Mitosis
Mitosis is the division of a cell nucleus that produces two genetically identical daughter cells, used for growth, repair, and asexual reproduction.
Definition
Mitosis is how one cell splits into two cells that are exact copies of each other: the cell first copies all its DNA, then divides so each new cell gets a full set. In a eukaryotic cell the duplicated chromosomes are divided equally into two nuclei through prophase (chromosomes condense), metaphase (chromosomes line up at the middle), anaphase (sister chromatids are pulled apart), and telophase (nuclei re-form), followed by cytokinesis, which splits the cytoplasm to give two genetically identical daughter cells used for growth, tissue repair, and asexual reproduction. Mitosis is the M phase of the cell cycle that follows DNA replication in S phase. Condensed sister chromatids, held together by cohesin, attach at their kinetochores to spindle microtubules from opposite poles, and the spindle assembly checkpoint delays anaphase until every kinetochore is under bipolar tension; separase then cleaves cohesin, chromatids move to the poles, the nuclear envelope reassembles, and cytokinesis proceeds by an actomyosin contractile ring in animals or a cell plate in plants. Progression is driven by cyclin-dependent kinases, and errors produce aneuploidy, which is common in tumors.
Example
When you get a cut, the skin cells around it divide by mitosis to make new skin cells and close the wound, each with the same DNA as the old one. A human skin cell with $46$ chromosomes copies them to make $92$ chromatids, then mitosis separates them so each daughter cell gets exactly $46$, and the cells lining the human intestine are replaced about every $3$ to $5$ days this way. A rapidly dividing human cell completes the cycle in about $24$ hours, of which mitosis itself takes about $1$ hour and S phase, copying $3.1$ billion base pairs, takes about $8$ hours; checkpoint failure can leave a daughter cell with $45$ or $47$ chromosomes.
Key Insight
You grew from one cell into trillions by mitosis, and your body still makes millions of new cells every second to replace old ones. Mitosis makes copies while meiosis makes variety: mitosis yields two identical body cells, meiosis four different sex cells with half the chromosomes. Mitosis is a mechanical process with a molecular safety inspection, and cancer drugs such as taxanes and vinca alkaloids work by jamming the spindle, which trips the checkpoint and pushes dividing cells into apoptosis.