During mitosis, a cell’s duplicated chromosomes divide and separate into two identical sets, each destined for a new daughter cell. The nucleus splits in two, and then the cytoplasm follows. By the end, one cell has become two, each carrying the same complete set of genetic information as the original.
Chromosomes Are the Main Event
Before mitosis begins, every chromosome in the cell has already been copied during an earlier phase called S phase. Each chromosome now consists of two identical copies, called sister chromatids, joined together at a connection point called the centromere. So while the cell still has the same number of chromosomes (46 in human cells), it holds double the usual amount of DNA.
The entire purpose of mitosis is to pull those sister chromatids apart so each new cell gets one complete copy of every chromosome. The chromatids are held together by a protein complex called cohesin, which acts like molecular glue. At the critical moment during anaphase, an enzyme cuts that glue, and the spindle fibers yank the separated chromatids toward opposite ends of the cell. Once they separate, each chromatid is considered its own independent chromosome. The result: two identical sets of 46 chromosomes, one at each pole of the cell.
How the Cell Pulls Chromosomes Apart
The physical machinery behind chromosome separation is the mitotic spindle, a structure made of protein filaments called microtubules. Two organizing centers, called centrosomes, duplicate before mitosis and migrate to opposite sides of the cell. From there, microtubules extend outward like cables, attaching to specialized docking sites on each sister chromatid.
One microtubule from one pole attaches to one sister chromatid, while a microtubule from the opposite pole attaches to the other. This ensures each side of the cell will receive exactly one copy. The cell doesn’t rush this step. A quality-control system called the spindle assembly checkpoint halts the process until every single chromosome is properly attached. Only when the checkpoint is satisfied does the cell proceed to cut the cohesin and separate the chromatids. The entire process of mitosis, from start to finish, typically takes 30 to 60 minutes in mammalian cells.
The Nucleus Breaks Down and Rebuilds
Chromosomes don’t just split on their own. The nuclear membrane, the barrier that normally surrounds and protects the cell’s DNA, has to be dismantled first. During prometaphase, this membrane breaks apart into small fragments that mix into the rest of the cell’s internal membranes. This gives the spindle fibers direct access to the chromosomes.
Once the chromosomes have been separated and pulled to opposite poles, a new nuclear membrane reassembles around each set during telophase. This rebuilding involves targeting membrane material to the surface of the clustered chromosomes, fusing it together, and installing pores that allow molecules to move in and out. By the end of telophase, the cell contains two fully formed, separate nuclei.
Cytokinesis Splits the Rest of the Cell
Mitosis technically refers only to the division of the nucleus. The physical splitting of the cytoplasm, all the fluid, organelles, and other cellular components outside the nucleus, happens through a separate but closely linked process called cytokinesis. It begins during late mitosis and finishes shortly after.
In animal cells, a band of protein filaments tightens around the cell’s midsection like a drawstring, pinching inward until the cell is squeezed into two. Plant cells handle it differently. Because they’re enclosed in a rigid cell wall, they can’t simply pinch inward. Instead, they build a new wall, called the cell plate, from the inside out, starting at the center and expanding outward until it divides the cell completely.
What Stays the Same After Division
A key feature of mitosis is that chromosome number doesn’t change. A human cell starts with 46 chromosomes and produces two daughter cells, each with 46 chromosomes. The DNA content does change across the process: it doubles during replication before mitosis, then gets split back to the normal amount. But the genetic information in each daughter cell is identical to the original. This is what makes mitosis fundamentally different from meiosis, which halves the chromosome number to produce eggs and sperm.
Every cell in your body that isn’t an egg or sperm was produced by mitosis. Skin cells, blood cells, liver cells, and the cells that heal a wound all arise this way. The fidelity of this process is essential. When chromosomes fail to separate properly, a condition called aneuploidy results, where daughter cells end up with the wrong number of chromosomes. Aneuploidy is a hallmark of cancer and a major cause of miscarriages. In human embryos produced through IVF, roughly 74% show signs of chromosome mis-segregation during the first few mitotic divisions after fertilization, though most of these errors are corrected or result in nonviable embryos rather than live births.
The Phases in Sequence
- Prophase: Chromosomes condense into visible, tightly coiled structures. Each consists of two joined sister chromatids. The centrosomes begin migrating to opposite poles.
- Prometaphase: The nuclear membrane breaks apart. Spindle microtubules attach to the chromatids at their centromeres.
- Metaphase: All chromosomes line up along the middle of the cell, pushed and pulled by the spindle until they’re aligned in a single plane.
- Anaphase: The cohesin holding sister chromatids together is cut. Spindle fibers contract, pulling the separated chromatids to opposite poles.
- Telophase: A new nuclear membrane forms around each cluster of chromosomes. The chromosomes begin to relax and decondense.
Cytokinesis overlaps with telophase and completes the physical division of the cell into two. The whole sequence, from prophase through cytokinesis, produces two genetically identical daughter cells ready to enter their own growth cycles.