Mitosis is the process of nuclear division in eukaryotic cells in which duplicated chromosomes are separated so that daughter nuclei generally receive equivalent chromosome complements. It is usually associated with somatic cell proliferation and is followed by cytokinesis, although the relationship between nuclear division and cytoplasmic division varies among cell types. Mitosis is conventionally divided into prophase, prometaphase, metaphase, anaphase and telophase, although the boundaries between these stages are conceptual descriptions of a continuous process.
Before mitosis begins, DNA has already been replicated during S phase, producing sister chromatids joined by cohesin. During mitosis, chromosomes become highly condensed, the nuclear envelope breaks down in many eukaryotic cells, spindle microtubules attach to kinetochores, chromosomes align and sister chromatids separate. Mitosis therefore preserves chromosome number in the usual case and enables genetically similar daughter cells to arise from a single parent cell.
Prophase begins with progressive chromosome condensation and reorganisation of the cytoskeleton. Each replicated chromosome consists of two sister chromatids associated along their length, with particularly important cohesion around the centromeric region. Centrosomes in animal cells have duplicated and begin organising the bipolar spindle. As the cell progresses towards prometaphase, the nuclear envelope breaks down and spindle microtubules gain access to chromosomes. Kinetochores assemble at centromeres and interact with spindle microtubules. Different classes of spindle microtubules contribute different mechanical functions.
Kinetochore microtubules connect chromosomes to spindle poles, interpolar microtubules overlap with microtubules from the opposite pole, and astral microtubules interact with the cell cortex. The spindle is therefore an integrated mechanical system that positions and separates chromosomes rather than simply a bundle of fibres attached randomly to DNA.
During metaphase, chromosomes become positioned near the equatorial region of the spindle in a configuration commonly called the metaphase plate. The critical objective is not merely alignment but establishment of appropriate bipolar attachment. Each pair of sister kinetochores should interact with spindle microtubules originating from opposite spindle poles. Tension and attachment status are monitored by the spindle assembly checkpoint, which delays anaphase until chromosomes satisfy required attachment conditions.
This checkpoint involves kinetochore associated proteins and signalling mechanisms that regulate the anaphase promoting complex or cyclosome. Once the checkpoint is satisfied, the anaphase promoting complex promotes degradation of specific regulatory proteins, leading to activation of separase and cleavage of cohesin components that hold sister chromatids together. This controlled transition ensures that sister chromatid separation is normally a one way event during mitosis.
Anaphase begins when sister chromatids separate and become individual daughter chromosomes. Kinetochore microtubules shorten or change their dynamics while chromosome movement towards opposite spindle poles occurs through coordinated interactions between microtubules, motor proteins and chromosome associated structures. At the same time, interpolar microtubules and motor proteins contribute to spindle elongation. Anaphase is often divided into anaphase A, involving chromosome movement towards poles, and anaphase B, involving increased distance between spindle poles, although the exact mechanics vary among organisms. Accurate anaphase requires both correct chromosome attachment and timely removal of sister chromatid cohesion. Errors can produce lagging chromosomes, chromosome bridges or unequal chromosome distribution. Such errors may lead to daughter cells with abnormal chromosome numbers and can contribute to genomic instability.
Telophase follows chromosome segregation and involves reorganisation of daughter nuclei. Chromosomes generally decondense, nuclear envelopes reform around separated chromosome sets and the mitotic spindle is disassembled or remodelled. Cytokinesis usually overlaps with late mitosis and physically separates the two daughter cells. In animal cells, actomyosin contraction forms a cleavage furrow, whereas plant cells construct a cell plate. The timing and coordination of these processes are essential because premature cytokinesis could cut through incompletely segregated chromosomes, while delayed cytokinesis could produce multinucleated cells. Mitosis is therefore not an isolated chromosome movement event but one component of a coordinated programme involving chromosome architecture, spindle mechanics, nuclear membrane dynamics and cytoplasmic restructuring.
Mitosis differs fundamentally from Meiosis even though both involve chromosome segregation machinery. Mitosis normally involves one round of chromosome duplication followed by one division, producing daughter nuclei with chromosome numbers comparable to the parent nucleus. Meiosis involves one round of DNA replication followed by two successive divisions and includes homologous chromosome pairing and recombination. Mitosis is generally used for growth, maintenance and asexual propagation, while meiosis contributes to sexual reproduction and produces haploid cells or their precursors.
Another important distinction is that mitosis separates sister chromatids after chromosome replication, whereas Meiosis I separates homologous chromosomes while sister chromatids remain associated. Meiosis II then resembles a mitotic division more closely. Understanding these distinctions prevents the common misconception that meiosis is simply a specialised version of ordinary mitosis.
Mitosis is regulated by cyclins, cyclin dependent kinases, ubiquitin mediated protein degradation, checkpoint pathways and cytoskeletal control systems. Cyclin dependent kinase activity drives entry into mitosis, while regulated destruction of mitotic cyclins and other proteins promotes exit. DNA damage checkpoints can delay progression before mitosis, while the spindle assembly checkpoint monitors chromosome attachment during mitosis itself. Mutations affecting these regulatory systems can permit cells with damaged or incorrectly replicated DNA to proliferate.
Chromosome missegregation during mitosis can generate aneuploid daughter cells and contribute to tumour evolution. Mitosis is therefore central to both normal tissue growth and the maintenance of genomic stability. Many anticancer drugs exploit mitotic vulnerabilities by interfering with microtubules or other components of the division machinery, although such interventions can also affect normal proliferating cells.
Mitosis has broad biological significance because it enables multicellular organisms to increase cell number and replace cells while maintaining genome continuity. It also supports vegetative reproduction in some organisms and contributes to tissue regeneration. The accuracy of mitosis depends on coordination between DNA replication, chromosome condensation, kinetochore assembly, spindle organisation, checkpoint signalling, chromatid cohesion and cytokinesis. The process is highly conserved but varies in molecular details between organisms and cell types. Modern microscopy has revealed that chromosome movements are dynamic and involve continuous microtubule turnover rather than simple static attachment. Mitosis therefore illustrates how molecular machines, mechanical forces and regulatory networks cooperate to achieve a precise cellular outcome.
