Mitosis (M Phase) - the Equational Division

Mitosis occurs in somatic (body) cells; the daughter cells receive the same number of chromosomes as the parent - hence it is called the equational division. The M phase has two events: karyokinesis (division of the nucleus) followed by cytokinesis (division of the cytoplasm). Karyokinesis has four stages:

1. Prophase

  • Chromatin condenses into compact chromosomes (each with two sister chromatids joined at the centromere).
  • The spindle apparatus begins to form; in animal cells the two centriole pairs move towards opposite poles.
  • By the end, the nuclear envelope and the nucleolus disappear and the ER and Golgi break down.

2. Metaphase

  • The chromosomes are the most condensed and line up at the equator (metaphase plate).
  • Spindle fibres attach to the kinetochores of the centromeres.

3. Anaphase

  • The centromeres split, so the two sister chromatids separate and move as daughter chromosomes towards opposite poles (centromere leading, arms trailing).

4. Telophase

  • The chromosomes reach the poles and decondense; the nuclear envelope and nucleolus reappear; two daughter nuclei form.

Cytokinesis

  • Animal cell - a cleavage furrow constricts the cell from the outside inward.
  • Plant cell - a cell plate forms in the middle and grows from the centre outwards (because the rigid wall cannot furrow).

Significance of mitosis

  • Produces genetically identical daughter cells; maintains the chromosome number.
  • Responsible for growth, repair and cell replacement (e.g. skin, gut lining) and for the healing of wounds. In plants it keeps meristems dividing.

High-yield one-liners: chromosomes most condensed at metaphase; centromeres split at anaphase; nuclear envelope disappears in prophase and reappears in telophase; plant cytokinesis = cell plate (centre → outward); animal = cleavage furrow.

Visual - The Stages of Mitosis

Prophase, metaphase, anaphase and telophase of mitosis with cytokinesis

Prophase (condensing chromosomes, forming spindle), metaphase (chromosomes at the equator), anaphase (sister chromatids separate), telophase and cytokinesis (cell plate in plants, cleavage furrow in animals).