Mitosis is the fundamental biological process by which a single cell divides into two genetically identical daughter cells. This form of division is the basis for growth, tissue repair, and asexual reproduction in many organisms. While often simplified into four major phases—prophase, metaphase, anaphase, and telophase—a more comprehensive understanding requires detailing six distinct stages. These six stages include the preparatory phase and the clear separation of the initial and final steps of chromosome movement. By examining these stages sequentially, we can track the precise mechanism a cell uses to ensure the faithful duplication and equal distribution of its entire genetic blueprint.
Interphase The Essential Preparation Period
Interphase represents the first stage in this six-step sequence, although it is the period of cell growth and DNA replication that precedes active division. During this phase, the cell is highly active, carrying out its normal functions while preparing the necessary components for division. Interphase is subdivided into three distinct sub-phases: G1, S, and G2.
The G1 phase, or “first gap,” is characterized by cellular growth and the synthesis of proteins and organelles. Following G1, the cell enters the S phase, or “synthesis” phase, where the cell’s entire DNA is replicated. This replication results in two identical copies of the genome, known as sister chromatids. The final stage is the G2 phase, or “second gap,” during which the cell synthesizes final proteins and checks the replicated DNA for errors, ensuring the cell is fully prepared to enter active mitosis.
Stages of Chromosome Condensation
The transition from preparation to active division begins with Prophase, the second stage. During prophase, the loosely arranged chromatin fibers within the nucleus begin to condense and coil tightly, becoming microscopically visible as distinct chromosomes. Each chromosome consists of the two identical sister chromatids held together along their length.
The mitotic spindle begins to form in the cytoplasm, organized by structures called centrosomes. These centrosomes, duplicated during interphase, move toward opposite sides of the cell, establishing the two poles of the developing spindle apparatus. This microtubule-based framework is necessary for the movement and separation of the chromosomes.
The third stage is Prometaphase, distinguished by the breakdown of the nuclear envelope. This dissolution allows the microtubules of the mitotic spindle to extend into the nuclear area. Specialized protein structures called kinetochores form on the centromere region of each sister chromatid.
Kinetochore microtubules, originating from the spindle poles, then attach to these kinetochore structures. The attachment process involves unstable interactions that transition into stable attachments to microtubules emanating from the opposite poles of the cell. This physical connection allows the chromosomes to begin moving erratically as the spindle machinery organizes them.
Stages of Chromosome Separation
The fourth stage, Metaphase, marks the point when the chromosomes are fully organized and positioned for separation. The spindle fibers manipulate the chromosomes until all are aligned along a single imaginary plane equidistant from the two spindle poles. This line is known as the metaphase plate.
The alignment at the metaphase plate is monitored by the spindle checkpoint. This checkpoint confirms that every kinetochore is properly attached to microtubules from opposing poles, a configuration known as bi-orientation. This arrangement guarantees that the two daughter cells will each receive an identical and complete set of genetic material.
Anaphase, the fifth stage, is characterized by the abrupt separation of the sister chromatids, which are now considered individual chromosomes. This separation is initiated by the activation of the protease separase. Separase cleaves the cohesin proteins that have been holding the sister chromatids together.
Once the cohesin is cleaved, the liberated chromosomes are pulled toward the opposite spindle poles. This movement is achieved primarily through the shortening of the kinetochore microtubules, which reels the chromosomes in. Simultaneously, the cell elongates as non-kinetochore microtubules push the poles further apart, contributing to the complete segregation of the two sets of chromosomes.
Telophase and Cytokinesis The Completion of Division
The final stage of active mitosis is Telophase, which begins once the two complete sets of chromosomes have arrived at their respective poles. This phase is the reversal of prophase and prometaphase events. The separated chromosomes begin to decondense, returning to their less compact chromatin state.
New nuclear envelopes form around each of the two chromosome sets, utilizing fragments of the old nuclear membrane. This process creates two distinct nuclei within the single, elongated parent cell, each containing an identical complement of genetic information. The mitotic spindle apparatus largely disassembles.
Simultaneously with telophase, the final physical division of the cell, known as Cytokinesis, is completed. In animal cells, this process involves the formation of a cleavage furrow, a shallow indentation that appears on the cell surface along the metaphase plate. This furrow is created by a contractile ring composed of actin microfilaments and myosin motor proteins that constricts the cell’s equator. The deepening of this ring eventually pinches the cell in two, resulting in the formation of two separate daughter cells, concluding the six-stage process.