Stages of Mitosis

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Chromosomes being dragged toward the metaphase plate.
Chromosomes (red) being dragged toward the metaphase plate. Image: Ted Salmon, NIH

During interphase, the chromosomes, which are made of heterochromatin and euchromatin, are contained in the nucleus (see video above).

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 upper two 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.

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Etymology: The name of this stage of mitosis is derived from the Latin word pro, meaning before.

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. The spindle apparatus >>
Centrosomes >>
DNA replication >>
Sister chromatids >>
Nuclear envelope >>

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