Your body contains trillions of cells, and almost every one of them is the product of a process that has been running continuously since you were a single fertilised egg. That process is mitosis, and it solves a deceptively hard problem: how to copy an entire set of chromosomes and split them perfectly in two.
Mitosis
Watch one cell divide into two identical cellsThe six stages
- Interphase — the cell is not really “resting”. It is copying its DNA. Each chromosome is now two identical sister chromatids joined at a centromere, though they are still loose chromatin inside the nucleus.
- Prophase — the chromatin condenses into the compact, visible chromosomes you see in textbook diagrams. The nuclear envelope breaks down and the spindle apparatus starts to form.
- Metaphase — chromosomes line up along the cell’s equator. Each one is attached to spindle fibres from both poles. This alignment is the checkpoint: the cell will not proceed until every chromosome is correctly attached.
- Anaphase — the sister chromatids are pulled apart. Once separated, each is considered a full chromosome in its own right. They travel centromere-first, which is why they appear to drag behind in a V shape.
- Telophase — two new nuclear envelopes assemble around the two chromosome sets, and the chromosomes relax back into chromatin.
- Cytokinesis — the cytoplasm divides. In animal cells a contractile ring pinches the membrane inward until the cell separates into two.
Why the result is identical
The elegance of mitosis is in the pairing. Because each chromosome was copied during interphase, and because the copies are separated to opposite poles during anaphase, each daughter cell receives one complete set — genetically identical to the parent and to its sister cell.
Watch the chromosome count in the readout: the cell starts with 4, briefly has 8 as the chromatids separate, and finishes with 4 + 4 across two cells. Nothing is gained and nothing is lost.
Try this
- Click 4. Anaphase and watch which direction the V shapes point. The centromere leads; the arms trail.
- Freeze on metaphase and notice that every chromosome is connected to both poles at once.
- Step from telophase into cytokinesis and watch the cell membrane pinch in the middle.
Why it matters
Mitosis is how you grow, how a cut heals, and how a single-celled zygote becomes a person. It is also what goes wrong in cancer: when the checkpoints that govern this sequence fail, cells divide without the usual controls. Understanding the normal sequence is the first step toward understanding what breaks.