Cell division is a fundamental biological process that allows organisms to grow, repair tissues, and reproduce. Mitosis is a specific type of cell division where a single parent cell divides into two genetically identical daughter cells. It ensures that genetic material is accurately passed from a parent cell to its daughter cells and is essential for maintaining the chromosome number across cell generations.
Preparing for Division
Before a cell enters the active stages of mitosis, it undergoes a preparatory phase known as interphase. This period accounts for the majority of a cell’s life. During interphase, the cell grows, synthesizes proteins, and carries out its normal metabolic functions.
Interphase is subdivided into three stages. The G1 phase is where the cell increases in size and duplicates its organelles. The S phase is when the cell’s entire DNA is replicated, ensuring that each chromosome consists of two identical sister chromatids. The G2 phase involves further growth and the synthesis of proteins necessary for cell division.
Prophase: The Beginning of Change
Prophase marks the initial stage of mitosis. During this phase, the diffuse genetic material within the nucleus, known as chromatin, condenses and coils, forming visible, compact chromosomes. Each condensed chromosome is composed of two identical sister chromatids joined at a central point called the centromere.
As chromosomes condense, the nucleolus, a structure involved in ribosome production, disappears. The nuclear envelope, which encloses the genetic material, starts to break down. In animal cells, centrosomes move towards opposite ends of the cell, initiating the formation of the mitotic spindle, a network of microtubules that guides chromosome movement.
Metaphase: Alignment for Division
Following prophase, the cell enters metaphase. During metaphase, the condensed chromosomes align along the cell’s central plane, often referred to as the metaphase plate. This alignment ensures each new daughter cell receives an identical set of chromosomes.
The formation of the mitotic spindle is complete, with spindle fibers extending from opposite poles of the cell. These spindle fibers, composed of microtubules, attach to specific protein structures called kinetochores. The attachment of spindle fibers to both sister chromatids helps maintain their precise alignment at the metaphase plate.
Anaphase: Separation of Chromosomes
Anaphase is a dynamic stage where the duplicated genetic material is separated. This process begins with the division of the centromeres that have been holding sister chromatids together. Once separated, each chromatid is considered an individual chromosome.
The newly independent chromosomes are pulled towards opposite poles of the cell by the shortening spindle fibers. This movement is driven by the depolymerization of microtubules at the kinetochore, effectively reeling in the chromosomes. As the chromosomes move apart, the cell often elongates.
Telophase and Cytokinesis: Two Cells Emerge
Telophase represents the final stage of nuclear division, essentially reversing many of the events that occurred during prophase. As the separated chromosomes arrive at opposite poles of the cell, they decondense, returning to a more diffuse chromatin state. New nuclear envelopes form around each set of chromosomes at the poles, creating two distinct nuclei within the single parent cell. The mitotic spindle fibers also disassemble during this phase.
Following telophase, cytokinesis completes cell division by dividing the cytoplasm. In animal cells, a cleavage furrow pinches the cell membrane inward until the cell divides into two daughter cells. Plant cells, with their rigid cell walls, employ a different mechanism; vesicles derived from the Golgi apparatus gather at the former metaphase plate and fuse to form a cell plate. This cell plate grows outward, forming a new cell wall that divides the parent cell into two daughter cells.