Mitosis is the process of somatic cell division, which organisms use for growth, tissue repair, and replacing old or damaged cells throughout the body. This mechanism ensures that a single parent cell accurately divides its genetic material to create two genetically identical daughter cells. While the nuclear division phase, known as karyokinesis, consists of four distinct stages (Prophase, Metaphase, Anaphase, and Telophase), the complete cell cycle involves six necessary steps, including the preparatory stage of Interphase and the final physical separation, Cytokinesis.
Interphase: The Preparation Stage
The cell spends the vast majority of its time in Interphase, often exceeding 95% of the total cycle duration, preparing for the upcoming division. Interphase is divided into three sub-phases: Gap 1 (G1), Synthesis (S), and Gap 2 (G2).
During the G1 phase, the cell grows physically, synthesizes various proteins, and duplicates most of its cellular contents and organelles. The cell must pass a specialized restriction point in G1 to commit to division; if it does not receive the proper signals, it may exit the cycle and enter a quiescent state known as G0.
The S phase follows G1 and is defined by DNA replication, where the cell copies all of its genetic material. At the end of the S phase, the amount of DNA per cell has effectively doubled, though the chromosome number remains unchanged. In animal cells, the centrioles are also duplicated during this phase, providing the structures needed for spindle formation later.
The G2 phase involves further cell growth, replenishing energy stores, and synthesizing proteins necessary for manipulating the chromosomes during mitosis. The cell employs a G2/M checkpoint to ensure that the DNA has been replicated completely and accurately before initiating the M phase. The genetic material exists as uncondensed chromatin throughout Interphase, making the individual chromosomes invisible under a light microscope.
Prophase: Chromatin Condensation
Prophase begins after Interphase, marking the start of the cell’s physical division process. The primary event is the condensation of the loose, thread-like chromatin fibers into discrete, compact chromosomes. Each replicated chromosome now consists of two identical strands, called sister chromatids, which are joined together at a central region known as the centromere.
Simultaneously, the mitotic spindle begins to assemble as the two centrosomes migrate toward opposite ends, or poles, of the cell. These centrosomes organize the production of microtubules that form the spindle fibers. The nucleolus also begins to disappear during this stage.
As Prophase concludes, the nuclear envelope breaks down into small vesicles, a stage sometimes referred to as Prometaphase, which allows the spindle microtubules to access the chromosomes. The spindle fibers then begin to attach to specialized protein structures called kinetochores, which are located at the centromere of each sister chromatid.
Metaphase: Alignment on the Plate
Following the breakdown of the nuclear envelope, the chromosomes are captured by spindle fibers and move toward the center of the cell. This movement results in the precise alignment of all chromosomes along the cell’s equatorial plane, an invisible line referred to as the metaphase plate. This alignment is mediated by kinetochore microtubules, which attach to the kinetochores on the sister chromatids.
Kinetochore microtubules originating from opposite poles attach to the sister kinetochores, creating a balanced, opposing tension across the centromere. The cell uses this tension to activate the spindle checkpoint, a surveillance mechanism that ensures every chromosome is correctly attached and aligned before division proceeds. This checkpoint prevents the cell from entering the next phase until all chromosomes are positioned properly, guaranteeing that each future daughter cell receives a complete genome.
Anaphase: Separation of Chromatids
Anaphase is typically the shortest but most rapid stage. It begins abruptly when specialized proteins, called cohesins, which hold the sister chromatids together, are cleaved. This cleavage allows the sister chromatids to separate simultaneously, turning each chromatid into an individual chromosome.
The newly separated chromosomes are then pulled forcefully toward opposite poles of the cell. This movement is driven primarily by the shortening of the kinetochore microtubules, which reel the chromosomes toward the centrosomes.
Simultaneously, non-kinetochore microtubules overlap and lengthen, pushing the poles further apart. This action causes the entire cell to elongate significantly, ensuring the two complete sets of chromosomes are well separated. By the conclusion of Anaphase, the two ends of the cell possess an equivalent, complete collection of chromosomes.
Telophase and Cytokinesis: The Conclusion of Division
Telophase represents a reversal of the events that characterized Prophase. As the separated sets of chromosomes arrive at the opposite poles of the cell, new nuclear envelopes begin to form around each group of genetic material.
The mitotic spindle fibers disassemble, and the chromosomes begin to de-condense, returning to their extended, chromatin state. The result of Telophase is two distinct nuclei, each containing an exact copy of the parent cell’s genetic information.
Cytokinesis, the physical division of the cytoplasm, usually overlaps with Telophase, starting late in Anaphase or early in Telophase. In animal cells, the first sign of this cytoplasmic division is the appearance of a pucker, or indentation, on the cell surface called the cleavage furrow.
This furrow is formed by a contractile ring composed of actin and myosin filaments located just beneath the plasma membrane. The contractile ring constricts like a drawstring, pulling the cell membrane inward until the cell is pinched into two. This separation results in the formation of two distinct, genetically identical daughter cells.