

1. Karyokinesis

Karyokinesis is the nuclear division part of mitosis. It is a complex process where the chromosomes get separated and move to opposite poles. It is divided into five proper stages. Each stage happens in a sequence and has its own specific role in dividing the chromosomes. These stages are: Prophase, Prometaphase, Metaphase, Anaphase and Telophase.

i. Prophase

  • This is the first stage. In this stage, the chromatin present inside the nucleus starts condensing and becomes thick. These change into visible chromosomes. Each chromosome has two sister chromatids which are attached at the centromere. The nucleolus becomes smaller and slowly disappears. The nuclear envelope also starts to break. Outside the nucleus, the centrosomes move to opposite poles and form spindle fibers.

ii. Prometaphase

  • This is the second stage. The nuclear membrane completely breaks down. The chromosomes are now free in the cytoplasm. At the centromere of each chromosome, a structure called kinetochore appears. Spindle fibers from opposite poles attach to the kinetochores. These fibers help in moving chromosomes to the middle of the cell.

iii. Metaphase

  • This is the third stage. The chromosomes now arrange themselves in a straight line at the center of the cell. This line is called the metaphase plate or equatorial plane. The centromeres of all chromosomes are aligned on this plane. Spindle fibers are fully attached to the kinetochores from both poles. This is the most stable and balanced stage of mitosis. Chromosomes are most visible in this stage under a microscope.

iv. Anaphase

  • This is the fourth stage. In this stage, the centromeres divide and the two sister chromatids of each chromosome are separated. These separated chromatids are now called daughter chromosomes. The spindle fibers pull them towards opposite poles. This movement is very fast. Now both poles get equal sets of chromosomes.

v. Telophase

  • This is the fifth and last stage of karyokinesis. The daughter chromosomes reach the poles and start uncoiling. They turn back into chromatin. A new nuclear membrane forms around each set of chromosomes. The nucleolus also reappears in each nucleus. Spindle fibers disappear. Now there are two complete nuclei in one cell.

2. Cytokinesis

Cytokinesis is the second and final stage of mitosis. It comes just after karyokinesis is complete. While karyokinesis divides the nucleus, cytokinesis divides the cytoplasm and other cell contents. The main purpose of cytokinesis is to fully separate the two new nuclei into two separate daughter cells, each having its own cytoplasm, organelles and cell boundary.

Cytokinesis is a very important step because without it, the mitotic division will remain incomplete. Even if the nucleus divides properly, the cell would not become two separate cells unless the cytoplasm and organelles also divide equally. Therefore, cytokinesis ensures that both daughter cells are not only genetically identical but also physically separated and ready to function independently.

The process of cytokinesis happens in different ways in animal and plant cells due to the presence or absence of the cell wall.
  • In animal cells, cytokinesis starts with the formation of a shallow groove in the middle of the cell surface. This groove is called the cleavage furrow. The furrow is formed by a ring of contractile proteins, mainly actin and myosin, present just under the plasma membrane. These proteins start contracting and pulling the membrane inward. As this furrow deepens, it pinches the cell into two equal parts. This process continues until the cell membrane joins in the middle, fully dividing the parent cell into two daughter cells.
  • In plant cells, there is a rigid cell wall, so a cleavage furrow cannot form. Instead, a structure called the cell plate is formed in the center of the dividing cell. The cell plate is made up of vesicles that come from the Golgi apparatus. These vesicles contain materials like pectin and cellulose which help in building the new wall. These vesicles fuse together in the center of the cell and gradually expand outward. As more vesicles join, the cell plate grows until it touches both sides of the cell wall. This forms a complete new dividing wall between the two daughter cells. Later, a new plasma membrane forms on both sides of the plate.
Mitosis is a type of equational cell division in which a single parent cell divides to produce two genetically identical daughter cells. It usually occurs in diploid cells and maintains the same number of chromosomes (2n) in daughter cells as present in the parent cell.







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