prophase /prō″fāz/ [Gk, pro + phasis, appearance] , the first of four stages of nuclear division in mitosis and in each of the two divisions of meiosis. In mitosis the chromosomes progressively shorten and thicken to form individually recognizable elongated double structures composed of two chromatids held together by a centromere. The nucleolus and nuclear membrane disappear, the spindle and polar bodies are formed, and the chromosomes begin to migrate toward the midplane of the developing spindle. In the first meiotic division, prophase is complex and subdivided into five stages: leptotene, zygotene, pachytene, diplotene, and diakinesis. In the second meiotic division the same processes occur as in mitotic prophase. See also anaphase, interphase, meiosis, metaphase, mitosis, telophase.
