Mitosis is the process by which a eukaryotic cell divides its nucleus to produce two daughter nuclei with essentially identical sets of chromosomes. It is central to growth, tissue repair, and the routine replacement of cells in the body.
Although mitosis is often described as a sequence of four stages—prophase, metaphase, anaphase, and telophase—the process is easier to understand when it is placed within the larger cell cycle. Before a cell divides, it must copy its DNA. After the chromosomes are separated into two nuclei, the cell usually divides its cytoplasm as well, a step called cytokinesis.
Where mitosis fits in the cell cycle
The cell cycle is the series of events through which a cell grows, copies its DNA, and divides. It is commonly divided into interphase and the M phase.
Interphase comes before mitosis and includes three stages. During G1, the cell grows and carries out its normal functions. During S phase, it replicates its DNA, so each chromosome is copied. During G2, the cell continues growing and prepares for division.
Interphase is sometimes mistaken for a resting period, but that description is misleading. The cell is metabolically active and performs the preparation required for successful division.
The M phase includes mitosis and cytokinesis. Mitosis separates the duplicated chromosomes into two nuclei; cytokinesis then divides the cell itself.
What happens to chromosomes before mitosis?
DNA replication occurs during S phase, before mitosis begins. After replication, each chromosome consists of two identical DNA copies called sister chromatids. The sister chromatids are joined at a region called the centromere.
This distinction is important because a chromosome is not the same thing as a single DNA molecule. After DNA replication, a chromosome contains two sister chromatids, but it is still counted as one chromosome until the sister chromatids separate.
The goal of mitosis is to distribute one copy of every chromosome to each new nucleus.
The stages of mitosis
Mitosis is traditionally divided into four main stages: prophase, metaphase, anaphase, and telophase. Some descriptions also identify prometaphase, the period between prophase and metaphase, as a distinct stage.
Prophase: chromosomes condense
During prophase, the duplicated chromosomes become increasingly compact and visible under a microscope. This condensation helps keep the DNA organized as it is moved within the cell.
The mitotic spindle also begins to form. The spindle is a structure made largely of microtubules, which are components of the cell’s internal framework. In animal cells, structures called centrosomes help organize the spindle.
As prophase progresses, the nucleolus disappears and the cell prepares for the breakdown of the nuclear envelope.
Prometaphase: the spindle attaches to chromosomes
In prometaphase, the nuclear envelope breaks down, allowing spindle microtubules to interact directly with the chromosomes.
Specialized protein structures called kinetochores form at the centromere region of each chromosome. Spindle microtubules attach to these kinetochores and begin positioning the chromosomes.
The cell must establish correct attachments before chromosome separation can safely occur. This is one reason chromosome movement is carefully regulated rather than simply happening as soon as the spindle forms.
Metaphase: chromosomes line up
During metaphase, the chromosomes become positioned near the center of the cell, along a region often called the metaphase plate.
Each chromosome’s sister chromatids are connected to spindle microtubules associated with opposite sides of the cell. This arrangement sets up the physical basis for distributing one chromatid from each chromosome to each daughter cell.
A spindle checkpoint helps prevent the cell from proceeding until chromosomes are appropriately attached to the spindle. Errors at this stage can lead to unequal chromosome distribution.
Anaphase: sister chromatids separate
Anaphase begins when the sister chromatids separate. Once separated, each former chromatid is considered an individual chromosome.
Spindle forces move the newly separated chromosomes toward opposite poles of the cell. Because each chromosome had been replicated before mitosis, this separation gives each side of the cell a corresponding set of chromosomes.
Anaphase is therefore the decisive stage for chromosome segregation: the duplicated genetic material is physically divided between the two future nuclei.
Telophase: new nuclei form
During telophase, the chromosomes reach opposite ends of the cell and begin to decondense. A new nuclear envelope forms around each set of chromosomes.
The spindle breaks down, and the structures characteristic of interphase nuclei begin to be restored. At the end of telophase, the cell has two nuclei containing corresponding sets of chromosomes.
Cytokinesis divides the cell
Mitosis divides the nucleus, but it does not by itself complete the division of the entire cell. That requires cytokinesis, the division of the cytoplasm.
In animal cells, cytokinesis usually begins with formation of a cleavage furrow. A ring of proteins contracts around the cell’s middle, causing the membrane to pinch inward until two cells form.
Plant cells use a different mechanism because their rigid cell walls prevent the membrane from simply pinching in two. Instead, a cell plate develops between the daughter nuclei and eventually becomes part of the new cell wall separating the daughter cells.
Cytokinesis often overlaps with late mitosis rather than occurring as a completely separate event.
Mitosis produces genetically similar daughter cells
The fundamental outcome of mitosis is the production of two daughter cells whose nuclei contain the same chromosome complement as the original cell, assuming the process occurs normally.
This is possible because DNA is copied before mitosis and the resulting sister chromatids are then separated so that each daughter nucleus receives one copy of each chromosome.
Mitosis does not normally create genetic diversity in the way meiosis does. Meiosis is the specialized cell division that produces eggs or sperm and reduces the chromosome number by half. It also includes processes that generate genetic variation.
Why mitosis matters in the human body
Mitosis is essential for increasing cell number during development and growth. It also allows tissues to replace cells that have worn out, become damaged, or reached the end of their normal lifespan.
Not every cell in the body divides continuously. Many highly specialized cells divide rarely or not at all under normal conditions. Other tissues, such as those that continually renew themselves, rely heavily on controlled cell division.
Mitosis is therefore not simply a mechanism for making more cells. It is part of a tightly regulated system that determines when cells should grow, copy their DNA, divide, or stop dividing.
What happens when mitosis goes wrong?
Accurate chromosome separation is critical. If chromosomes are distributed incorrectly, daughter cells can receive abnormal numbers of chromosomes, a condition known as aneuploidy.
Cells have multiple safeguards that help detect DNA damage and problems with chromosome attachment before division proceeds. These controls can delay the cell cycle or, when damage is severe, contribute to elimination of the affected cell.
Cancer illustrates why regulation of cell division matters. Cancer cells can acquire changes that allow them to bypass normal controls on proliferation, survive when they should not, or accumulate chromosome abnormalities. Mitosis itself is a normal and necessary process; the problem arises when the systems controlling cell growth and division become disrupted.
Mitosis at a glance
| Stage | Main event |
|---|---|
| Prophase | Chromosomes condense and the mitotic spindle begins to form. |
| Prometaphase | The nuclear envelope breaks down and spindle microtubules attach to chromosomes. |
| Metaphase | Chromosomes align near the center of the cell. |
| Anaphase | Sister chromatids separate and move toward opposite poles. |
| Telophase | Chromosomes decondense and two nuclear envelopes form. |
| Cytokinesis | The cytoplasm divides, producing two daughter cells. |
The key idea is that mitosis is a coordinated chromosome-separation process, not simply a cell splitting in half. DNA is replicated beforehand, duplicated chromosomes are organized and attached to the spindle, sister chromatids are separated, and two nuclei are assembled around the resulting chromosome sets. Cytokinesis then completes the physical division of the cell.
