Mitosis is the process of nuclear division in eukaryotic cells that ensures a parent cell divides to produce two genetically identical daughter cells. This mechanism is fundamental to nearly all multicellular life, serving as the basis for growth, development, and tissue repair. Mitosis allows the body to replace old or damaged cells, such as those lining the skin or digestive tract. Mitosis itself represents the distinct period of chromosome segregation, known as the M phase, which is only one part of the overall cell cycle.
Interphase: Preparing the Cell for Division
The cell spends the majority of its life in interphase, the stage occurring before the initiation of mitosis. This phase is a highly active sequence of growth and preparation divided into three sub-stages: G1, S, and G2. During the G1 phase, the cell grows, synthesizes proteins, and accumulates the energy reserves required for DNA replication.
This preparation transitions into the S phase, or Synthesis phase, where the cell’s entire genome is duplicated. Each chromosome is copied, resulting in two identical DNA molecules called sister chromatids, which remain joined together. Following DNA replication, the G2 phase begins, where the cell replenishes energy stores, synthesizes proteins for the mitotic machinery, and ensures all DNA has been replicated without damage.
Prophase: Chromosome Condensation and Nuclear Disassembly
The onset of prophase marks the beginning of mitosis. The diffuse chromatin fibers within the nucleus begin coiling and folding, condensing into compact chromosomes. This condensation makes the genetic material easier to manage and separate in later stages.
Simultaneously, the cell begins to assemble the mitotic spindle, a framework of microtubules originating from the centrosomes. These centrosomes, duplicated during interphase, start to migrate toward opposite poles of the cell, extending microtubule fibers between them. This movement establishes the two poles for cell division.
The nuclear envelope begins to break down into small vesicles, marking the transition into prometaphase. This disassembly allows the spindle microtubules to gain access to the condensed chromosomes. The spindle fibers then begin to attach to protein complexes called kinetochores, located at the centromere of each sister chromatid.
Metaphase: Alignment at the Center
Once the nuclear envelope has dissipated, the spindle fibers lead the cell into metaphase. Kinetochore microtubules, extending from opposite poles, attach to the kinetochores on sister chromatids, creating a bi-oriented attachment. This arrangement establishes a balanced pulling force on each chromosome.
The tension generated by these opposing forces causes all the chromosomes to align precisely along the cell’s imaginary equatorial plane, known as the metaphase plate. This alignment is necessary to ensure equal distribution of genetic material to the daughter cells. Before proceeding, the cell activates the spindle assembly checkpoint, which verifies every chromosome is properly attached to microtubules from both poles and is under tension.
If the checkpoint detects unattached or misaligned chromosomes, it pauses the cell cycle for correction. Only upon successful passage of this checkpoint does the cell proceed into the next stage of division. This regulatory step is a quality control mechanism that prevents errors in chromosome number.
Anaphase: Separation of Sister Chromatids
The completion of the metaphase checkpoint triggers anaphase, where the sister chromatids separate. The cohesin proteins that held the sister chromatids together since the S phase are cleaved by an enzyme called separase. This cleavage dissolves the connection between the identical DNA copies.
The former sister chromatids are now considered individual chromosomes and are pulled toward the opposite poles of the cell. The primary force for this movement is generated by the shortening of the kinetochore microtubules, pulling the chromosomes toward the centrosomes. Non-kinetochore microtubules also lengthen, pushing the poles further apart and elongating the entire cell.
This movement results in two complete and identical sets of chromosomes migrating to opposite ends of the cell. This separation ensures that each pole receives a full complement of genetic information. Anaphase concludes once all the separated chromosomes have arrived at their respective poles.
Telophase and Cytokinesis: Nuclear Reformation and Cell Splitting
Telophase represents a reversal of the events of prophase, focusing on the reformation of two distinct nuclei. As the chromosomes cluster at the poles, they begin to decondense, unwinding back into chromatin. Simultaneously, new nuclear envelopes form around each set of chromosomes using the membrane vesicles generated earlier.
The mitotic spindle apparatus is dismantled, and the nucleoli reappear within the newly formed nuclei. By the end of telophase, the cell contains two separate nuclei, each housing an identical copy of the full genome. This stage completes the process of nuclear division, or mitosis.
Cytokinesis, the division of the cytoplasm, usually begins during late anaphase and continues through telophase. In animal cells, this process involves the formation of a cleavage furrow, an indentation that appears on the cell surface midway between the two poles. This furrow is caused by the contraction of a contractile ring positioned beneath the plasma membrane.
The contractile ring is composed of actin filaments and myosin motor proteins, which work together to pinch the cell in two. The furrow deepens until it severs the connection, resulting in two separate daughter cells. Each new daughter cell is now ready to enter the G1 phase of the next cell cycle.