Metaphase

Metaphase is the second phase of mitosis, and its onset is marked by the complete disintegration of the nuclear envelope. Once the nuclear envelope breaks down, the chromosomes are freed and become spread out through the cytoplasm of the cell.

By this stage the condensation of the chromosomes is complete, so they can be seen very clearly under the microscope. For this reason, metaphase is the stage at which the morphology of chromosomes is most easily studied — their shape and structure are at their most distinct.

At metaphase, each chromosome is made up of two sister chromatids held together by the centromere. On the surface of the centromeres are small disc-shaped structures called kinetochores. These kinetochores act as the points of attachment where the spindle fibres bind to the chromosomes, allowing the chromosomes to be moved into position at the centre of the cell.

As a result, metaphase is characterised by all the chromosomes coming to lie at the equator of the cell. One chromatid of each chromosome is linked, through its kinetochore, to spindle fibres coming from one pole, while its sister chromatid is linked, through its own kinetochore, to spindle fibres from the opposite pole. The plane along which the chromosomes line up at this stage is called the metaphase plate. …