Mitosis is the process by which a single cell copies its chromosomes and divides its nucleus into two, producing two genetically identical daughter cells. It takes about one hour in a typical human cell, yet it’s responsible for building and maintaining your entire body, from the earliest days of embryonic development through every wound you heal as an adult.
How Mitosis Fits Into the Cell Cycle
Cells don’t spend most of their time dividing. In a human cell that completes one full cycle roughly every 24 hours, mitosis occupies only about one of those hours. The other 23 hours make up interphase, the period when the cell grows, copies all of its DNA, and prepares the molecular machinery it needs for division. Interphase breaks down into a growth phase (about 11 hours), a DNA-copying phase (about 8 hours), and a final preparation phase (about 4 hours). Only after all of that groundwork is complete does the cell enter mitosis.
The Four Stages of Mitosis
Mitosis is divided into four stages: prophase, metaphase, anaphase, and telophase. Each one accomplishes a specific physical task, and they always happen in the same order.
Prophase
Prophase is the starting signal. The long, loose strands of copied DNA condense into tightly packed chromosomes that are visible under a microscope. Each chromosome at this point is actually a pair of identical copies, called sister chromatids, joined together at a point called the centromere. While the chromosomes are condensing, two structures called centrosomes move to opposite sides of the cell and begin building the mitotic spindle, a network of protein fibers that will eventually pull the chromosomes apart. Near the end of prophase, the membrane surrounding the nucleus breaks down, giving the spindle access to the chromosomes.
Metaphase
Before full metaphase begins, there’s a brief transition period where the spindle fibers reach out in many directions, growing and shrinking until they connect to a landing pad on each chromosome called the kinetochore. This “search and capture” process is somewhat random. A fiber from one side of the cell grabs one sister chromatid, and a fiber from the opposite side grabs its twin. The tug of war between the two sides pulls each chromosome to the middle of the cell. When all chromosomes are lined up along this central plane (called the metaphase plate), the cell has reached metaphase. This lineup is critical: it ensures that when the chromosomes separate, each side of the cell gets exactly one complete set.
Anaphase
Anaphase begins when the proteins holding each pair of sister chromatids together are broken down. The instant that bond is cut, the spindle fibers reel each chromatid toward its respective pole. What was one X-shaped chromosome becomes two independent chromosomes moving in opposite directions. By the end of anaphase, each half of the cell contains a full, identical collection of chromosomes.
Telophase
In telophase, the process essentially runs in reverse. The chromosomes at each pole begin to unwind back into their loose, threadlike form. Small membrane fragments bind to the surface of the chromosomes and fuse together, rebuilding a nuclear envelope around each set. Nuclear pores reassemble, the internal scaffolding of the nucleus reforms, and two distinct nuclei now exist inside one cell.
Cytokinesis: Splitting the Cell in Two
Mitosis divides the nucleus, but the cell itself still needs to split. That job belongs to cytokinesis, which usually begins during late anaphase or telophase and finishes shortly after. In animal cells, a ring of protein filaments pinches the cell membrane inward like a drawstring until the cell is cleaved into two separate daughter cells. Each one receives one nucleus and roughly half of the original cell’s contents. The result: two cells with identical genomes, each carrying the full human set of 46 chromosomes (22 pairs of autosomes and one pair of sex chromosomes).
Why Your Body Depends on Mitosis
Mitosis serves three major roles in the body: growth, maintenance, and repair.
During embryonic development, a single fertilized egg divides over and over through mitosis to build every tissue and organ. Stem cells in the embryo give rise to all the different cell lineages in the body, and mitosis is the engine behind that expansion.
In adults, certain tissues never stop dividing. Your skin, the lining of your gastrointestinal tract, the surface of your cornea, and your blood-forming tissue are all continuously replacing cells that wear out or slough off. These tissues rely on a steady supply of new cells produced through mitosis. When you cut your skin, cells at the wound’s edge ramp up their division rate to close the gap, whether the injury heals by regeneration (growing back the original tissue) or by scarring.
How the Cell Prevents Mistakes
Getting 46 chromosomes sorted perfectly into two groups is a high-stakes task, and cells have a built-in quality control system called the spindle assembly checkpoint. This checkpoint works by monitoring whether every single chromosome is properly attached to spindle fibers from both sides of the cell. If even one kinetochore is unattached or incorrectly connected, the checkpoint halts the entire process and prevents the cell from moving into anaphase. Only when all chromosomes are stably connected does the checkpoint release its block and allow separation to proceed.
What Happens When Mitosis Goes Wrong
When a chromosome fails to separate correctly, one daughter cell ends up with an extra copy and the other is missing one. This condition is called aneuploidy, and it’s far more common than most people realize. About 74% of human embryos created through in vitro fertilization show signs of chromosome errors from the first few rounds of mitotic division after fertilization. Many of these errors are incompatible with life, which is a major reason early miscarriages are so common.
Aneuploidy is also one of the defining features of cancer. When cells gain or lose chromosomes, they can acquire growth advantages, resist therapies, or gain the ability to spread to other parts of the body. Specific patterns of chromosome gain and loss show up repeatedly in certain tumor types. Sometimes a misplaced chromosome gets trapped in a small bubble of membrane called a micronucleus. About half of these micronuclei rupture, exposing the trapped DNA to damage and triggering extensive rearrangements on that chromosome. This cascade can fuel further genomic instability in later cell divisions.
The aneuploidies that survive to birth in humans tend to involve smaller chromosomes, where the extra or missing genetic material causes less disruption. Trisomies of chromosomes 13, 18, and 21, along with a missing Y chromosome, are the most common examples.
Mitosis vs. Meiosis
Mitosis and meiosis are both forms of cell division, but they serve completely different purposes. Mitosis produces two identical cells with the same chromosome count as the parent. It handles growth and repair in every tissue of the body. Meiosis, on the other hand, produces four cells that each have half the normal chromosome count. It only occurs in the cells that become eggs and sperm. Meiosis also shuffles genetic material between chromosomes, creating new combinations of traits. Mitosis does not. Every daughter cell is, in principle, a genetic clone of the original.