Mitosis is the fundamental process of cell division that allows a single parent cell to create two genetically identical daughter cells. This mechanism is central to life, enabling growth and facilitating the continuous repair and replacement of worn-out cells throughout the body. Mitosis is a short, dynamic sequence of events that follows a preparatory phase, ensuring that the duplicated genetic material is precisely divided before the cell physically splits.
Interphase: The Preparatory Phase
Before a cell can enter mitosis, it spends the majority of its life in a state of intense preparation known as interphase. Interphase is not technically one of the mitotic stages, but it is a period of high metabolic activity that is necessary for division to occur. This preparatory time is divided into three distinct sub-phases: G1, S, and G2.
The G1 phase, or “First Gap,” is when the cell grows physically larger, synthesizes proteins, and produces new organelles. The cell then enters the S phase, which stands for “Synthesis,” where the entire genome is accurately duplicated. This DNA replication results in each chromosome consisting of two identical strands, known as sister chromatids, which remain joined together. The final phase, G2, or “Second Gap,” involves continued cell growth and the synthesis of proteins required for the division process.
Prophase: Condensing the Chromosomes
Prophase marks the beginning of mitosis, initiating the structural reorganization of the cell’s contents. The most visible event is the condensation of the replicated genetic material, which transitions from diffuse chromatin into compact, visible chromosomes. This coiling makes the chromosomes easier to move and separate without tangling. Each chromosome appears as an “X” shape, comprising two identical sister chromatids held together at a central region called the centromere.
Simultaneously, the cell begins to construct the mitotic spindle, an intricate framework made of protein fibers called microtubules. The centrosomes move toward opposite sides of the cell, establishing the two poles of the division. The nuclear envelope surrounding the DNA also starts to break down, allowing the spindle fibers access to the compact chromosomes.
Metaphase: Alignment on the Plate
Following the structural changes of prophase, the cell enters metaphase, a stage defined by the precise alignment of all chromosomes. The mitotic spindle fibers fully extend from the poles and attach to specialized protein structures on the chromosomes called kinetochores. Spindle fibers from opposite poles attach to the two kinetochores on each chromosome. This bi-oriented attachment creates tension as the fibers pull the chromosome in two directions.
This balanced tension pulls the chromosomes into a single, straight line exactly halfway between the two spindle poles, known as the metaphase plate. The cell pauses here for the spindle assembly checkpoint, which verifies that every chromosome is correctly attached to fibers from both poles. This alignment is essential for guaranteeing that each resulting daughter cell receives an identical, complete set of genetic instructions.
Anaphase: Separating the Sister Chromatids
Anaphase is the shortest stage of mitosis, characterized by the instantaneous separation of the sister chromatids. Once the metaphase checkpoint confirms alignment, the protein complex holding the sister chromatids together at the centromere is cleaved. This cleavage liberates the sister chromatids, which are now considered individual chromosomes.
The newly separated chromosomes are immediately pulled toward opposite spindle poles by the shortening of the kinetochore microtubules. This movement is driven by the depolymerization of the microtubules. By the end of anaphase, two complete and identical sets of chromosomes have been successfully moved to opposite ends of the cell, ensuring equal genetic distribution.
Telophase and Cytokinesis: Reformation and Division
Telophase represents the final stage of nuclear division, where the separated chromosomes begin to revert to their uncoiled state. As the chromosomes arrive at their poles, the mitotic spindle apparatus begins to disassemble. A new nuclear envelope forms around each of the two sets of chromosomes, creating two distinct nuclei. The chromosomes decondense once more, returning to the loose chromatin form typical of a non-dividing cell.
The process of cytokinesis, the physical division of the cytoplasm, overlaps with telophase and completes cell division. In animal cells, separation is accomplished by the formation of a contractile ring made of actin and myosin filaments beneath the plasma membrane. This ring tightens like a drawstring, creating a visible indentation called the cleavage furrow. The furrow deepens until it pinches the parent cell completely in two, resulting in two separate, genetically identical daughter cells that immediately enter the G1 phase of interphase.