Mitosis is the division of the nucleus, the process by which a cell duplicates its chromosomes and sorts them into two identical sets. Cytokinesis is the physical splitting of the cell itself, the step that pinches or walls off the cytoplasm to produce two separate daughter cells. They are distinct processes with different molecular machinery, but they happen in close succession and are tightly coordinated, which is why they are so often confused with each other. The distinction matters because one can actually occur without the other, and when that happens, the consequences for the cell range from normal physiology to cancer.
What Mitosis Handles
Mitosis is entirely about the chromosomes. Before it begins, the cell has already copied its DNA during a preparatory phase. Mitosis is the process of sorting those copies so each future daughter cell will get a complete genome. To do this, the cell assembles a structure called the mitotic spindle, built from protein filaments called microtubules. These microtubules attach to the chromosomes and generate the tiny forces needed to pull them apart, first lining them up along the cell’s midplane and then dragging one copy of each chromosome toward each end of the cell.1PubMed Central. Mitotic force generators and chromosome segregation
In most animal cells and many other organisms, mitosis also involves a dramatic demolition and rebuilding project for the nuclear envelope, the membrane that normally surrounds the nucleus. This membrane is completely disassembled at the start of mitosis so the spindle can access the chromosomes, and then it is reassembled around each set of separated chromosomes at the end.2PubMed Central. Nuclear envelope remodelling during mitosis This style, sometimes called “open” mitosis, is the most radical solution to the problem of dividing a nucleus, and it is the version you go through every time one of your cells divides.3PubMed. Orchestrating nuclear envelope disassembly and reassembly during mitosis
By the end of mitosis, the cell has two distinct nuclei sitting at opposite ends, each enclosed in a freshly built nuclear membrane. The cell’s cytoplasm, though, has not been divided yet. It is still one continuous space, shared by two nuclei. That is where cytokinesis comes in.
What Cytokinesis Handles
Cytokinesis splits everything else. Once the chromosomes are safely sorted, the cell needs to divide its cytoplasm, its organelles, its membranes, and its internal fluids into two separate packages. In animal cells, this happens through a contractile ring, a belt of protein filaments (primarily actin and myosin, the same proteins that let your muscles contract) that assembles just beneath the cell surface at the cell’s equator. This ring tightens like a drawstring, pinching the membrane inward in what is called a cleavage furrow, until the cell is squeezed into two.4PubMed Central. The contractile ring
The organelles that end up in each daughter cell are not always divided with surgical precision. Some organelles, like mitochondria, are distributed somewhat randomly between the two halves as the cell pinches apart.5PubMed. Random partitioning of cytoplasmic organelles at cell division: the effect of organelle and cell volume This is generally fine because a healthy cell has enough of each type to ensure both daughters get a workable share.
The key conceptual difference is scope. Mitosis is about information: making sure each daughter gets a perfect copy of the genome. Cytokinesis is about physical separation: creating two independent cells that can each function on their own. Mitosis uses a spindle made of microtubules. Cytokinesis in animal cells uses a contractile ring made of actin and myosin. They rely on entirely different structural machinery.
How the Two Processes Are Coordinated in Time
In a typical animal cell, cytokinesis begins during the late stages of mitosis, not after mitosis is completely finished. The cleavage furrow starts to form while the chromosomes are still being pulled apart, and it deepens as the final steps of mitosis wrap up. The two processes overlap, which is a big part of why people think of them as a single event.
The spindle itself plays a role in telling the cell where to divide. Research in zebrafish embryos has shown that microtubules between the two separating chromosome sets transmit positional signals to the cell surface, essentially telling the cortex exactly where to assemble the contractile ring.6PubMed. Characterization of mid-spindle microtubules during furrow positioning in early cleavage period zebrafish embryos This makes biological sense: the cell needs the furrow to form precisely between the two chromosome sets, not off to one side, or one daughter would get both copies and the other would get none.
So even though mitosis and cytokinesis use different machinery, the mitotic spindle acts as a signaling hub that triggers and positions cytokinesis. The spindle does double duty: it pulls the chromosomes apart and then, through its remnants, tells the cell surface where to cut. This is why the two processes look seamless from the outside even though they are mechanistically separate.
Plants Do Cytokinesis Differently
The contractile ring mechanism described above is an animal cell strategy. Plant cells are encased in a rigid cell wall, so they cannot simply pinch themselves in two. Instead, plant cells build a new wall from the inside out. After mitosis, a structure called the phragmoplast forms at the cell’s midplane. It directs small vesicles, essentially little membrane-bound packages carrying wall-building materials, to fuse together at the center of the cell and spread outward until they reach the existing cell walls on either side.7PubMed. Class II kinesin-12 facilitates cell plate formation by transporting cell plate materials in the phragmoplast The result is a new dividing wall, called the cell plate, that partitions the cytoplasm into two compartments.
Fungi take yet another approach. In budding yeast, for instance, the cell builds a septum, a partition wall whose construction depends heavily on the synthesis and deposition of chitin, a tough polysaccharide that also gives insect exoskeletons their rigidity.8PubMed. Monitoring Chitin Deposition During Septum Assembly in Budding Yeast The details of septum assembly differ from plant cell plate formation, but the underlying principle is the same: a physical barrier is built to divide the cytoplasm.
What stays constant across animals, plants, and fungi is the mitotic spindle. The chromosome-sorting machinery is remarkably conserved. It is the cytokinesis step that has evolved different solutions depending on the physical constraints of the cell type. This is a vivid illustration of why the two processes are conceptually distinct: you can pair the same chromosome-segregation system with fundamentally different cell-division strategies.
When Cytokinesis Fails
Because mitosis and cytokinesis are separate processes, one can succeed while the other fails. When mitosis finishes normally but cytokinesis does not complete, the result is a single cell with two nuclei, or sometimes one nucleus with double the normal chromosome count. This tetraploid state, where a cell has four copies of each chromosome instead of the usual two, is one of the recognized pathways toward cancer.
Cytokinesis failure is, in fact, listed as one of five main routes by which cells become tetraploid. While brief episodes of tetraploidy occur in some normal physiological contexts, persistent tetraploidy tends to cause chromosomal instability, meaning the cell starts gaining or losing chromosomes unpredictably during subsequent divisions. That instability generates the kind of genetic diversity within a tumor that drives it to become more aggressive and harder to treat.9PubMed. Tetraploidy in cancer: Diagnostic and therapeutic perspectives
Healthy cells have a safeguard against this. When cytokinesis fails and a cell becomes tetraploid, a tumor-suppression pathway kicks in. A protein called LATS2 is activated, which stabilizes p53, the cell’s most important emergency brake. LATS2 works by blocking the molecule that normally tags p53 for destruction, so p53 accumulates and halts the cell cycle. This prevents the tetraploid cell from continuing to divide and potentially becoming cancerous.10Cell. Cytokinesis Failure Activates Hippo Tumor Suppression Pathway The mechanism is distinct from how p53 responds to DNA damage. The cell has a separate alarm system specifically for detecting that cytokinesis went wrong.
This is one of the strongest reasons to understand mitosis and cytokinesis as independent processes. The cell itself treats them as independent checkpoints, running different surveillance programs for each one. A failure of chromosome segregation during mitosis triggers one set of alarms. A failure of physical division during cytokinesis triggers a completely different set.
Can Mitosis Happen Without Cytokinesis on Purpose?
Yes, and it does routinely. Some cells in your body deliberately skip cytokinesis after completing mitosis, and this is a normal part of their development. Megakaryocytes, the bone marrow cells that produce blood platelets, go through multiple rounds of mitosis without ever dividing their cytoplasm. The result is a single enormous cell with a highly lobed nucleus containing many copies of the genome. This large cell then sheds thousands of platelet fragments into the bloodstream. The whole strategy depends on uncoupling mitosis from cytokinesis on purpose.
Skeletal muscle cells are another example. During development, precursor cells fuse together to form long, multinucleated fibers. While the fusion mechanism is different from skipped cytokinesis, the end result is functionally similar: a single cell with many nuclei, something that only makes sense if you understand that nuclear division and cell division are not the same thing.
Certain liver cells (hepatocytes) also become binucleated or polyploid as a normal feature of aging and regeneration. These cells went through mitosis but did not finish cytokinesis, and they function perfectly well. The cell uses tetraploidy strategically in these contexts, even though it triggers alarm systems in most other cell types.
Asymmetric Division and Unequal Cytokinesis
When most people picture cell division, they imagine a cell splitting into two roughly equal halves. That is the most common scenario, but cytokinesis does not have to be symmetric. In some biologically critical situations, cytokinesis is deliberately lopsided.
The most dramatic example is egg cell (oocyte) maturation. When an oocyte divides during meiosis, which shares mitosis-like chromosome segregation, it needs to keep almost all of its cytoplasm for the future egg. The division is accomplished by migrating the spindle to one edge of the cell and then performing cytokinesis in a way that pinches off a tiny cell, called a polar body, while the large oocyte retains the vast majority of the cellular contents.11PubMed. WDR62 is a novel participator in spindle migration and asymmetric cytokinesis during mouse oocyte meiotic maturation The asymmetry depends on precise positioning of the spindle and localized changes at the cell surface. If the spindle does not migrate correctly, the division becomes more symmetric and the oocyte loses too much cytoplasm to be viable.
Stem cells in various tissues also use asymmetric division, partitioning specific proteins or signaling molecules unequally so that one daughter cell remains a stem cell while the other begins to specialize. The chromosome segregation during mitosis is symmetric in these cases (both daughters get the same genome), but cytokinesis distributes the cytoplasm unevenly. Again, this only makes sense if you think of the two processes as distinct, because they can be regulated independently.
Watching Division in Real Time
One reason the distinction between mitosis and cytokinesis remained blurry for so long is that, under a standard microscope, the whole process looks like a single fluid event. Modern imaging techniques have changed this. Researchers have developed fluorescent markers that specifically label the cytokinesis machinery, making it possible to watch chromosome segregation and cell splitting as visually separate events.
One such system uses a fluorescently tagged version of a protein called anillin, which is recruited to the contractile ring during cytokinesis. By imaging this marker in living tissue, researchers have been able to resolve the exact timing and direction of cleavage furrow formation in individual cells, even in complex tissues like the developing brain. In neural progenitor cells, for example, the cleavage furrow was observed to ingress from one side in a specific direction, and the division of the cell’s long basal process happened before the cell body itself split.12Nature Communications. Direct visualization of cell division using high-resolution imaging of M-phase of the cell cycle These are details that only become visible when you have tools that can distinguish the mitosis machinery from the cytokinesis machinery in a living cell.
This kind of imaging has reinforced what the molecular biology already told us: the two processes are mechanistically separable, proceed with their own timing, and can behave differently even in cells that are side by side in the same tissue. The cleavage furrow does not simply appear when mitosis ends and chop the cell in half. It forms in a specific place, moves in a specific direction, and progresses at a rate that is regulated independently of what the chromosomes are doing.
Why the Confusion Persists
Textbooks often present “cell division” as a single process with sub-steps, listing the phases of mitosis (prophase, metaphase, anaphase, telophase) and then tacking cytokinesis on at the end as if it were just one more phase. This framing makes it easy to think of cytokinesis as the final act of mitosis rather than a parallel process with its own molecular logic. Some older textbook diagrams even label the combined event as “M phase,” lumping mitosis and cytokinesis together under one umbrella term.
The reality is more like two overlapping but independent programs. Mitosis runs its program of condensing chromosomes, building a spindle, and separating chromosome copies. Cytokinesis runs its own program of assembling a contractile ring (or cell plate, or septum), positioning it correctly, and constricting it. The two programs communicate with each other, mainly through signals from the spindle midzone that tell cytokinesis where to happen, but they use different proteins, different structural frameworks, and different regulatory checkpoints. A cell can complete one without the other. They can be symmetric or asymmetric independently of each other. And their failure modes trigger different cellular alarm systems.
If you take one thing away from the distinction, it is that chromosome sorting and cell splitting are solved by two different molecular machines that happen to work in close coordination. That coordination is so tight in most cell divisions that it looks like a single event, but evolution has kept the two systems modular, allowing cells to combine them in creative ways depending on what the tissue or organism needs.