Mitosis is a type of cell division that produces two genetically identical daughter cells from a single parent cell. Each daughter cell ends up with the same number of chromosomes as the original, making it the body’s primary method for growth, tissue repair, and replacing cells that wear out. The actual division phase takes only 30 to 60 minutes, though the full cycle of preparing for and completing division can stretch from about 6 hours to much longer depending on the cell type.
The Genetic Outcome Is an Exact Copy
The defining truth of mitosis is its result: two cells that are genetic clones of the parent. A human cell starts with 46 chromosomes (two copies of each, making it “diploid”). Before mitosis begins, the cell copies all of its DNA during a preparation phase called interphase, so it temporarily holds double the usual DNA content. Mitosis then splits that duplicated material evenly, and each daughter cell walks away with the original 46 chromosomes.
This is fundamentally different from meiosis, the division process that creates sperm and egg cells. Meiosis involves two rounds of division, cuts the chromosome count in half (down to 23 in humans), and shuffles genetic material so that every resulting cell is unique. Mitosis involves one division, preserves the full chromosome count, and happens in cells throughout the body. Meiosis occurs only in reproductive organs.
Four Stages Drive the Division
Mitosis unfolds in a predictable sequence: prophase, metaphase, anaphase, and telophase. Each stage has a specific mechanical job.
During prophase, the long, loose strands of DNA condense into compact, visible chromosomes that are easier to move. A structure called the mitotic spindle, made of protein filaments called microtubules, starts forming. The membrane around the nucleus breaks down, freeing the chromosomes into the cell’s interior. Some microtubules begin capturing chromosomes by attaching to a protein patch on each one called the kinetochore.
In metaphase, all chromosomes line up along the middle of the cell. Each chromosome’s two identical halves (sister chromatids) are attached to microtubules pulling from opposite sides. This tug-of-war alignment is critical: it ensures that when the chromatids separate, one copy goes to each side.
During anaphase, the protein glue holding sister chromatids together dissolves. The microtubules pull the separated chromatids toward opposite poles of the cell, while other microtubules push the poles apart, stretching the cell into an elongated shape.
Telophase is essentially prophase in reverse. New nuclear membranes form around each set of chromosomes. The chromosomes relax back into their loose, stringy form, and the spindle breaks down. By the end of telophase, there are two distinct nuclei inside one cell.
How the Cell Actually Splits in Two
After the nuclei form, the cell still needs to physically divide its cytoplasm, a step called cytokinesis. How this happens depends on the type of organism.
In animal cells, a ring of protein filaments tightens around the cell’s equator like a drawstring, creating an indentation called a cleavage furrow. The furrow deepens until it pinches the cell into two separate daughter cells.
Plant cells can’t do this because they’re surrounded by a rigid cell wall. Instead, they build a new wall from the inside out. Small vesicles carrying wall-building materials travel along leftover spindle microtubules to the center of the cell, where they fuse into a growing disc called the cell plate. The plate expands outward until it reaches the existing cell wall, dividing the cell in two.
Why Your Body Depends on It
Mitosis serves three main purposes in multicellular organisms: growing larger, replacing worn-out cells, and repairing damaged tissue. Some cell types divide almost constantly. Skin cells, the lining of your digestive tract, salivary gland cells, and blood-forming cells in bone marrow are all “labile” cell types that continuously proliferate through mitosis to replace cells that die or get sloughed off. Your gut lining, for instance, replaces itself roughly every few days, entirely through mitotic division.
Other tissues divide more slowly or only when triggered by injury. But the mechanism is the same: one cell becomes two identical copies, and the tissue grows or heals.
Built-In Quality Control
Cells don’t enter mitosis casually. A checkpoint at the boundary between the preparation phase and mitosis verifies that DNA has been copied correctly and isn’t damaged. If problems are detected, a protein called p53 activates multiple braking systems that block the cell from proceeding. These brakes work through several overlapping pathways, so even if one fails, others can still halt division. Separate, p53-independent sensors also monitor for DNA damage and can stop the process on their own.
Once mitosis is underway, another checkpoint operates at metaphase. The cell verifies that every chromosome is properly attached to spindle microtubules from both poles before allowing the chromatids to separate. Tension from the two-sided pull on each chromosome stabilizes the attachment and signals that everything is correctly positioned. If a chromosome isn’t properly connected, division pauses until the error is corrected.
What Happens When Mitosis Goes Wrong
When chromosomes don’t separate evenly, the daughter cells end up with too many or too few chromosomes, a condition called aneuploidy. This is not rare in disease: aneuploidy appears in nearly 70% of solid human tumors. The connection between mitotic errors and cancer was first proposed over a century ago, and modern research has confirmed that mistakes during chromosome separation are a major source of the abnormal chromosome counts seen in cancer cells.
Lagging chromosomes that don’t move to their proper pole during anaphase are a commonly observed error in chromosomally unstable cancer cells. These missegregated chromosomes can drive both gains and losses of whole chromosomes and contribute to structural rearrangements in the genome. The resulting genetic diversity within a tumor population gives cancer cells more raw material for evolving resistance to treatment or acquiring new growth advantages. At the individual cell level, the imbalanced chromosome dosage also disrupts normal biological networks, making cell behavior more unpredictable.
Mitotic errors earlier in development, such as during embryonic growth, can have different consequences, ranging from miscarriage to conditions like Down syndrome, where an extra copy of chromosome 21 is present in every cell. Errors that accumulate later in life have been linked to both aging and the initiation of tumors.