Mitosis is a single division. One cell splits into two genetically identical daughter cells, and that is the entire event. The confusion usually arises because people mix up mitosis with meiosis, which involves two rounds of division, or because the phases within mitosis (prophase, metaphase, anaphase, telophase) look like separate events when they are really stages of one continuous process. Despite its apparent simplicity, that single division has more going on beneath the surface than the textbook version suggests.
What Happens in That One Division
Mitosis is specifically the division of the nucleus. A cell copies its entire set of chromosomes during an earlier phase called S phase, and then mitosis parcels one complete copy to each side of the cell. The process flows through a series of stages: chromosomes condense and become visible, they line up at the cell’s midline, they split apart, and then two new nuclei form around each set. Researchers tracking the process with fluorescent probes use landmarks like the release of condensed chromosomes into the cytoplasm, their alignment at the spindle equator, and the moment sister chromatids pull apart to measure how long each stage lasts and whether anything has gone wrong.1PubMed Central. Live Cell Fluorescence Imaging for Phenotypic Analysis of Mitosis All of these events, from the first chromosome condensation to the formation of two nuclei, constitute that one mitotic division.
After the nucleus has split, the cell typically goes through cytokinesis, where the cytoplasm pinches apart to produce two physically separate cells. This is technically a distinct step from mitosis itself. The distinction matters because the two processes do not always happen together.
When Nuclear Division and Cell Division Part Ways
In most animal cells, cytokinesis follows mitosis so quickly that the two seem inseparable. But biology has plenty of situations where a nucleus divides without the cell cleaving in two, or where multiple rounds of nuclear division happen inside one shared cytoplasm. The result is a cell with more than one nucleus, and this is not a mistake.
Multinucleated cells and syncytia, where many nuclei share a common cytoplasm, play important roles in normal development and tissue maintenance.2PubMed Central. Nuclear movement in multinucleated cells Skeletal muscle fibers are a classic example: they form when individual muscle cells fuse together, creating long fibers with dozens or even hundreds of nuclei. The placenta contains a massive syncytium that helps transfer nutrients from mother to fetus. In these cases, mitosis happened, sometimes many times, but cytokinesis was incomplete or absent. Even in single-celled organisms, nuclear division without cytokinesis has been documented, as observed in the protozoan Paranotila lata, where the nucleus divides and the resulting pronuclei fuse again.3PubMed. Nuclear division without cytokinesis followed by fusion of pronuclei in Paranotila lata gen. et sp. nov.
So while the answer to “how many divisions occur in mitosis” is always one nuclear division, whether that nuclear division leads to two separate cells depends on whether cytokinesis follows through.
Why People Confuse Mitosis with Meiosis
The most common reason people wonder about the number of divisions in mitosis is that meiosis involves two. In meiosis, a cell goes through two consecutive rounds of division to produce four cells, each with half the original chromosome count. These are the divisions that create eggs and sperm. Mitosis, by contrast, produces two cells that are chromosomally identical to the parent. The fact that both processes share similar-looking stages (both have a metaphase, both have an anaphase) feeds the confusion. But the core difference is arithmetic: mitosis gives you two from one, meiosis gives you four from one.
The evolutionary relationship between these two processes is itself a subject of research. The two-division structure of meiosis is thought to have evolved from the single-division mitotic framework, with extra regulatory steps layered on to achieve chromosome reduction.4PubMed Central. The evolution of meiosis from mitosis In other words, meiosis is the elaboration; mitosis is the original template.
Endomitosis and Endoreplication
There is a twist that complicates the clean “one division, two cells” story. Some cells enter mitosis but deliberately skip the final step, ending up with extra copies of their genome inside a single cell. This process is called endomitosis, and it is not a malfunction. It is a programmed part of how certain cell types mature.
Megakaryocytes, the giant bone marrow cells that produce blood platelets, are the most striking example. They go through endomitosis repeatedly, entering and exiting mitosis without completing cytokinesis, and can accumulate huge amounts of DNA (sometimes 64 or 128 copies of the genome) before shedding platelets from their massive cytoplasm.5PubMed Central. Endomitosis: a new cell fate in the cell cycle leading to polyploidy in megakaryocytes and hepatocytes Liver cells (hepatocytes) also undergo endomitosis during normal development, which is why a healthy adult liver contains a substantial fraction of cells with more than the standard two sets of chromosomes.
A related process, endoreplication, skips mitosis entirely: the cell copies its DNA over and over without ever entering the division phase. Both endoreplication and endomitosis produce polyploid cells containing more than two chromosome sets, but through different routes. Endomitosis involves entering and then aborting mitosis, while endoreplication just repeats the DNA-copying phase without attempting to divide at all.6Utrecht University. Regulation and function of developmentally controlled polyploidization For the reader keeping count, mitosis itself still equals one division event in these cells. They just run through it multiple times, or in the case of endoreplication, skip it.
The Molecular Trigger That Starts Mitosis
A cell does not stumble into mitosis by accident. Entry is controlled by a molecular switch that has been conserved across nearly all eukaryotic life. The key player is a protein called CDK1 (cyclin-dependent kinase 1), originally discovered in yeast and later found to serve the same function in human cells. CDK1 partners with proteins called cyclins, particularly cyclin B, and when this complex becomes active it flips the cell from its growth-and-preparation phase into the dramatic reorganization of mitosis.7PubMed Central. Cyclin-dependent protein kinases and cell cycle regulation in biology and disease
The activation of CDK1 is not a gradual ramp-up. It works more like a switch: CDK1 activity rises, and once it passes a threshold it triggers a feedback loop that amplifies its own activation while shutting down the enzymes that had been keeping it in check.8PubMed. CDK1-Cyclin B mediated phosphorylation of PP2A-B56 on B56Par1.T73T75 controls mitotic entry timing and Schizosaccharomyces pombe cell size The cyclin partner itself also influences how quickly and in what order CDK1 modifies its target proteins, which helps establish the orderly timing of mitotic events.9PubMed Central. Phosphate-binding pocket on cyclin B governs CDK substrate phosphorylation and mitotic timing This is why the single division of mitosis unfolds in a predictable sequence rather than all at once: the molecular machinery orchestrates each stage in order.
The Built-In Error Checker
Because distributing chromosomes correctly is so important, cells have a surveillance system called the spindle assembly checkpoint. This checkpoint prevents the cell from proceeding to the chromosome-separation step (anaphase) until every single chromosome is properly attached to the spindle fibers that will pull the copies apart.10eLife. Spindle assembly checkpoint-dependent mitotic delay is required for cell division in absence of centrosomes If even one chromosome pair has not achieved the correct two-sided attachment, the checkpoint keeps the brakes on.11PubMed Central. The chromosomal passenger complex and the spindle assembly checkpoint: kinetochore-microtubule error correction and beyond
This is the cell’s way of ensuring that one division produces two cells with exactly the right number of chromosomes. When the checkpoint fails, as it sometimes does in cancer cells, the consequences range from cells that end up with too many or too few chromosomes to cells that die outright from the imbalance.
What Goes Wrong in Cancer Cells
Cancer cells frequently have extra copies of the centrosome, the organelle that helps organize the spindle during division. With more than two centrosomes, the spindle can form with three or more poles instead of the usual two, creating what is called a multipolar division. If the cell actually went through with it, the chromosomes would be split among three or more clusters, almost guaranteeing that each daughter cell would receive the wrong number. Most cells that attempt this die.12PubMed Central. Mechanisms to suppress multipolar divisions in cancer cells with extra centrosomes
Tumor cells get around this problem by clustering their extra centrosomes together so that the spindle still forms two poles and the division looks mostly normal. This centrosome-clustering trick is one reason cancer cells can survive with abnormal centrosome numbers while still accumulating chromosome errors at a low enough rate to stay viable. It is also a potential therapeutic target: if you could block centrosome clustering, cancer cells with extra centrosomes would be forced into lethal multipolar divisions while normal cells (which have just two centrosomes) would be unaffected.
Some cancer drugs already exploit mitosis from a different angle. Microtubule-targeting compounds like taxol interfere with the spindle fibers themselves, preventing proper chromosome attachment and trapping the cell in mitotic arrest.13Blood. Pharicin a, a Novel Natural Compound That Induces Mitotic Arrest and Catastrophe of Cancer Cells by Perturbing Microtubule Dynamics and the Spindle Checkpoint A cell stuck in mitosis eventually either dies or exits in a damaged state. Because cancer cells divide more frequently than most healthy cells, they are more likely to encounter a mitosis-blocking drug at the vulnerable moment.
Open Versus Closed Mitosis
Not all organisms handle that single division the same way at the structural level. In human cells, the nuclear envelope, the membrane surrounding the nucleus, breaks down completely at the start of mitosis. The chromosomes are released into the open cytoplasm where the spindle can grab them. This is called open mitosis.
In some yeasts and other single-celled organisms, the nuclear envelope stays intact throughout mitosis. The spindle forms inside the nucleus, and the chromosomes are separated without the nuclear contents ever mixing with the cytoplasm. This is closed mitosis.14Current Biology. Deciphering the Evolutionary History of Open and Closed Mitosis There are also intermediate forms where the envelope partially breaks down or stretches without fully disintegrating. The evolutionary history of which form came first remains a subject of debate, but the key point for the reader is that the number of divisions does not change: open or closed, it is still one division producing two nuclei.
Symmetric and Asymmetric Division
When we say mitosis produces two identical daughter cells, that is only half the story. In many contexts, a single mitotic division intentionally produces two cells with different fates. Stem cells do this routinely: one daughter remains a stem cell, while the other begins to specialize. The mechanisms that create this asymmetry are linked to how the mitotic machinery is oriented within the cell and how certain proteins or other molecules are distributed unevenly before the cell splits.15PubMed Central. Cellular and molecular mechanisms of asymmetric stem cell division in tissue homeostasis
In the fruit fly nervous system, for instance, neural stem cells divide asymmetrically to generate one large self-renewing stem cell and one smaller sibling that goes on to differentiate into a neuron or supporting cell.16bioRxiv. Asymmetric nuclear division of neural stem cells contributes to the formation of sibling nuclei with different identities Genetically, the two daughter cells carry the same DNA. But the unequal distribution of signaling molecules means they read different parts of that DNA and become different cell types. The number of nuclear divisions is still one. What changes is the outcome.
Asymmetric division is critical for tissue maintenance in adults too. Your gut lining, your skin, and your blood-forming system all rely on stem cells that balance self-renewal with differentiation through asymmetric mitotic divisions. If every stem cell division were perfectly symmetric, you would either exhaust your stem cell pool or end up with a mass of undifferentiated cells, neither of which is compatible with a healthy tissue.
Telomeres and the Limits of Repeated Division
Each time a cell goes through mitosis, a small amount of DNA is lost from the ends of its chromosomes. These chromosome tips, called telomeres, act as protective caps. They do not contain genes; they are repetitive sequences whose job is to be expendable. But after many rounds of division, telomeres become critically short, and the cell receives a signal to stop dividing. This is one of the fundamental limits on how many times a normal cell can go through mitosis over its lifetime.
Progressive telomere shortening, along with declining activity of the enzyme that rebuilds telomeres, contributes to cellular aging, tissue deterioration, and the onset of age-related diseases.17PubMed Central. Targeting telomere dynamics with plant-derived compounds: Molecular strategies against aging Most human cells can divide somewhere in the range of 40 to 60 times before telomere shortening triggers a permanent exit from the cell cycle. Cancer cells often get around this limit by reactivating the telomere-rebuilding enzyme, which is one reason they can divide indefinitely.
The connection to the original question is this: each of those 40 to 60 lifetime divisions is a single mitotic event. The limit is not on how many divisions happen within mitosis (that number is always one) but on how many times a cell can repeat the whole mitotic process before its telomeres run out.
How Long One Mitotic Division Takes
In a typical human cell growing in culture, mitosis itself lasts roughly 30 minutes to an hour. The rest of the cell cycle, where the cell grows, copies its DNA, and prepares for division, takes far longer, often 12 to 24 hours or more depending on the cell type. DNA replication during S phase is one of the major factors controlling overall cell cycle length. Experiments in which replication was shortened by manipulating the licensing machinery showed that cells could re-enter mitosis as early as six to eight hours after the previous division, cutting the cycle by about a quarter, which highlights how much time the preparation phase normally takes relative to mitosis itself.18Molecular Cell. DNA Replication Determines Timing of Mitosis by Restricting CDK1 and PLK1 Activation
In a living organism, the timing varies enormously. Early embryonic cells in a frog embryo can complete the entire cycle in about 25 minutes, with mitosis accounting for a large share. Adult liver cells, by contrast, might sit in a resting phase for months or years before dividing at all. The speed of mitosis is tuned to the cell’s needs, but the fundamental pattern, one round of chromosome separation producing two nuclei, is the same regardless of whether it takes 15 minutes or an hour.