But as mitosis begins, the nuclear envelope starts to break up and disappear. Each chromosome has replicated during interphase and is therefore composed of two sister chromatids containing identical genetic information.
Early during prophase, the first stage of mitosis, the chromosomes become visible with a light microscope as they condense (that is, as they shorten, coil, and thicken). Also, a spindle apparatus (blue strands in the figures at left) begins to extend outward from each of the two centrosomes. These starlike configurations, composed of radiating microtubules, are also known as asters — Greek for stars — (see photomicrograph of asters >>).
After the nuclear envelope has disappeared, proteins
bind to the centromeres to make the kinetochores. Microtubules attach at the kinetochores and the chromosomes begin to move.
The name of this stage of mitosis
is derived from the Latin word pro,
Prometaphase. Some teachers like to distinguish a fifth stage of mitosis called prometaphase, which corresponds to that portion of mitosis occurring after
the disappearance of the nuclear envelope, but before
arrival of the chromosomes at the metaphase plate.