Mitosis in Five Phases: How One Cell Becomes Two

One cell, faithfully, into two

Mitosis is the process by which a eukaryotic cell duplicates its entire genome and then divides so that each of the two resulting daughter cells receives one complete, identical copy of every chromosome. Get it right and two healthy cells result; get it wrong and you get a daughter cell with too many or too few chromosomes — aneuploidy, a hallmark of many cancers and of conditions like Down syndrome when errors occur in the related process of meiosis. Biologists conventionally split the process into five named, continuous phases, each defined by a distinct, visually identifiable event.

Prophase: the chromosomes condense

Before mitosis begins, the cell has already duplicated its DNA during S phase of interphase, so each chromosome now exists as two identical sister chromatids joined at the centromere. In prophase, this loosely tangled chromatin coils and condenses into the compact, visible chromosome shapes recognisable under a microscope, the nuclear envelope begins to break down, and two centrosomes — the cell's microtubule-organising centres — start migrating toward opposite poles of the cell, nucleating the mitotic spindle as they go.

Prometaphase and metaphase: capture and alignment

Once the nuclear envelope fully dissolves in prometaphase, spindle microtubules invade the former nuclear space and attach to each chromosome at a protein structure called the kinetochore, assembled on the centromere. Correct attachment means each sister chromatid is captured by microtubules from opposite spindle poles — a configuration called amphitelic attachment. The cell does not proceed until every chromosome is captured this way and pulled to the cell's equatorial plane, the metaphase plate; this alignment is actively policed by the spindle assembly checkpoint, which halts the cycle if even a single kinetochore remains unattached or improperly attached.

spindle assembly checkpoint, simplified:
  for each chromosome:
      if kinetochore not correctly attached to both poles:
          block anaphase onset (delay cell cycle)
  once all chromosomes report correct attachment -> release the brake -> anaphase begins

Anaphase: the fastest, most irreversible step

Anaphase begins the instant the checkpoint is satisfied, triggered by an enzyme complex (APC/C) that tags the protein gluing sister chromatids together (cohesin) for destruction. Once cohesin is cleaved, the two sister chromatids of every chromosome separate simultaneously and are pulled toward opposite spindle poles by shortening kinetochore microtubules, while the poles themselves are pushed further apart by overlapping microtubules sliding against each other — a coordinated, essentially irreversible event that takes only a few minutes.

Telophase and cytokinesis: two cells emerge

In telophase the two separated sets of chromosomes arrive at opposite poles, decondense back into loose chromatin, and a new nuclear envelope re-forms around each set, producing two nuclei within what is still, briefly, one cell. The physical split into two separate cells — cytokinesis — usually begins already during anaphase in animal cells, via a contractile ring of actin and myosin that pinches the cell membrane inward at the equator like a drawstring, finally severing the two daughter cells apart.

What the simulation shows

This simulation renders the five phases in sequence — condensing chromosomes, spindle formation and kinetochore capture, metaphase alignment, sister chromatid separation, and the final pinch of cytokinesis — so the otherwise invisible mechanics of chromosome segregation, and the strict checkpoint that keeps errors from propagating, are visible as continuous motion rather than five static textbook diagrams.

Frequently asked questions

Try it live

Everything above runs in your browser — open Mitosis 3D and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.