Mitosis
Mitosis is the eukaryotic process that divides one replicated nucleus into two genetically matched nuclei, helping cells grow, replace worn tissue, and reproduce asexually.
What mitosis is
Mitosis is nuclear division in eukaryotic cells. Before it begins, the cell has already copied its DNA during interphase. Mitosis then organizes and separates those duplicated chromosomes so each new nucleus receives one complete set. Cytokinesis, the division of the cytoplasm, usually follows or overlaps the end of mitosis.
Before mitosis
A cell does not enter mitosis from a blank state. During the cell cycle, it grows, copies its DNA, checks for problems, and prepares division machinery. By the time mitosis starts, each chromosome consists of two sister chromatids joined at a centromere.
Prophase and prometaphase
In prophase, chromosomes condense into visible structures, and the mitotic spindle begins to form from microtubules. During prometaphase, the nuclear envelope breaks down in many cells, allowing spindle fibers to attach to chromosome structures called kinetochores.
Metaphase
In metaphase, chromosomes line up near the cell's middle, often called the metaphase plate. This alignment is not cosmetic; it helps ensure that each sister chromatid is attached to spindle fibers from opposite poles before separation begins.
Anaphase
In anaphase, sister chromatids separate and move toward opposite poles of the cell. Once separated, each chromatid is considered an individual chromosome. Motor proteins, spindle microtubules, and chromosome attachments coordinate this movement.
Telophase and cytokinesis
In telophase, chromosomes arrive at opposite poles and begin to decondense. New nuclear envelopes form around each chromosome set. Cytokinesis then divides the cytoplasm: animal cells typically use a cleavage furrow, while plant cells build a cell plate that becomes a new cell wall region.
Accuracy and checkpoints
Mitosis must be accurate because misplaced chromosomes can damage cell function or development. Checkpoint systems help delay division if DNA is damaged or chromosomes are not properly attached to the spindle. These controls connect mitosis to cancer biology, aging, development, and tissue repair.
Why it matters
Mitosis matters because multicellular life depends on reliable cell replacement, growth, wound healing, and development. It also explains how many single-celled eukaryotes reproduce asexually. At the same time, errors in mitotic control can contribute to tumors, infertility, developmental problems, and genetic instability.