Mitosis is the process by which one eukaryotic cell divides its nucleus to produce two nuclei with the same set of chromosomes as the original nucleus. It is essential for growth, tissue repair, and the replacement of cells.
Mitosis is commonly divided into four main stages: prophase, metaphase, anaphase, and telophase. Each stage has a distinct job. In simple terms, the chromosomes are prepared, lined up, separated, and enclosed in two new nuclei.
Although these stages are often taught as four sharply separated steps, mitosis is actually a continuous process. The changes that define one stage develop gradually and lead directly into the next.
Prophase: Chromosomes become visible and the cell prepares to divide
During prophase, the cell begins organizing its duplicated genetic material for separation.
Before mitosis starts, the cell has already copied its DNA during the S phase of the cell cycle. Each duplicated chromosome consists of two identical sister chromatids, which are joined at a region called the centromere. Early in prophase, the DNA becomes tightly packaged around proteins, causing the chromosomes to condense enough to become distinct structures under a microscope.
At the same time, the mitotic spindle begins to form. The spindle is a framework of microtubules that will eventually move the chromosomes. In animal cells, centrosomes move toward opposite sides of the cell as the spindle develops.
The nucleolus, a structure inside the nucleus involved in producing ribosomal components, disappears. Later in mitosis, the nuclear envelope surrounding the chromosomes will also break down, allowing spindle microtubules to interact with the chromosomes.
In many textbook descriptions, the breakdown of the nuclear envelope marks the beginning of prometaphase, a stage between prophase and metaphase. Some treatments include prometaphase as part of prophase, while others list it separately.
Metaphase: Chromosomes line up in the middle
During metaphase, the duplicated chromosomes become positioned near the center of the cell.
Spindle microtubules attach to protein structures called kinetochores, which form at the centromere region of each chromosome. Microtubules from opposite sides of the cell attach to the sister chromatids in a way that establishes tension and prepares the chromatids for separation.
The chromosomes then align along an imaginary plane called the metaphase plate. The metaphase plate is not a physical structure; it is simply the approximate middle line where the chromosomes are arranged.
This alignment is important because it helps ensure that each daughter cell receives one copy of every chromosome. The cell has checkpoints that help prevent chromosome separation from proceeding until the chromosomes are properly attached to the spindle.
Anaphase: Sister chromatids separate
Anaphase begins when the sister chromatids of each duplicated chromosome separate.
Once the connections holding the sister chromatids together are released, each chromatid becomes an individual chromosome. Spindle microtubules then help move the newly separated chromosomes toward opposite ends of the cell.
This is the stage in which the duplicated genetic material is physically divided between the two future daughter cells.
The chromosomes move toward opposite poles as the spindle changes. At the same time, microtubules that extend between the two poles can help push the poles farther apart, contributing to elongation of the cell.
Because the sister chromatids were originally identical copies produced during DNA replication, their separation distributes equivalent genetic information to the two sides of the cell under normal conditions.
Telophase: Two new nuclei form
During telophase, the chromosomes arrive at opposite ends of the cell and begin to decondense.
A new nuclear envelope forms around each group of chromosomes, producing two separate nuclei. The chromosomes become less tightly packed, returning toward the less condensed form of DNA found during most of the cell’s life. The mitotic spindle is dismantled, and structures such as the nucleolus reappear.
Telophase therefore reverses many of the visible nuclear changes that occurred during prophase. Instead of one nucleus containing condensed chromosomes, the cell now has two nuclei, each containing a complete set of chromosomes.
Cytokinesis completes the division
Mitosis divides the nucleus, but the cell itself must also divide. This process is called cytokinesis.
Cytokinesis usually begins during late anaphase or telophase and may overlap with them. In animal cells, a contractile ring constricts the cell membrane, producing a cleavage furrow that eventually separates the cell into two daughter cells.
Plant cells divide differently because their rigid cell walls prevent the membrane from simply pinching inward. Instead, vesicles gather at the center of the cell and form a cell plate, which develops into a new section of cell wall between the daughter cells.
The result is two daughter cells, each with its own nucleus and, under normal mitotic division, the same chromosome number as the parent cell.
The four stages at a glance
| Stage | What happens |
|---|---|
| Prophase | Chromosomes condense, the spindle forms, and the cell prepares the duplicated chromosomes for separation. |
| Metaphase | Chromosomes align near the center of the cell, with spindle fibers attached to their kinetochores. |
| Anaphase | Sister chromatids separate and move toward opposite sides of the cell. |
| Telophase | Chromosomes reach opposite ends, decondense, and become enclosed in two new nuclei. |
A useful way to remember the sequence is PMAT: Prophase → Metaphase → Anaphase → Telophase.
Why chromosome separation has to be so precise
The central challenge of mitosis is not simply making two cells. It is making two cells that receive the correct genetic information.
DNA replication creates two copies of each chromosome, but those copies must then be distributed accurately. The spindle provides the machinery for doing this. Its microtubules interact with kinetochores and help position and separate the duplicated chromosomes.
Errors in chromosome segregation can produce daughter cells with abnormal numbers of chromosomes, a condition known as aneuploidy. Cells therefore use regulatory mechanisms, including the spindle assembly checkpoint, to delay chromosome separation when the necessary attachments have not been properly established.
Mitosis is different from meiosis
Mitosis should not be confused with meiosis, the specialized type of cell division that produces eggs or sperm.
Mitosis involves one round of chromosome separation and normally produces two genetically similar daughter cells with the same chromosome number as the parent cell. Meiosis involves two successive divisions and produces cells with half the chromosome number, while also creating substantial genetic variation through processes such as crossing over and the independent assortment of chromosomes.
The PMAT sequence appears in both mitotic division and the individual divisions of meiosis, but what happens to the chromosomes differs substantially between the two processes.
The key idea behind PMAT
The four stages describe a coordinated sequence rather than four unrelated events. Prophase packages and organizes the duplicated chromosomes. Metaphase positions them for accurate separation. Anaphase separates the copies. Telophase establishes two new nuclei around them. Cytokinesis then divides the cell itself.
Together, these events allow a single cell to produce two daughter cells with accurately distributed genetic material.
