What happens during mitosis (M phase)?

Mitosis, also called the M phase, is the most active and visible part of the cell cycle. In this phase, a single mother cell divides into two daughter cells and each new cell receives the exact copy of genetic material as present in the original cell. This phase is mainly responsible for growth, repair of damaged tissues and maintaining genetic continuity. Mitosis ensures that the chromosome number remains the same in both daughter cells. The entire process is divided into two main stages: karyokinesis, which is the division of the nucleus and cytokinesis, the division of the cytoplasm. These two stages occur in a highly coordinated sequence to ensure the proper distribution of cellular components and genetic material.

1. Karyokinesis (Division of Nucleus)

Karyokinesis is the first major step of mitosis, in which the nucleus of the parent cell divides. It ensures that each daughter cell will receive an equal and exact set of chromosomes. This process is further divided into five clear stages, each with its own specific changes inside the cell:

a. Prophase:

  • This is the beginning stage. Here, the long thread-like chromatin condenses and becomes thick and visible as chromosomes. Each chromosome has two identical sister chromatids joined at the centromere. The nucleolus disappears and spindle fibers begin to form as the centrosomes move toward opposite poles of the cell.

b. Prometaphase:

  • In this stage, the nuclear envelope completely breaks down. The spindle fibers reach the chromosomes and attach to the centromeres at a region called the kinetochore. The chromosomes begin to move and prepare to line up in the center of the cell.

c. Metaphase:

  • This is the stage of perfect alignment. All the chromosomes line up exactly in the middle of the cell, forming the metaphase plate. This proper alignment is important to ensure equal distribution of chromosomes during the next step.

d. Anaphase:

  • At this stage, the two sister chromatids of each chromosome are pulled apart by the spindle fibers. They start moving toward opposite poles of the cell. Now each chromatid becomes a separate chromosome. This separation ensures that both sides will receive identical genetic material.

e. Telophase:

  • In this final nuclear step, the chromosomes reach the opposite poles and begin to uncoil into thin chromatin again. New nuclear membranes form around each set of chromosomes and the nucleolus reappears. The cell now has two separate nuclei.

2. Cytokinesis (Division of Cytoplasm)

After nuclear division, the cell moves to its final step i.e., cytokinesis. This is the process by which the entire cytoplasm divides, completing the formation of two separate daughter cells. Though it begins during telophase, it is often considered a separate event. Cytokinesis differs slightly between plant and animal cells:
  • In animal cells, a contractile ring made of actin filaments forms at the center of the cell, pulling the membrane inward. This process, known as cleavage, deepens until the cell is pinched into two separate daughter cells, each with a full set of organelles and genetic material.
  • In plant cells, due to the presence of a rigid cell wall, a cell plate forms between the two daughter nuclei. This plate grows outward until it fuses with the cell membrane, forming a new cell wall that separates the daughter cells.
The entire process is divided into two main stages: karyokinesis, which is the division of the nucleus and cytokinesis, the division of the cytoplasm. These two stages occur in a highly coordinated sequence to ensure the proper distribution of cellular components and genetic material.

By the end of mitosis and cytokinesis, two genetically identical daughter cells have been formed, each with an identical set of chromosomes. These cells are now ready to enter the interphase phase of the cell cycle, where they will prepare for their next round of division or differentiation.






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