Figure 1. Structure of the mitotic spindle.
Mitosis can be staged into individual phases. In interphase, most of the chromatin is decondensed in the nucleus so that individual chromosomes cannot be seen, and the microtubules are organized in a radial array from the centrosome. During prophase, the chromosomes become highly condensed, and the centrosomes begin to separate. Nuclear envelope breakdown manifests the transition between prophase and prometaphase so that the individual chromosomes are no longer constrained in the nucleus. During prometaphase, kinetochore (k)-fibres (bundles of stabilized microtubules) connect the spindle microtubules and the kinetochores on the chromosomes, such that the chromosome can align at the spindle equator, which defines metaphase. The microtubules are uniformly oriented with their minus ends at the centrosome and their plus ends extending towards the spindle equator, where they often overlap. The astral microtubules emanate from the centrosomes and extend their plus ends towards the cell cortex. The movement of the chromosomes towards the poles occurs during anaphase A, and the two spindle poles separate during anaphase B. The nuclear envelope begins to reform and the DNA begins to decondense during telophase. An organized central spindle bundle of microtubules is also present. Cytokinesis divides the cytoplasm of the cell so that the two daughter nuclei are segregated into individual cells, which enter interphase and begin the process again.