Mitosis is the process by which a single cell divides to produce two genetically identical daughter cells. This form of cell division allows the body to grow, replace old or damaged cells, and repair tissues. It is an equational division, meaning the resulting cells maintain the same total number of chromosomes as the parent cell. Mitosis is the final stage of the cell cycle, which governs the entire life of a cell.
Preparing the Cell for Division
The cell spends the vast majority of its time in Interphase, a preparatory phase that occurs before mitosis begins. Interphase is divided into three subphases: G1, S, and G2, during which the cell grows and accumulates necessary materials for division. The G1 phase, or first gap, is a period of active growth and protein synthesis where the cell increases its size and volume.
The S phase (synthesis phase) is where the cell’s entire genetic material is duplicated. This replication results in two identical copies of each chromosome, known as sister chromatids, which remain temporarily joined. The cell then enters the G2 phase (second gap), where it finalizes preparation by replenishing energy stores and synthesizing proteins needed for chromosome manipulation and spindle formation. This ensures the cell is ready to enter the M phase, which encompasses mitosis and the final physical split.
The Initial Stages of Mitosis
The mitotic phase begins with Prophase, where the loosely packed DNA (chromatin) coils and condenses into the compact, visible structures recognized as chromosomes. Centrosomes, duplicated during Interphase, move toward opposite poles of the cell, initiating the formation of the mitotic spindle, a framework of microtubules.
Prometaphase is marked by the breakdown of the nuclear envelope. This allows the spindle microtubules to access the condensed chromosomes. Each sister chromatid pair possesses a specialized protein structure called the kinetochore, which serves as the attachment point for the spindle fibers. Microtubules from opposing poles attach to the kinetochores, moving the chromosomes toward the cell’s center.
Chromosome Alignment and Segregation
The next stage, Metaphase, is defined by the precise alignment of all chromosomes at the cell’s imaginary midpoint, known as the metaphase plate. Kinetochore microtubules from opposing poles exert forces that position the chromosomes exactly along this equatorial plane. This alignment is maintained by a balance of tension, ensuring each sister chromatid is properly connected to a microtubule originating from opposite poles.
A quality control mechanism, the metaphase checkpoint, operates at this stage to ensure genetic fidelity. This checkpoint prevents the cell from progressing until every kinetochore is correctly attached and every chromosome is aligned. Once satisfied, the cell triggers Anaphase, characterized by the sudden separation of the sister chromatids. Cohesin proteins, which held the chromatids together since the S phase, are rapidly cleaved, freeing the individual chromatids.
Upon separation, each former sister chromatid is now considered a full, independent chromosome. These newly separated chromosomes are then quickly pulled toward their respective poles as the kinetochore microtubules shorten. Motor proteins working along the microtubules drive this movement, while non-kinetochore microtubules lengthen, which contributes to the overall elongation of the cell. This synchronized movement ensures an equal set of genetic information is delivered to each end of the dividing cell.
Completing the Division
The final stage of nuclear division is Telophase, which reverses the events of Prophase. Once the two complete sets of chromosomes arrive at opposite poles, they decondense, reverting to the looser chromatin structure. New nuclear envelopes begin to form around each set of chromosomes, utilizing fragments of the old nuclear membrane. The mitotic spindle disassembles, and the cell is temporarily binucleated, containing two distinct nuclei.
Cytokinesis, the physical division of the cytoplasm, typically begins during late Anaphase and continues through Telophase. In animal cells, this occurs through the formation of a contractile ring composed of actin and myosin filaments. This ring forms beneath the plasma membrane and contracts inward, creating the cleavage furrow. The furrow eventually pinches the parent cell completely in two, resulting in two separate, genetically identical daughter cells.