Mitosis is the process by which one cell splits into two genetically identical daughter cells, and it underpins nearly every aspect of multicellular life. A human body contains roughly 37 trillion cells, virtually all of them produced through successive rounds of mitosis starting from a single fertilized egg. The same process replaces worn-out skin, heals a broken bone, and regenerates liver tissue after injury. When mitosis works correctly, it sustains growth, powers repair, and maintains the tissues that keep an organism alive. When it fails, the consequences range from developmental defects to cancer.
From One Cell to Trillions
Growth in any multicellular organism depends on cells dividing and multiplying. In early embryonic development, this happens at a breathtaking pace. After fertilization, the single-celled embryo undergoes rapid cleavage divisions, and coordinating those divisions across the large expanse of the embryo requires chemical waves and physical flows within the cytoplasm.1PubMed Central. Cell cycle control during early embryogenesis These early rounds of mitosis don’t add much overall mass; they mostly subdivide the original egg into smaller and smaller cells. True growth comes later, when the embryo begins importing nutrients and the daughter cells both divide and enlarge.
Once an organism moves beyond the embryonic stage, mitosis continues to drive the increase in cell number needed for a child’s bones to lengthen, muscles to thicken, and organs to reach functional size. Each division produces two cells with the same complete genome as the parent cell, meaning every new cell carries the full set of instructions needed to participate in the organism’s tissues. This stands in contrast to meiosis, which halves the genetic material to produce sex cells.2Cell Biology: Research & Therapy. Mitosis and Meiosis: Understanding the Differences and Similarities in Cell Division Growth in height, weight, and organ capacity throughout childhood and adolescence is, at its core, a story of mitosis producing more and more identical copies of the body’s working cells.
How Organs Know When to Stop Growing
If mitosis is responsible for building organs, the body also needs a way to stop building once an organ reaches the right size. A liver shouldn’t keep growing once it’s large enough to handle its metabolic load. This kind of size control is extraordinarily precise, and researchers have identified signaling pathways that regulate it by controlling how many cells an organ contains and how large each cell grows.3PubMed Central. Organ size control by Hippo and TOR pathways
One of the best-studied brakes on organ growth involves a protein called Yap1 and its counterparts. When Yap1 activity is high, cells keep dividing. When opposing proteins outcompete Yap1, division slows and the organ stops expanding. In zebrafish, researchers found that a precise balance between Yap1 and two related proteins ensures the right number of cells form in a developing sensory organ: too much Yap1 produced excess cells, while too much of its competitors left the organ undersized.4bioRxiv. Vgll4 Proteins limit Organ Size in Zebrafish through Yap1-Dependent and -Independent Mechanisms This tug-of-war between growth-promoting and growth-limiting signals is how the body calibrates organ size with striking accuracy. When that calibration breaks down, either through mutation or chronic signaling errors, it can contribute to tissue overgrowth or tumor formation.
Tissue Repair and the Role of Stem Cells
Beyond building the body in the first place, mitosis is how the body fixes itself. Your skin is a good example: the outermost layer is constantly shed and replaced. Multiple populations of stem cells residing in the epidermis divide to replenish what is lost, whether from everyday wear or from a wound.5PubMed Central. A Beginner’s Introduction to Skin Stem Cells and Wound Healing Cut your finger, and the inflammatory response recruits these stem cells to ramp up mitosis at the wound margins. New cells migrate inward, fill the gap, and eventually restore a functional barrier.
The liver takes repair even further. After surgical removal of a large portion of liver tissue, the remaining hepatocytes re-enter the cell cycle. In human liver cells, the Wnt/β-catenin signaling pathway plays an essential role in triggering this mitotic re-entry; expression of the key protein β-catenin rises within minutes of tissue loss and stays elevated for up to a day, pushing cells toward division.6PubMed Central. Molecular mechanisms in liver repair and regeneration: from physiology to therapeutics The liver can regenerate enough mass to restore function even after losing more than half its volume, making it one of the most regeneration-capable organs in mammals.
Stem cells in the blood-forming system illustrate another dimension of mitotic repair. Under normal, steady-state conditions, blood stem cells tend to divide asymmetrically: one daughter cell remains a stem cell while the other begins to mature into a red blood cell, white blood cell, or platelet. This keeps the stem cell pool stable while continuously supplying fresh blood cells. Analysis of paired daughter cells in culture found that the probability of a blood stem cell undergoing this asymmetric division was about 99.5%.7Blood. Obligatory Asymmetric Cell Division Regulates Self-Renewal In Hematopoietic Progenitor/Stem Cells But during periods of heavy demand, such as after chemotherapy or major blood loss, those same stem cells can switch to symmetric divisions that expand the stem cell pool rapidly before resuming asymmetric division once the crisis passes.8Blood. An RNA Interference Screen Reveals Fate Determinants Implicated in Asymmetric Cell Division as Potential Regulators of Hematopoietic Stem Cell Self-Renewal This flexibility shows that mitosis is not a one-size-fits-all process; the body can tune the mode of division to match the urgency of the repair.
The Built-In Error-Checking System
Getting mitosis right is not trivial. Every time a cell divides, it must duplicate all of its chromosomes and then distribute one complete set to each daughter cell. The physical machinery that accomplishes this, the mitotic spindle, is built from protein filaments called microtubules that attach to each chromosome at a specialized structure and physically pull the copies apart.9PubMed Central. Mitotic force generators and chromosome segregation Those attachment points have been reconstituted in the lab from 21 purified human protein subunits, revealing just how complex the connection between a chromosome and the spindle actually is.10Current Biology. Dispatch Chromosome Segregation: Reconstituting the Kinetochore
Given this complexity, errors are inevitable without some form of quality control. The cell provides one in the form of the spindle assembly checkpoint, a surveillance system that monitors whether every chromosome has properly attached to the spindle before the cell proceeds to pull them apart. If even a single chromosome is unattached, the checkpoint halts progress by generating a signal that blocks the next phase of division.11PubMed Central. BubR1 recruitment to the kinetochore via Bub1 enhances spindle assembly checkpoint signaling This delay gives the cell time to correct the problem. The checkpoint prevents chromosomes from being distributed unevenly, a defect that would leave daughter cells with the wrong number of chromosomes.12PubMed. The spindle assembly checkpoint: preventing chromosome mis-segregation during mitosis and meiosis
Researchers have also found that the chromosome attachment targets themselves are not static. At the onset of mitosis, the outer layer of these structures expands, making them a larger target for the searching spindle fibers. Computational modeling shows that this expansion, combined with early side-on contacts between chromosomes and microtubules, allows the spindle to assemble quickly while still keeping the error rate low enough for the checkpoint to handle any mistakes.13PubMed Central. Adaptive changes in the kinetochore architecture facilitate proper spindle assembly It’s an elegant engineering solution: widen the net early on, then refine the attachments before committing to separation.
When Mitosis Goes Wrong
Cancer is, fundamentally, a disease of uncontrolled cell division. When the checkpoints and growth-limiting signals described above are disabled by mutations, cells can divide without restraint. Mutations in checkpoint proteins are found across all types of cancer, because disabling cell cycle control opens the door to genetic instability.14PubMed Central. Cell cycle checkpoints and their inactivation in human cancer Once a cell accumulates enough mutations to evade these controls, it can form a tumor that grows independently of the normal signals telling it to stop.
The connection between mitotic errors and cancer goes beyond simply dividing too fast. Mistakes during chromosome segregation can cause a cell to end up with an abnormal number of chromosomes, a condition linked to tumor initiation and progression. Recent work has shown that these errors can also shatter mis-segregated chromosomes into fragments that get reassembled in scrambled order, creating dramatic rearrangements in a single cell division event.15Molecular Cell. Boveri and beyond: Chromothripsis and genomic instability from mitotic errors This kind of catastrophic reshuffling can activate cancer-promoting genes or disable tumor suppressors in one stroke, accelerating the path from a normal cell to a malignant one.16PubMed Central. The impact of mitotic errors on cell proliferation and tumorigenesis
Another route to instability involves whole-genome doubling, where a cell ends up with twice the normal chromosome count. This doesn’t automatically cause cancer, but it creates a permissive state. Cells that have doubled their genome show variable chromosome losses and gains over subsequent divisions, and some develop increased tolerance to further mitotic errors, partly by suppressing the p53 pathway that normally kills cells with too many mistakes.17PubMed Central. Chromosomal instability, tolerance of mitotic errors and multidrug resistance are promoted by tetraploidization in human cells The result is a cell population that evolves rapidly, acquiring new mutations and drug resistance. This helps explain why many advanced cancers are so difficult to treat: their mitotic machinery is broken in ways that paradoxically give them an evolutionary advantage.
Antimitotic Drugs and Cancer Treatment
The centrality of mitosis to cancer growth is exactly what makes it a therapeutic target. Antimitotic drugs, which interfere with the spindle or other components of the division machinery, are among the most widely used and well-validated chemotherapy agents.18PubMed Central. Antimitotic drugs in the treatment of cancer Drugs like taxanes stabilize microtubules so they can’t disassemble, while vinca alkaloids prevent microtubules from assembling in the first place. Either way, the spindle can’t function, the checkpoint halts the cell, and prolonged arrest usually triggers cell death.
The catch is that antimitotic drugs also affect normal cells that divide frequently, which is why chemotherapy often causes side effects in the gut lining, bone marrow, and hair follicles. These are among the most mitotically active tissues in the body, and they bear the brunt of a drug that doesn’t distinguish between a cancer cell mid-division and a healthy one. This collateral damage has driven decades of research into more targeted approaches, including drugs aimed at specific mitotic regulators rather than the spindle itself, though the classic spindle poisons remain foundational treatments for many cancers.
Environmental Chemicals That Interfere with Mitosis
It isn’t only mutations and drugs that can disrupt mitosis. Environmental exposures can interfere with the division machinery as well. Bisphenol A (BPA), a chemical found in some plastics and food-contact materials, has been shown to increase chromosomal instability by disrupting spindle formation and the attachment of spindle fibers to chromosomes. In cell culture, BPA prolonged the time cells spent in mitosis by activating the spindle assembly checkpoint, and it caused abnormal multi-polar spindles by triggering extra duplication of the structures that organize the spindle.19PubMed. Bisphenol A disrupts mitotic progression via disturbing spindle attachment to kinetochore and centriole duplication in cancer cell lines
Air pollution particles pose a similar threat. Fine particulate matter (PM2.5) can enter lung cells and over-stabilize the microtubule proteins that make up the spindle, reducing the dynamic behavior the spindle needs to function. The result is mitotic failure and the formation of cells with multiple nuclei, a hallmark of disrupted division.20PubMed Central. PM2.5-Induced Multinucleation of Bronchial Epithelial Cells due to Microtubule Overacetylation and Disruption of Microtubule-Associated Proteins Multinucleated cells are generally nonfunctional and can be precursors to further genetic instability. These findings suggest that some of the long-term health effects of pollution exposure may trace back to interference with the basic mechanics of cell division.
How Physical Forces Guide Where a Cell Divides
Mitosis isn’t just a biochemical event; it’s a mechanical one. The orientation of the mitotic spindle determines where the dividing cell splits, and that orientation matters for tissue architecture. In a skin layer, for example, a cell that divides parallel to the surface adds another cell to the sheet, while one that divides perpendicular to the surface sends one daughter cell deeper into the tissue. Mechanical forces play a key role in setting this orientation.21PubMed Central. Force and the spindle: mechanical cues in mitotic spindle orientation
The motor protein dynein, anchored at the cell’s outer edge, pulls on the spindle’s microtubules to rotate it into position. The amount of dynein at the cortex and how tightly it clusters determine how strongly the spindle resists being pushed off-axis.22Developmental Cell. Mechanics of mitotic spindle orientation in human cells Cells also have a backup system: when a cell rounds up completely during mitosis, a force-sensing mechanism involving focal adhesion kinase aligns the spindle with the direction of greatest mechanical tension. But when a cell retains an elongated shape during division, a separate shape-sensing mechanism takes over instead.23PubMed Central. Spindle Orientation Regulation Is Governed by Redundant Cortical Mechanosensing and Shape-Sensing Mechanisms This dual system ensures that cells divide in the right plane regardless of how their local environment deforms them, which is critical for maintaining the organized structure of tissues during growth and repair.
Aging and the Slowdown of Cell Division
If mitosis is what keeps tissues replenished, it follows that a decline in mitotic capacity would contribute to aging, and that appears to be the case. As organisms age, stem cells accumulate DNA damage, their telomeres shorten, their mitochondria become less efficient, and their epigenetic patterns shift.24Theoretical and Natural Science. Molecular Mechanisms of Stem Cell Aging and Their Challenges and Applications in Regenerative Medicine The cumulative effect is that aged stem cells divide less frequently and are more prone to errors when they do. Tissues that depend on constant replenishment, such as the blood, gut lining, and skin, show measurable declines in regenerative capacity with age.
This isn’t simply a matter of running out of cells. Aged stem cells often enter a state of quiescence, remaining alive but not dividing, or they accumulate enough damage to trigger a permanent growth arrest called senescence. Senescent cells don’t divide at all and instead secrete inflammatory signals that can impair the function of their neighbors. The slowing of mitosis in aging tissues is therefore both a cause and a consequence of tissue decline: less division means less repair, and less repair means more accumulated damage, which further slows division.
Evolutionary Roots of the Mitotic Machinery
The molecular machinery that drives mitosis is extraordinarily ancient. The protein tubulin, which forms the microtubule fibers of the mitotic spindle, is related to a protein called FtsZ that bacteria and archaea use to divide. Phylogenetic analysis of an FtsZ protein from the archaeon Halobacterium salinarium found it to be more closely related to eukaryotic tubulin than the FtsZ proteins of other bacteria, supporting the idea that the spindle’s core component evolved from a prokaryotic cell-division protein.25PubMed Central. Isolation of an ftsZ homolog from the archaebacterium Halobacterium salinarium: implications for the evolution of FtsZ and tubulin In other words, the basic principle of using protein filaments to segregate genetic material during cell division predates the evolution of eukaryotes themselves. Mitosis as we recognize it, with condensed chromosomes, a spindle, and the structured phases of division, evolved later, but it was built on components that have been in the toolkit of life for billions of years.
When Cells Skip Mitosis Entirely
Not every cell in a multicellular organism divides by standard mitosis, and some deliberately bypass it. In a process called endoreplication, cells replicate their DNA but skip the mitotic phase entirely, resulting in a single cell with multiple copies of its genome.26Trends in Plant Science. Molecular control and function of endoreplication in development and physiology This strategy is especially common in plants, where endoreplicated cells can grow to enormous sizes without dividing. The giant cells in a tomato fruit or the specialized nutrient-transfer cells in seeds use endoreplication to increase their volume and metabolic output far beyond what a normal-sized cell could achieve.
Animals use endoreplication too, though less widely. Human megakaryocytes, the bone marrow cells that produce platelets, replicate their DNA multiple times without dividing, accumulating enough genetic material and cytoplasm to fragment into thousands of platelets at once. Liver hepatocytes in some species also undergo endoreplication, which may serve as a buffer against DNA damage: with extra copies of every gene, the cell can tolerate mutations that would be lethal in a cell with only two copies. Endoreplication highlights that while mitosis is the default path for growth and repair, evolution has found situations where skipping it entirely gives a cell exactly the properties the tissue needs.