Cytokinesis begins during late anaphase and continues through telophase, completing after the nuclear division stages of mitosis are finished. It is the physical splitting of one cell into two, and while it overlaps with the final phases of mitosis, it is sometimes considered a separate sixth phase because it involves dividing the cytoplasm rather than the chromosomes.
Where Cytokinesis Fits in the Mitotic Timeline
Mitosis is traditionally divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Each stage handles the job of duplicating and separating chromosomes. Cytokinesis follows telophase and overlaps with it, handling the final task of physically pinching or partitioning the cell in two. The key trigger happens during anaphase, when the chromosomes have fully separated and moved to opposite poles of the cell. At that point, signaling molecules begin assembling the machinery that will divide the cytoplasm.
The reason cytokinesis doesn’t start earlier is simple: the cell needs to be certain that each daughter cell will receive a complete set of chromosomes. Cells have a built-in checkpoint system that monitors whether the chromosomes have been properly positioned. In yeast, this is called the spindle position checkpoint, which blocks the cell from exiting mitosis until the spindle is correctly oriented. Only when the chromosomes are safely at opposite ends does the cell greenlight the physical split. If the spindle is mispositioned, inhibitory signals keep the division machinery turned off.
How Animal Cells Divide: The Cleavage Furrow
In animal cells, cytokinesis works from the outside in. A band of protein filaments called the contractile ring assembles just beneath the cell membrane at the cell’s equator, directly between the two sets of separated chromosomes. This ring is built from actin filaments and a motor protein that pulls on them, generating force the same way muscles contract. As the ring tightens, it draws the membrane inward, creating a visible groove called the cleavage furrow. The furrow deepens until the cell is pinched into two separate daughter cells.
The assembly of this ring is tightly controlled by a signaling protein called RhoA. During anaphase, a complex of proteins that sits on the spindle structure between the separating chromosomes recruits and activates RhoA specifically at the cell’s midpoint. RhoA then does two things: it triggers the assembly of actin filaments through a protein called formin, and it activates the motor proteins that will generate the squeezing force. Additional structural proteins crosslink the actin filaments and stabilize the ring so it stays focused at the correct division site rather than drifting.
The precision of this system is remarkable. The spindle itself tells the cell exactly where to divide. Because the spindle sits between the two chromosome sets, the signaling proteins that activate along its midzone guarantee the furrow forms in the right place, ensuring each daughter cell gets one complete nucleus.
How Plant Cells Divide: The Cell Plate
Plant cells can’t pinch inward because they’re surrounded by a rigid cell wall. Instead, they build a new wall from the inside out. After the chromosomes separate, a structure called the phragmoplast forms between them. The phragmoplast is made of microtubules, actin filaments, and membrane compartments, and it acts as a scaffold and delivery system.
Motor proteins travel along the phragmoplast’s microtubules like cargo on rails, carrying small vesicles derived from the cell’s internal membrane factories. These vesicles converge at the center of the cell and fuse together, forming tubes that extend and connect into a flat, disc-like network called the cell plate. The cell plate grows outward from the center toward the existing cell walls on either side.
As the cell plate matures, it fills in with the polysaccharides that make up a real cell wall. The earliest and most abundant material deposited is callose, followed by cellulose, hemicellulose, and pectin. Once the cell plate reaches the outer walls and fuses with them, the two daughter cells are fully separated, each with its own intact cell wall.
Animal vs. Plant Cytokinesis at a Glance
- Direction: Animal cells divide from the outside in (centripetal). Plant cells divide from the inside out (centrifugal).
- Main structure: Animal cells use an actin-based contractile ring. Plant cells use a phragmoplast that delivers vesicles.
- Visible sign: Animal cells show a cleavage furrow. Plant cells form a cell plate.
- Building material: Animal cells simply pinch the existing membrane. Plant cells must synthesize an entirely new cell wall.
What Happens When Cytokinesis Fails
If the chromosomes separate normally but the cell never physically divides, the result is a single cell with two nuclei and twice the normal number of chromosomes. This is called a tetraploid cell. Tetraploid cells are inherently unstable. When they attempt to divide again, the extra chromosomes often get distributed unevenly, producing daughter cells with abnormal chromosome counts, a condition called aneuploidy.
This matters because aneuploidy is a hallmark of cancer. Tetraploidy and near-tetraploidy have been detected in precancerous conditions like Barrett’s esophagus and early-stage cervical cancer. In mouse experiments, impaired cytokinesis directly leads to multinucleated cells, chromosomal abnormalities, and tumor formation. Research has shown that repeatedly failed cytokinesis is sufficient on its own to generate aneuploid cells that behave like cancer cells in lab dishes and form tumors when implanted in animals. The generation of tetraploid cells through failed division is now considered a possible early step in how chromosomally abnormal tumor cells arise in many human solid cancers.
Even in normal biology, cytokinesis doesn’t always happen. Cells that detach from the tissue surrounding them often fail to complete division, ending up binucleated. This is one of the body’s context-dependent controls: cells that aren’t properly anchored in tissue shouldn’t be proliferating in the first place.