Prophase and metaphase

Mitosis is called equational division because parent and daughter cells have the same number of chromosomes. Its nuclear division (karyokinesis) is split into four stages, prophase, metaphase, anaphase and telophase, but the process is continuous and the boundaries between stages are not sharp.

Prophase follows the S and G2 phases. Straight after S and G2 the new DNA molecules are intertwined and not distinct; prophase begins as the chromosomal material starts to condense and gets untangled.

By the end of prophase the chromosomes are compact, and each is seen to consist of two chromatids joined at the centromere.

The centrosome, duplicated during S phase, starts moving towards opposite poles. Each centrosome sends out microtubules called asters, and the two asters with the spindle fibres form the mitotic apparatus.

Under the microscope a cell at the end of prophase shows no Golgi complex, no endoplasmic reticulum, no nucleolus and no nuclear envelope.

Metaphase begins with the complete disintegration of the nuclear envelope, so the chromosomes lie spread in the cytoplasm. Condensation is now complete.

Metaphase is the stage at which chromosome morphology is easiest to study. Each metaphase chromosome is two sister chromatids held together by the centromere.

Kinetochores are small disc-shaped structures on the surface of the centromere, where spindle fibres attach.

All the chromosomes come to lie at the equator: one chromatid of each chromosome is attached by its kinetochore to fibres from one pole, and its sister to fibres from the other pole. The plane of alignment is the metaphase plate.