Mitosis

Mitosis is an equational division: the daughter cells get exactly the same number of chromosomes as the parent cell. It is a continuous process, but is described in four stages that run into one another: prophase, metaphase, anaphase, telophase.

Prophase. The long, thread-like chromatin condenses into visible chromosomes. Each chromosome is now two chromatids held together at the centromere. The two centrosomes (doubled in interphase) move to opposite sides of the nucleus, each sending out rays (asters). The asters and the spindle fibres between them make up the mitotic apparatus. By the end of prophase, the Golgi complex, ER, nucleolus and nuclear envelope can no longer be seen.

Metaphase. The nuclear envelope has completely gone and the chromosomes are at their most condensed, so this is the best stage to study their shape. Spindle fibres attach to small discs on the centromere, the kinetochores. The chromosomes are pulled to line up across the middle of the cell, on the metaphase plate.

Anaphase. Each centromere splits, and the two chromatids of every chromosome separate. Now called daughter chromosomes, they move to opposite poles, centromere first, with the arms trailing behind.

Telophase. The chromosomes reach the poles and uncoil again into chromatin. A nuclear envelope forms around each group, giving two nuclei. The nucleolus, Golgi complex and ER form again.