Mitosis is a fundamental biological process where a single parent cell divides to create two genetically identical daughter cells. This cell division is how organisms grow, repair damaged tissues, and, in some cases, reproduce asexually. Mitosis ensures that each new cell receives an exact, complete set of chromosomes, thereby preserving genetic continuity across generations. This process is universal among eukaryotic organisms, from microscopic single-celled life forms to complex multicellular beings.
The Four Stages of Mitosis
The process of nuclear division in mitosis unfolds through four stages: Prophase, Metaphase, Anaphase, and Telophase. Each stage involves specific cellular rearrangements that ensure the accurate distribution of genetic material. The sequential progression through these stages is controlled to maintain cellular integrity.
During prophase, the cell’s genetic material, which exists as chromatin fibers, condenses and coils to form visible chromosomes. Each chromosome consists of two identical sister chromatids joined at the centromere. Simultaneously, the nuclear envelope, which encloses the genetic material, begins to break down, and the mitotic spindle, a structure made of microtubules, starts to form from centrosomes.
As the cell transitions into metaphase, the condensed chromosomes align themselves along the cell’s equatorial plane, known as the metaphase plate. The spindle fibers, extending from the centrosomes at opposite poles of the cell, attach to the centromere of each sister chromatid. This attachment ensures that each chromatid is properly oriented for separation.
Anaphase marks where the sister chromatids separate from each other. Once separated, these are no longer called chromatids but are considered individual chromosomes. Motor proteins, associated with the spindle fibers, pull these newly individualized chromosomes towards opposite poles of the cell. Concurrently, the cell itself begins to elongate in preparation for division.
The final stage of nuclear division is telophase, which reverses the events of prophase. The chromosomes arrive at the opposing poles of the cell and begin to decondense, returning to their thread-like chromatin form. A new nuclear envelope reforms around each complete set of chromosomes, creating two distinct nuclei. The mitotic spindle also disassembles.
Cytokinesis: The Final Split
Following the nuclear division of mitosis, the entire cell divides through cytokinesis, which involves the division of the cytoplasm and its contents. While discussed alongside mitosis, cytokinesis is a distinct process that completes the formation of two separate daughter cells. This cytoplasmic division begins during the late stages of mitosis, overlapping with anaphase or telophase.
In animal cells, cytokinesis involves the formation of a cleavage furrow, an indentation on the cell surface. This furrow is created by a contractile ring composed of actin and myosin filaments, which are similar to the proteins found in muscle cells. This ring contracts inward, much like a drawstring, pinching the parent cell into two daughter cells.
Plant cells, possessing rigid cell walls, undergo cytokinesis differently. Instead of a cleavage furrow, a cell plate forms in the middle of the cell. This cell plate originates from vesicles that migrate to the equatorial plane and fuse. The cell plate then grows outward from the center, fusing with the parent cell walls and plasma membrane, establishing a new cell wall that divides the original cell into two daughter cells.