What Is Mitosis?
Mitosis is the process by which a single cell divides to produce two genetically identical daughter cells. Each daughter cell contains the same number and type of chromosomes as the original parent cell. Mitosis is how your body grows, repairs damaged tissue, and replaces old cells — right now, millions of your cells are undergoing mitosis.
Mitosis is part of the larger cell cycle, which includes interphase (the period between divisions when the cell grows and copies its DNA) and the mitotic phase (M phase), which includes mitosis itself plus cytokinesis (the physical splitting of the cell in two). When biologists say a cell is "dividing," they usually mean the entire M phase — mitosis followed by cytokinesis.
It is important to understand that mitosis divides the nucleus and its chromosomes. Cytokinesis divides the cytoplasm and the rest of the cell. These are technically separate processes, though they usually happen in sequence. In some organisms — certain fungi, for example — the nucleus divides without the cell splitting, producing multinucleated cells.
Before Mitosis: Interphase
Before a cell can divide, it must prepare. During interphase — which accounts for about 90% of the cell cycle — the cell grows, produces proteins, and most importantly, replicates its DNA. By the time a cell enters mitosis, every chromosome has been duplicated, forming two identical copies called sister chromatids, joined at a region called the centromere.
Interphase has three sub-phases: G1 (cell growth and normal function), S phase (DNA synthesis — this is when chromosomes are copied), and G2 (additional growth and preparation for division). The cell also duplicates its centrosomes during S phase, which will be needed to organize the mitotic spindle.
A human cell in G1 has 46 chromosomes (23 pairs). After S phase, it still has 46 chromosomes, but each chromosome consists of two sister chromatids. The total amount of DNA has doubled, but the chromosome count has not — this distinction confuses many students. Think of it this way: a chromosome with two sister chromatids is still one chromosome, like a pair of socks still counts as one pair.
Prophase
Prophase is the first stage of mitosis, and the most time-consuming. The chromatin (loose DNA) condenses into tightly coiled chromosomes that become visible under a microscope. Each chromosome appears as an X shape because it consists of two sister chromatids joined at the centromere.
The nuclear envelope (membrane surrounding the nucleus) begins to break down. The nucleolus — a structure inside the nucleus that produces ribosomes — disappears. The centrosomes migrate to opposite poles (ends) of the cell and begin forming the mitotic spindle, a structure made of protein filaments called microtubules.
In late prophase (sometimes called prometaphase), the nuclear envelope fully disintegrates, and spindle fibers from each centrosome attach to the chromosomes at structures called kinetochores — protein complexes located at each centromere. Each sister chromatid has its own kinetochore, and the kinetochores of sister chromatids attach to spindle fibers from opposite poles. This ensures that when the chromatids separate, one goes to each side.
Metaphase
During metaphase, the chromosomes line up along the middle of the cell at a plane called the metaphase plate (or equatorial plate). This alignment is not random — the spindle fibers from opposite poles pull on each chromosome with equal force, positioning it precisely at the center.
Metaphase is the stage where chromosomes are easiest to see and photograph under a microscope because they are maximally condensed and neatly aligned. This is why karyotypes (images of a cell's complete set of chromosomes) are prepared from cells arrested in metaphase.
There is an important quality-control checkpoint here called the spindle assembly checkpoint (SAC). The cell verifies that every chromosome is properly attached to spindle fibers from both poles before proceeding. If any chromosome is unattached or incorrectly attached, the checkpoint halts the cell cycle until the error is fixed. This prevents daughter cells from getting the wrong number of chromosomes, a condition called aneuploidy that can lead to diseases like Down syndrome (trisomy 21).
Anaphase
Anaphase is the shortest and most dramatic stage of mitosis. The enzyme separase cleaves the cohesin proteins holding sister chromatids together at their centromeres. Once freed, each chromatid — now called an individual chromosome — is pulled toward the opposite pole of the cell by the shortening spindle fibers.
The movement happens because the microtubules attached to kinetochores depolymerize (shorten) at the pole end, reeling the chromosomes in like fishing line. At the same time, other microtubules between the poles elongate, pushing the poles farther apart and stretching the cell into an oval shape.
By the end of anaphase, the two sets of identical chromosomes have been separated to opposite ends of the cell. A human cell that entered mitosis with 46 chromosomes (each made of two sister chromatids) now has 46 chromosomes at each pole — but each chromosome is now a single chromatid. The total chromosome count per side is unchanged; only the DNA content per chromosome has changed.
Telophase
Telophase is essentially the reverse of prophase. The chromosomes at each pole decondense (uncoil) back into loose chromatin. New nuclear envelopes form around each set of chromosomes, creating two distinct nuclei within the same cell. The nucleolus reappears in each new nucleus. The spindle fibers disassemble.
At the end of telophase, the cell has two complete nuclei, each with a full set of chromosomes identical to the original cell. However, the cell itself has not yet divided — that is the job of cytokinesis.
Some textbooks combine late telophase with cytokinesis because they overlap in time. The nuclear envelope is forming while the cell is already beginning to pinch apart. Conceptually, though, they are separate events: telophase reorganizes the nuclear contents, while cytokinesis splits the cytoplasm.
Cytokinesis: Splitting the Cell
Cytokinesis is the physical division of the cell's cytoplasm into two daughter cells. In animal cells, a contractile ring made of actin and myosin filaments forms at the metaphase plate and tightens like a drawstring, pinching the cell in two. The indentation that forms is called the cleavage furrow, and it deepens until the cell is completely divided.
In plant cells, cytokinesis works differently because the rigid cell wall prevents pinching. Instead, vesicles from the Golgi apparatus accumulate at the center of the cell and fuse to form a cell plate — a new section of cell wall that grows outward until it reaches the existing cell wall, dividing the cell in two.
After cytokinesis, the two daughter cells enter G1 of interphase and begin the cell cycle again (or exit the cycle into a resting state called G0 if they are not actively dividing). Each daughter cell is genetically identical to the parent cell and to each other.
Mitosis vs. Meiosis: Key Differences
Mitosis produces 2 genetically identical diploid (2n) daughter cells. Meiosis produces 4 genetically unique haploid (n) daughter cells. This is the fundamental difference, and everything else flows from it.
Mitosis involves one division cycle (PMAT once). Meiosis involves two sequential divisions (PMAT I followed by PMAT II), which is why it produces four cells instead of two.
Mitosis does not involve crossing over or independent assortment. Meiosis includes both: during prophase I, homologous chromosomes exchange segments (crossing over), and during metaphase I, homologous pairs align randomly (independent assortment). These processes generate genetic diversity, which is why sexually reproducing organisms produce offspring that are genetically different from both parents.
Mitosis is used for growth, repair, and asexual reproduction. Meiosis is used exclusively to produce gametes (sex cells — sperm and eggs in animals, spores in plants). A skin cell divides by mitosis. A cell in the ovary or testis divides by meiosis.
A common exam question: how many chromosomes does a human skin cell have after mitosis? 46. After meiosis I? 23 (but each chromosome still has two chromatids). After meiosis II? 23 (single chromatids). Understanding these numbers and why they change is essential for biology exams.
Remembering the Stages: PMAT
The mnemonic PMAT helps you remember the four stages of mitosis in order: Prophase, Metaphase, Anaphase, Telophase. Some students use the phrase "People Meet And Talk" or "Please Make Another Taco" to remember the sequence.
For meiosis, the stages are PMAT I and PMAT II — the same four stages happening twice, with key differences in the first division (homologous pairs separate in meiosis I) versus the second division (sister chromatids separate in meiosis II, just like mitosis).
Quick summary to remember: Prophase = chromosomes condense, nuclear envelope breaks down. Metaphase = chromosomes line up at the middle. Anaphase = chromosomes are pulled apart to opposite poles. Telophase = nuclear envelopes reform, chromosomes decondense.
If you have a biology exam coming up and need practice with mitosis and meiosis questions, snap a photo of any problem from your textbook and send it to ScanSolve. The AI will walk you through the answer with detailed explanations of each stage.
Why Mitosis Matters: Cancer and Beyond
Cancer is fundamentally a disease of uncontrolled mitosis. Normal cells divide only when they receive growth signals and stop when they receive stop signals. Cancer cells ignore these signals and divide continuously, forming tumors. Many cancer treatments — chemotherapy, radiation — work by disrupting mitosis in rapidly dividing cells. Drugs like taxol (paclitaxel) stabilize spindle fibers so they cannot shorten during anaphase, preventing cell division.
Understanding mitosis also explains wound healing (skin cells divide to close a cut), growth (bones grow because cartilage cells divide at growth plates), and regeneration (your liver can regrow because liver cells retain the ability to divide). Some animals, like salamanders, can regenerate entire limbs through controlled mitotic division.
Mitosis is also the basis of cloning. Dolly the sheep was created by transferring a somatic cell nucleus into an enucleated egg cell, which then underwent mitotic divisions to develop into a complete organism. Every cell in Dolly was produced by mitosis from that single starting cell, making her genetically identical to the donor.