The four phases of mitosis are prophase, metaphase, anaphase, and telophase. Together, they take a single cell’s duplicated DNA and split it into two identical nuclei, each with a complete set of 46 chromosomes (in humans). The entire process is fast: mitosis takes roughly 45 to 50 minutes on average, which is only about 5% of a cell’s total division cycle. The other 95% is spent in interphase, where the cell grows and copies its DNA in preparation.
Prophase: Chromosomes Appear and the Spindle Forms
Prophase is when the cell visibly shifts from its normal working state into division mode. The copied DNA, which has been loosely spread throughout the nucleus, begins to coil and condense into compact structures you can see under a microscope. Each chromosome at this point consists of two identical copies, called sister chromatids, joined together at a pinch point called the centromere. This condensation also untangles the DNA strands that became intertwined when they were copied.
At the same time, structures called centrosomes (which duplicated earlier during interphase) start migrating to opposite sides of the nucleus. These centrosomes act as anchor points for the mitotic spindle, a network of protein fibers that will eventually pull the chromosomes apart. By the end of prophase, the nuclear envelope, the membrane surrounding the nucleus, begins to break down, giving the spindle fibers access to the chromosomes.
Metaphase: Chromosomes Line Up at the Center
Metaphase is the alignment phase. Each chromosome must attach to spindle fibers from both poles of the cell before division can proceed. This attachment happens at the kinetochore, a protein complex built on each chromatid’s centromere that acts as a docking site for spindle fibers.
The process starts somewhat randomly. A spindle fiber from one pole latches onto a kinetochore on one side of a chromosome, pulling it toward that pole. Once the opposite kinetochore captures a fiber from the other pole, the chromosome is pulled equally from both directions and settles at the cell’s equator, a position called the metaphase plate. The chromosomes don’t just sit still once they arrive. They oscillate back and forth in small movements as the opposing forces stay in tension.
This phase has a built-in safety check called the spindle assembly checkpoint. The cell will not proceed to the next phase until every single chromosome is properly attached to fibers from both poles. If even one kinetochore is unattached, the checkpoint blocks the signals that would trigger separation. This prevents daughter cells from ending up with the wrong number of chromosomes, a defect linked to cancer and developmental disorders.
Anaphase: Sister Chromatids Split Apart
Anaphase is the shortest and most dramatic phase. Once the spindle assembly checkpoint confirms that all chromosomes are correctly attached, the cell activates an enzyme called separase. This enzyme cuts the molecular glue (a protein complex called cohesin) holding each pair of sister chromatids together. The moment that bond is severed, the spindle fibers reel the now-separated chromatids toward opposite poles of the cell.
The movement is rapid. Each chromatid, now considered an individual chromosome, is pulled centromere-first toward its respective pole while the spindle fibers shorten. At the same time, the cell itself begins to elongate as the two poles push farther apart. By the end of anaphase, each pole of the cell has a complete set of 46 chromosomes.
Telophase: Two New Nuclei Form
Telophase essentially reverses what happened in prophase. The separated chromosomes arrive at opposite poles and begin to decondense, loosening back into the spread-out form used during normal cell activity. A new nuclear envelope assembles around each group of chromosomes. This starts with small membrane vesicles binding to the surface of the chromosomes, then fusing together to form a double membrane. Nuclear pore complexes, the gateways that control what enters and exits the nucleus, reassemble within this new envelope.
The structural scaffolding inside the nucleus also rebuilds, and the nucleolus (the region responsible for producing the cell’s protein-building machinery) reappears as the relevant genes become active again. By the end of telophase, the cell contains two fully formed, distinct nuclei, each with a complete copy of the organism’s genome.
Cytokinesis: Dividing the Cell Itself
Mitosis technically ends with telophase, but the cell isn’t truly two cells yet. That requires cytokinesis, the physical division of the cytoplasm, which typically overlaps with late anaphase and telophase.
In animal cells, a ring of protein filaments tightens around the cell’s midsection like a drawstring, pinching the membrane inward until the cell splits in two. Plant cells handle this differently because their rigid cell walls can’t be pinched. Instead, vesicles from internal compartments gather at the center of the cell and fuse together to build a new cell wall from the inside out, gradually extending until the two daughter cells are fully separated. Despite these surface-level differences, recent research has shown that both animal and plant cells rely on similar underlying machinery in the final moments of separation, particularly the use of interdigitating protein fibers to guide vesicles to the division site.
What Each Phase Accomplishes
It helps to think of the four phases as solving four distinct problems. Prophase packages the DNA so it can be moved without tangling or breaking. Metaphase ensures every chromosome is accounted for and properly anchored. Anaphase executes the actual separation with precision. Telophase rebuilds the nuclear architecture so each new cell can resume normal function.
The timing is tightly controlled. In cultured human cells, the entire mitotic process averages about 48 minutes, with relatively little variation from cell to cell (a standard deviation of only about 6 minutes). This consistency exists because internal feedback systems keep mitosis insulated from fluctuations in the longer phases of the cell cycle. A cell that took longer to copy its DNA in interphase still completes mitosis in roughly the same amount of time as one that was faster. The result is two genetically identical daughter cells, each ready to enter their own interphase and begin the cycle again.