Mitosis – A Dramatic Phase of Cell Cycle

Mitosis – A Dramatic Phase of the Cell Cycle

Mitosis is a crucial part of the cell cycle, also referred to as karyokinesis. It is an equational division, meaning the number of chromosomes in the parent and daughter cells remains the same. Mitosis mainly occurs in diploid cells of animals, though it may also take place in some haploid cells of plants.

Stages of Karyokinesis

Mitosis consists of four main stages:

1. Prophase

      Early Prophase: The chromosomal material condenses to form visible chromosomes. Each chromosome is attached to a centromere. The centrosomes (containing centrioles) begin migrating towards opposite poles and radiate microtubules called asters.

      Mitotic Apparatus: Formed by two asters and spindle fibers.

      Late Prophase: The nuclear envelope and nucleolus gradually disappear along with other cell organelles.
    

2. Metaphase

The nuclear envelope is completely disintegrated, marking the start of metaphase. Chromosomes become most visible and are made up of two sister chromatids joined at the centromere. They align at the metaphase plate or equatorial plane. Each chromatid connects to spindle fibers from opposite poles via kinetochore discs.

3. Anaphase

The centromeres split, and sister chromatids (now called daughter chromosomes) move towards opposite poles. The centromere leads the way, while the arms trail behind.

4. Telophase

The chromosomes decondense and lose their identity, forming a chromosome cluster. The nuclear envelope reforms around each set, and the nucleolus, Golgi apparatus, and endoplasmic reticulum reappear.

Cytokinesis

Cytokinesis is the division of the cytoplasm, which follows karyokinesis:

In animal cells: It occurs via a cell furrow method involving the plasma membrane.
In plant cells: It occurs by the formation of a cell plate, which becomes the middle lamella between two daughter cells. This begins from the center and moves outward.

Organelles like mitochondria and plastids are equally distributed during cytokinesis. In some cases, karyokinesis is not followed by cytokinesis, resulting in a multinucleated condition (e.g., endosperm in coconut).

Significance of Mitosis

Responsible for growth in multicellular organisms.
Maintains the nucleo-cytoplasmic ratio by producing new cells.
Essential for repair and regeneration (e.g., epidermis, gut lining, blood cells).
Enables continuous growth in plants through meristematic tissues (apical and lateral cambium).
Haploid cells of lower plants and some social insects like honeybees may also divide by mitosis.

Conclusion: Mitosis ensures that each daughter cell receives the same genetic material as the parent cell, making it fundamental for growth, repair, and development in living organisms.

Mitosis