Cytokinesis is the physical splitting of one cell into two, and it looks different depending on the type of cell. In animal cells, you’d see the cell pinching inward at its middle, like a belt tightening around a balloon. In plant cells, there’s no pinching at all. Instead, a new wall builds outward from the center. The process begins during anaphase (when chromosomes are pulling apart) and finishes as the cell enters its resting phase between divisions.
The Pinch: How Animal Cells Split
The first visible sign of cytokinesis in an animal cell is a small pucker on the cell surface, called the cleavage furrow. If you were watching a dividing cell under a microscope, you’d see the membrane start to indent around the cell’s equator, roughly halfway between the two sets of chromosomes that have already moved to opposite ends.
What’s driving that pinch is a ring of protein filaments assembling just beneath the membrane. This ring works like a drawstring: motor proteins pull on structural filaments, causing them to slide past each other and tighten the loop. As the ring constricts, the furrow deepens, and the cell looks increasingly like a figure eight or a peanut shell. The whole process can move at several micrometers per minute, which means in a typical mammalian cell it takes roughly 10 to 20 minutes from the first visible indent to final separation.
Near the end, the two halves of the cell are connected only by a thin cytoplasmic bridge. At the center of that bridge sits a dense structure called the midbody, a bundle of leftover spindle fibers and regulatory proteins. Under an electron microscope, the midbody appears as a dark, compact mass with the narrowing membrane on either side. The bridge eventually severs in a final step called abscission, and the two daughter cells pull apart completely.
Building a Wall: How Plant Cells Divide
Plant cells can’t pinch inward because they’re enclosed in a rigid cell wall. Instead, they build a brand-new wall from the inside out. The visual result is strikingly different from animal cell division.
Starting in late anaphase, a structure called the phragmoplast forms between the two new nuclei. It’s made of overlapping protein fibers and acts like a delivery track, guiding tiny vesicles (membrane-wrapped packages of wall-building materials) toward the center of the cell. These vesicles fuse together, and under a microscope, you can see a thin, disc-shaped structure called the cell plate begin to appear at the midline.
The cell plate goes through several visible stages as it matures. It starts as a cluster of fused vesicles, then transitions into a network of connected tubes. Excess membrane is recycled away, and the structure flattens into a smoother sheet with small holes, or fenestrae, scattered across it. Those holes are where strands of the cell’s internal membrane system pass through, and many of them become plasmodesmata, the tiny channels that let neighboring plant cells communicate. The cell plate expands outward until it reaches the existing cell walls on all sides, fusing with them and completing the partition. Where an animal cell took a few minutes to pinch, a plant cell may spend 30 minutes or more constructing this new wall.
What Happens to Organelles During the Split
While the cell is dividing, all of its internal machinery needs to be shared between the two daughters. Mitochondria, the cell’s energy producers, prepare for this ahead of time. During the phases leading up to division, the cell’s mitochondrial network breaks apart into many smaller individual units that spread evenly throughout the cell. Because they’re tethered to the internal scaffolding and to the endoplasmic reticulum (a sprawling membrane network used for protein processing), they end up roughly evenly distributed on both sides of the division plane. When the cell finally splits, each daughter inherits a similar share without the cell needing an active sorting mechanism.
If this fragmentation step fails and mitochondria remain as large, interconnected networks during division, the splitting machinery can still physically sever them, but the distribution between daughter cells becomes less even. The endoplasmic reticulum, which is already spread throughout the cell, gets partitioned in a similar passive way.
Not All Divisions Are Equal
Most textbook diagrams show cytokinesis producing two identical-looking cells, but many divisions are deliberately unequal. During egg cell development, for instance, the dividing cell positions its cleavage furrow off-center, producing one large cell that keeps most of the cytoplasm and a much smaller cell called a polar body. Under a microscope, the size difference is dramatic. Stem cells in the brain also divide asymmetrically, placing specific proteins on only one side of the dividing cell so that one daughter stays a stem cell while the other goes on to become a neuron. Even in seemingly equal divisions, structures like the midbody are inherited by only one of the two daughter cells.
When Cytokinesis Doesn’t Happen
Sometimes a cell copies its DNA and pulls its chromosomes apart but never actually splits. The result is a single cell with two (or more) nuclei. This isn’t always a mistake. In the human liver, 30 to 40 percent of cells are naturally polyploid, containing double the usual amount of DNA because they went through mitosis without forming a contractile ring. Heart muscle cells do the same thing. These multinucleated or polyploid cells function normally in those tissues.
In other contexts, failed cytokinesis is a problem. A cell that doesn’t split ends up with extra copies of its centrosome (the structure that organizes chromosome separation), and that raises the risk of errors during future divisions. Cells with the wrong number of chromosomes can become cancerous, which is why cytokinesis failure in most tissues is tightly prevented by the cell’s internal checkpoints.
What Researchers See in the Lab
If you’ve searched for images or videos of cytokinesis, you’ve likely seen cells glowing green, magenta, or red. Those colors come from fluorescent proteins that scientists fuse to specific molecules involved in division. Tagging the structural filaments with a fluorescent marker, for example, lets researchers watch the contractile ring assemble and tighten in real time. Newer super-resolution microscopy techniques use photoactivatable fluorescent proteins that only light up when hit with a specific wavelength of light, allowing scientists to track individual molecules within the ring and measure how fast they move.
Under a standard light microscope without any special labeling, cytokinesis in an animal cell simply looks like a round cell slowly narrowing at its center until it becomes two. In a time-lapse video, the whole process from first furrow to final separation takes only a few minutes of real time. Plant cytokinesis is harder to see without staining because the cell plate is thin and translucent, but fluorescent dyes that bind to the sugar callose (deposited during wall construction) make the growing plate glow brightly against a dark background.