Mitosis is the process by which a eukaryotic cell divides its duplicated chromosomes so that each new cell receives a complete, matching set of genetic material. It is essential for growth, tissue repair, and the routine replacement of cells.
The process is commonly divided into four main stages: prophase, metaphase, anaphase, and telophase. A useful way to remember their sequence is PMAT. Although these stages are often taught as separate steps, mitosis is a continuous process in which one event leads into the next.
Before mitosis begins, the cell has already copied its DNA. During mitosis, the challenge is to separate those duplicated chromosomes accurately and distribute one copy of each chromosome to each daughter cell.
What happens before mitosis?
Mitosis is part of the cell cycle, the series of events through which a cell grows, copies its DNA, and divides. DNA replication occurs during the S phase of interphase, before mitosis begins.
After DNA replication, each chromosome consists of two identical copies called sister chromatids. The sister chromatids are attached to one another at a region called the centromere. They contain the same genetic information because one was produced as a copy of the other.
The cell must then organize these duplicated chromosomes, line them up, separate the sister chromatids, and form two nuclei. Mitosis accomplishes the nuclear division; cytokinesis, which usually overlaps with the later stages of mitosis, divides the cell’s cytoplasm to produce two separate daughter cells.
Prophase: chromosomes become organized for division
During prophase, the duplicated chromosomes begin to condense. DNA that was relatively extended inside the nucleus becomes tightly packaged into distinct chromosomes that can be moved without becoming tangled.
Each visible duplicated chromosome contains two sister chromatids joined at the centromere. At the same time, the mitotic spindle begins to form. The spindle is a framework of microtubules that will ultimately move the chromosomes.
In animal cells, the centrosomes move toward opposite sides of the cell as the spindle develops. The nucleolus disappears, and the nuclear envelope eventually breaks down, allowing spindle microtubules to interact with the chromosomes.
Some textbooks distinguish prometaphase from prophase. During prometaphase, the nuclear envelope breaks down completely and spindle microtubules attach to chromosomes at protein structures called kinetochores. Other descriptions treat these events as part of prophase. This difference in terminology does not change the underlying sequence of events.
Metaphase: chromosomes line up in the middle
During metaphase, the duplicated chromosomes become positioned near the center of the cell. This region is often called the metaphase plate, although it is not an actual physical structure.
Spindle microtubules attach to the kinetochores of the sister chromatids from opposite sides of the cell. This arrangement is crucial: each sister chromatid must be connected to the spindle apparatus in a way that allows the two copies to move toward opposite poles.
The cell has several mechanisms for checking chromosome attachment before separation occurs. These safeguards help prevent chromosomes from being distributed unevenly.
Anaphase: sister chromatids separate
Anaphase begins when the connections holding sister chromatids together are released. The sister chromatids then separate and move toward opposite ends, or poles, of the cell.
Once separated, each former chromatid is considered an individual daughter chromosome.
Spindle microtubules help drive this movement. At the same time, the cell elongates as other spindle components contribute to pushing the two chromosome groups farther apart.
Anaphase is particularly important because it is the stage at which the duplicated genetic material is physically divided between the two future daughter cells. If chromosomes fail to separate correctly, daughter cells can receive abnormal numbers of chromosomes.
Telophase: two new nuclei form
During telophase, the chromosomes arrive at opposite ends of the cell. They begin to decondense, returning toward the less tightly packaged form characteristic of chromosomes between divisions.
A new nuclear envelope forms around each set of chromosomes, producing two separate nuclei. The mitotic spindle is dismantled as its work is completed.
At roughly the same time, cytokinesis divides the cytoplasm. In animal cells, the cell membrane typically pinches inward to form a cleavage furrow. In plant cells, a cell plate forms between the two nuclei and develops into a new dividing wall.
The result is two daughter cells, each with its own nucleus containing a complete set of chromosomes.
The four stages at a glance
| Stage | Main event | What happens to the chromosomes? |
|---|---|---|
| Prophase | Chromosomes condense and the spindle forms | Duplicated chromosomes become clearly organized |
| Metaphase | Chromosomes align at the cell’s center | Sister chromatids are positioned for separation |
| Anaphase | Sister chromatids separate | Daughter chromosomes move to opposite poles |
| Telophase | Two nuclei form | Chromosomes arrive, then begin to decondense |
Why chromosome duplication must happen first
Mitosis does not copy DNA. The DNA has already been replicated before mitosis begins.
This distinction is important. During S phase, each chromosome is copied, producing two sister chromatids. Mitosis then separates those copies. If a cell attempted to divide without first replicating its DNA, the resulting daughter cells would not receive the normal complete complement of genetic material.
The sequence therefore follows a simple logic: copy the genetic material first, then separate the copies accurately.
Mitosis and meiosis are not the same
Mitosis is sometimes confused with meiosis, but they serve different purposes.
Mitosis generally produces two genetically matching daughter cells and maintains the chromosome number of the original cell. It is used by multicellular organisms for growth, maintenance, and tissue repair.
Meiosis, by contrast, produces cells involved in sexual reproduction and includes two successive divisions after a single round of DNA replication. It reduces the chromosome number by half and generates genetically different cells.
The separation of sister chromatids during mitosis is therefore different from the chromosome-separation events that characterize the earlier division of meiosis.
Why mitosis matters
The accuracy of mitosis is fundamental to multicellular life. A growing organism needs to produce new cells without continually losing or gaining chromosomes. Similarly, tissues must replace damaged or worn-out cells while preserving the appropriate genetic information.
Mitosis is not simply a four-step checklist. It is a carefully coordinated process involving chromosome condensation, spindle formation, chromosome attachment, movement, separation, and reconstruction of the nuclei. The four traditional stages—prophase, metaphase, anaphase, and telophase—provide a useful framework for understanding that sequence.
The key idea is straightforward: prophase prepares the chromosomes, metaphase aligns them, anaphase separates them, and telophase establishes two new nuclei. Cytokinesis then completes the physical division of the cell.
