Cancer is, at its core, a disease of uncontrolled cell division. Every tumor begins when cells lose the ability to stop dividing on cue, and mitosis is the final act of that division process. Mutations in the genes that regulate when and how cells enter mitosis allow cancer cells to keep multiplying when they should have stopped, paused for repairs, or died. But the relationship runs deeper than simple overproduction of cells. Errors during mitosis itself generate the chromosome-level chaos that makes tumors more aggressive, harder to treat, and capable of evolving resistance to drugs.
Why Normal Cells Do Not Just Keep Dividing
Before a cell splits in two, it passes through a tightly regulated cycle of growth, DNA copying, and preparation. Mitosis is the phase where duplicated chromosomes are physically separated into two daughter cells. In healthy tissue, the decision to enter this cycle is governed by a network of proteins that act as accelerators and brakes. Growth signals push a cell forward; tumor suppressor proteins pull it back if something is wrong.
The key gatekeeping step happens before mitosis even begins. A group of proteins including p16, cyclin D1, and the retinoblastoma protein (RB) control whether a cell commits to dividing. Damage to the genes encoding these proteins is so common in human cancers that researchers consider the disruption of this pathway nearly universal in tumor development.1PubMed. Cancer cell cycles The mutations do not always make cells divide faster in an absolute sense. Rather, they compromise the cell’s ability to exit the division cycle when it should, locking it into a loop of replication.2PubMed Central. Cell cycle control in cancer
Layered on top of this are growth-signaling networks outside the cell. Mutations that hyperactivate these signals, combined with loss of tumor suppressor function, create a one-two punch: the gas pedal is stuck and the brakes are cut.3PubMed. The cell cycle and cancer The result is not just extra mitoses but mitoses that proceed despite DNA damage, chromosomal abnormalities, or signals from neighboring cells telling the cell to stop.
Checkpoints That Fail
Cells have built-in quality checks before and during mitosis. One of the most important is the spindle assembly checkpoint, which prevents chromosomes from being pulled apart until every single one is properly attached to the molecular machinery that separates them. Think of it as a safety latch: if even one chromosome is not lined up correctly, the latch stays engaged and mitosis pauses. In many cancers, this checkpoint is weakened or broken. When it fails, chromosomes get distributed unevenly between daughter cells, a condition called aneuploidy.4PubMed Central. An overview of the spindle assembly checkpoint status in oral cancer
Certain viruses exploit this vulnerability directly. The HPV E6 protein, for instance, disrupts spindle assembly checkpoint signaling so that infected cells exit mitosis prematurely even when chromosomes are misaligned.5PubMed Central. Spindle assembly checkpoint signaling and sister chromatid cohesion are disrupted by HPV E6-mediated transformation The downstream consequence is the same: cells end up with the wrong number of chromosomes, which can accelerate tumor progression.
The tumor suppressor p53, often called the “guardian of the genome,” operates its own mitotic safeguard. When DNA is damaged or telomeres are critically short, p53 arrests the cell before it enters mitosis by lowering levels of cyclin B1, a protein the cell absolutely needs to begin dividing.6PubMed Central. Cell death during crisis is mediated by mitotic telomere deprotection Cells that lack functional p53 skip this safeguard and enter mitosis with uncapped telomeres and unrepaired breaks, which is a recipe for genetic scrambling.7Current Biology. A p53-Dependent Response to Telomere Loss Ensures Proper Mitotic Entry Since p53 is mutated in roughly half of all human cancers, this particular brake failure is extremely widespread.
Chromosome Chaos and Why It Matters
Abnormal chromosome numbers are not just a side effect of cancer. They actively drive the disease forward. Most solid tumors are aneuploid, and many keep mis-segregating chromosomes with every new division in a process researchers call chromosomal instability, or CIN. Higher rates of CIN correlate with worse outcomes for patients, suggesting that the ongoing scrambling of genetic material makes tumors harder to control.8PubMed Central. Mechanisms of chromosomal instability
The reason is evolutionary. A tumor with CIN is not one clone of identical cells. It is a diverse population of cells with different chromosome combinations, and that diversity gives the tumor raw material for adaptation. When the immune system attacks, or when chemotherapy kills the most vulnerable cells, the surviving oddball clones with favorable chromosome arrangements can repopulate the tumor. CIN is a key driver of therapy resistance and metastasis precisely because it fuels this kind of rapid natural selection within a tumor.9Journal of Cell Science. Characterizing chromosomal instability-driven cancer evolution and cell fitness at a glance Some tumors even undergo whole-genome doubling events that provide a fast track toward favorable chromosome arrangements, accelerating this evolutionary process.10PubMed Central. Chromosomal Instability as a Driver of Tumor Heterogeneity and Evolution
The idea that abnormal mitosis causes cancer is not new. Over a century ago, the biologist Theodor Boveri proposed that faulty chromosome distribution during cell division could trigger tumor growth, based on his experiments with sea urchin eggs. He even suggested that normal cells carry an inhibitory mechanism that prevents division until the right stimulus arrives.11Trends in Cell Biology. The significance of Boveri’s work on mitosis and cancer Modern cancer genetics has largely confirmed his intuition.12Nature Reviews Molecular Cell Biology. Boveri revisited: chromosomal instability, aneuploidy and tumorigenesis
Extra Centrosomes and Multipolar Spindles
Centrosomes are the organelles that organize the spindle fibers pulling chromosomes apart during mitosis. A normal cell has two, producing a neat bipolar spindle. Many cancer cells accumulate extra centrosomes, which should theoretically produce a multipolar spindle and tear the cell apart into three or more fragments, killing it. Yet cancer cells frequently survive this situation through a trick called centrosome clustering: they bundle the extra centrosomes together into two functional poles so that division can proceed.13PubMed. Spindle multipolarity is prevented by centrosomal clustering
This clustering process is itself a potential drug target. A genome-wide screening approach identified several proteins, including a motor protein called HSET, that are essential for clustering extra centrosomes. HSET is normally dispensable in healthy cells with two centrosomes, but cancer cells with extra centrosomes depend on it to survive mitosis.14PubMed Central. Mechanisms to suppress multipolar divisions in cancer cells with extra centrosomes Recent work has identified additional regulators of this pathway, and blocking them forces cancer cells into multipolar division that leads to cell death.15Oncogene. LIMK2 promotes centrosome clustering and cancer progression by activating MST4-mediated phosphorylation of NPM1 The appeal of this strategy is its selectivity: healthy cells with normal centrosome numbers would be unaffected.
How Cancer Drugs Exploit Mitosis
Many of the oldest and most widely used chemotherapy drugs work by sabotaging mitosis. The two main classes, taxanes and vinca alkaloids, both target the microtubule fibers that form the spindle. Taxanes freeze microtubules in place so they cannot shorten; vinca alkaloids prevent them from assembling properly. Either way, the spindle cannot function, and the cell gets stuck in mitotic arrest and typically dies.16Seminars in Oncology. Exploring the Mechanisms of Action of the Novel Microtubule Inhibitor Vinflunine – Section: Microtubule Inhibitors (MTIs)
Beyond these classic drugs, researchers are developing inhibitors that target the kinase enzymes cancer cells need for proper mitotic progression. Aurora kinase A and Polo-like kinase 1 (PLK1) are two such enzymes that are overexpressed in many aggressive tumors. Inhibiting both simultaneously can push cancer cells into mitotic catastrophe, a form of cell death triggered by severe mitotic failure, while sparing normal cells that do not rely as heavily on these enzymes.17PubMed Central. Co-inhibition of polo-like kinase 1 and Aurora kinases promotes mitotic catastrophe In pediatric brain tumors called diffuse midline gliomas, Aurora kinase A knockout specifically killed tumor cells but not other brain tumor cell types, suggesting a cancer-specific vulnerability.18iScience. Combined inhibition of Aurora kinase A and Polo-like kinase 1 is a therapeutic strategy for diffuse midline glioma
Mitotic Catastrophe as a Safety Net
The body has a built-in defense against cells that attempt mitosis with severe problems. When a cell’s division goes badly enough wrong, it triggers a response called mitotic catastrophe, which drives the cell toward either death or permanent growth arrest. The International Nomenclature Committee on Cell Death recognizes mitotic catastrophe as a genuine tumor-suppressive mechanism.19PubMed Central. Targeting the Mitotic Catastrophe Signaling Pathway in Cancer
Under the microscope, cells undergoing mitotic catastrophe are distinctive: they become abnormally large and contain multiple nuclei rather than one, reflecting a failed attempt to divide.20PubMed. Mechanisms of drug-induced mitotic catastrophe in cancer cells Several chemotherapy drugs and radiation treatments deliberately trigger this process. For example, tubulin-disrupting compounds can cause cancer cells to arrest in the G2/M phase, develop mitotic catastrophe features within about a day, and then die by non-apoptotic mechanisms.21PubMed Central. A New Inhibitor of Tubulin Polymerization Kills Multiple Cancer Cell Types and Reveals p21-Mediated Mechanism Determining Cell Death after Mitotic Catastrophe The challenge is that cancer cells with disabled p53 or other survival tricks sometimes find ways to escape mitotic catastrophe and keep going.
Mitotic Slippage and Drug Resistance
This escape route has a name: mitotic slippage. When cancer cells are held in mitotic arrest by a drug, some manage to exit mitosis without ever properly dividing their chromosomes. They slip back into interphase as a single oversized cell with double the normal DNA content. These polyploid cells can survive, and some go on to proliferate again, fueling resistance to treatment.22PubMed Central. Mitotic slippage: an old tale with a new twist
The mechanics involve a slow leak of cyclin B1, the protein that maintains the mitotic state. Even while the spindle assembly checkpoint is screaming “do not proceed,” cyclin B1 gradually degrades until the cell falls out of mitosis.23Molecular Oncology. Mitotic arrest and slippage induced by pharmacological inhibition of Polo-like kinase 1 In laboratory experiments with liver cancer cells treated with paclitaxel, slippage became the dominant outcome at lower doses: after about four days, more than half of treated cells had become polyploid rather than dying. Over a week, these polyploid cells became the predominant surviving population.24PLOS ONE. Cell Division Cycle 6 Promotes Mitotic Slippage and Contributes to Drug Resistance in Paclitaxel-Treated Cancer Cells Drug resistance from slippage also complicates the use of newer microtubule-targeting agents. Some breast cancer cells resistant to taxanes show cross-resistance to vinca alkaloids but, intriguingly, become more sensitive to a different class of microtubule disruptors that bind at the colchicine site.25PLOS ONE. Sensitivity of docetaxel-resistant MCF-7 breast cancer cells to microtubule-destabilizing agents including vinca alkaloids and colchicine-site binding agents Findings like these suggest that combining drugs with different mitotic targets might outflank slippage-based resistance.
Counting Dividing Cells to Grade Tumors
The connection between mitosis and cancer is not just academic; pathologists use it every day to diagnose and stage tumors. The mitotic index, a count of how many cells in a tissue sample are actively dividing, is one of the oldest and most reliable markers in cancer pathology. In breast cancer, a high mitotic index predicts increased risk of death and recurrence regardless of tumor size or whether cancer has reached the lymph nodes.26British Journal of Cancer. Prognostic impact of discrepant Ki67 and mitotic index on hormone receptor-positive, HER2-negative breast carcinoma
A related tool is Ki67 staining, which labels a protein present in cells throughout the active division cycle but absent in resting cells. In certain tumor types, especially neuroendocrine tumors, the Ki67 proliferative index is mandatory for diagnosis and directly determines how the tumor is graded and treated.27PubMed Central. Diagnostic, Prognostic, and Predictive Role of Ki67 Proliferative Index in Neuroendocrine and Endocrine Neoplasms: Past, Present, and Future Both tools are essentially measuring the same thing from different angles: how aggressively a tumor’s cells are entering and completing mitosis.
The Immune System Watches for Mitotic Mistakes
One of the more surprising recent discoveries is that the immune system can detect cells that have fumbled mitosis. When chromosomes fail to separate cleanly, they sometimes form thin bridges of stretched chromatin connecting the two daughter cells, or get left behind as small membrane-bound fragments called micronuclei. Research has shown that stretched chromatin bridges activate the cGAS-STING signaling pathway, which is the same innate immune sensor that normally detects foreign DNA from viruses. Once triggered, this pathway initiates inflammatory signaling that can alert the immune system to the presence of defective cells.28PubMed Central. Chromatin bridges, not micronuclei, activate cGAS after drug-induced mitotic errors in human cells
This connection has implications for how cancer drugs work. Taxanes and other mitosis-disrupting agents do not just directly kill tumor cells; they may also make them visible to the immune system by generating the chromatin bridges and micronuclei that trip cGAS-STING signaling. Separately, researchers found that loss of certain quality-control proteins causes aberrant mitosis and chromosomal instability that activates this same immune-sensing pathway, suggesting the link between mitotic failure and immune detection is a fundamental surveillance mechanism.29Nature Communications. DNAJA2 deficiency activates cGAS-STING pathway via the induction of aberrant mitosis and chromosome instability
DNA Repair That Happens During Mitosis Itself
For a long time, biologists assumed that DNA repair was essentially shut off during mitosis, since most repair pathways are indeed suppressed while chromosomes are condensed. But it turns out that cells have an emergency backup called mitotic DNA synthesis, or MiDAS, which completes the replication of DNA regions that were not finished before the cell entered mitosis. These unfinished spots tend to occur at fragile sites in the genome, stretches of DNA that are particularly hard to copy.
In normal human cells, this last-minute synthesis depends on a protein called FANCD2 and protects fragile-site stability.30PubMed Central. Mitotic DNA synthesis in untransformed human cells preserves common fragile site stability via a FANCD2-driven mechanism that requires HELQ Cancer cells, however, use a different version of MiDAS that depends on the RAD52 protein.31Molecular Cell. RAD52 Facilitates Mitotic DNA Synthesis Following Replication Stress The cancer-cell version appears to prioritize preventing chromosomes from breaking apart during segregation, even at the expense of fragile-site integrity. When this emergency synthesis fails, the consequences are severe: chromosome instability and missegregation during mitosis.32PubMed Central. Folate stress induces SLX1- and RAD51-dependent mitotic DNA synthesis at the fragile X locus in human cells This difference between the normal and cancer-cell versions of MiDAS is another potential vulnerability that researchers are beginning to explore.
Cancer Stem Cells and Asymmetric Division
Not all cells within a tumor divide the same way. A subpopulation known as cancer stem cells can perform asymmetric division: one daughter cell keeps the stem-like ability to keep dividing indefinitely, while the other differentiates into a non-stem tumor cell.33PubMed Central. Molecular mechanisms controlling asymmetric and symmetric self-renewal of cancer stem cells This mirrors how normal tissue stem cells maintain organs, but in a tumor it means the stem-cell pool is never exhausted. Even if a drug wipes out the bulk of the tumor’s dividing cells, the cancer stem cells that divided asymmetrically may have shielded their most dangerous daughter from harm. In glioblastoma, the most lethal brain cancer, cancer stem cells shift between symmetric and asymmetric division modes in response to therapy, and their ability to do so contributes to treatment resistance.34PubMed Central. Asymmetric Division Promotes Therapeutic Resistance in Glioblastoma Stem Cells
Physical Squeezing Can Cause Mitotic Errors
Tumors grow in crowded tissue environments, and the physical forces cells experience can directly interfere with mitosis. When cancer cells migrate through tight spaces within tissue, the mechanical compression distorts the mitotic spindle. Tumor cells dividing under confinement frequently showed altered spindle organization, chromosome missegregation, and formation of micronuclei, even though their DNA-damage checkpoints were fully functional.35PubMed Central. Mechanical stress during confined migration causes aberrant mitoses and c-MYC amplification Perhaps most strikingly, this mechanical stress led to amplification of the c-MYC oncogene, one of the most potent drivers of cancer growth. The implication is that a tumor’s physical environment does not just passively house cancer cells; it actively generates new genetic changes by disrupting mitosis.
Experiments confining individual cancer cells in tiny tubes confirmed that even moderate compression changes the shape and dynamics of the mitotic spindle and the arrangement of chromosomes.36PubMed Central. Molecular Insights into Division of Single Human Cancer Cells in On-Chip Transparent Microtubes Compression to roughly 60% of a cell’s normal height prolonged the early stages of mitosis and increased chromosome loss, and these losses persisted in the descendants of the compressed cells.37Cell Reports. Confinement-driven chromosome loss and its mitotic memory in cancer This “mitotic memory” of mechanical stress adds another layer to how tumors accumulate genetic diversity without any new mutations in cell-cycle genes.
Metabolism and the Energy Demands of Mitotic Cancer Cells
Mitosis is energetically expensive. A cell undergoing division must reorganize its entire interior, move chromosomes along spindle fibers, and physically pinch itself in two, all of which require large amounts of energy. Cancer cells, which already reprogram their metabolism to sustain rapid proliferation, face a particular challenge coordinating energy production with the demands of mitosis.38PubMed Central. Energy restriction causes metaphase delay and chromosome mis-segregation in cancer cells
One metabolic adaptation recently uncovered involves a burst of the antioxidant molecule NADPH during mitotic entry. Aneuploid cancer cells, which carry high levels of damaging reactive oxygen species, depend on this NADPH surge to protect the kinase enzymes that drive chromosome segregation. Without it, reactive oxygen species inactivate those kinases and chromosomes mis-segregate. The enzyme responsible, G6PD, is activated by a specific phosphorylation event that occurs only as cells enter mitosis, making it a potentially targetable vulnerability unique to aneuploid tumors.39Nature Metabolism. A mitotic NADPH upsurge promotes chromosome segregation and tumour progression in aneuploid cancer cells This finding ties together two hallmarks of cancer, metabolic reprogramming and chromosomal instability, into a single mechanism that operates right at the moment of cell division.