Mitosis

Mitosis is the stage of the cell cycle in which a eukaryotic cell separates its already copied chromosomes into two identical nuclei. It is usually followed by cytokinesis, the splitting of the cytoplasm, which produces two daughter cells with the same chromosome number and the same genetic information as the parent. Mitosis is how the body grows, replaces worn-out cells and repairs wounds, and how many single-celled eukaryotes reproduce.

It runs through a continuous series of stages. In prophase, the chromosomes condense, the nuclear envelope starts to break apart and a spindle of microtubules forms. In prometaphase, spindle fibers attach to each chromosome at a protein structure called the kinetochore. In metaphase, the chromosomes line up across the middle of the cell; in anaphase, the two sister chromatids of each chromosome are pulled to opposite poles; and in telophase, new nuclear envelopes form around the two sets.

Errors that leave a daughter cell with extra or missing chromosomes are rare but can damage cells and are common in cancers. Mitosis differs from meiosis, which halves the chromosome number to make eggs and sperm.