Prometaphase

The nuclear membrane dissolves as the microtubules invade the nuclear space.

The 23 human chromosome territories during prometaphase in fibroblast cells.

Prometaphase is the phase of mitosis following prophase and preceding metaphase, in eukaryotic somatic cells.

In Prometaphase, the nuclear membrane disintegrates. Microtubules emerging from the centrosomes at the poles (ends) of the spindle reach the chromosomes, now highly condensed. At the centromere region, each sister chromatid has a protein structure called a kinetochore. Some of the spindle microtubules attach to the kinetochores, throwing the chromosomes into agitated motion. Other spindle microtubules make contact with microtubules coming from opposite pole. Forces exerted by protein "motors" associated with spindle microtubules move the chromosomes toward the center of the cell.

Open and closed mitosis

In "open mitosis", microtubules which have radiated from the two centrosomes located at the opposite poles of the cells invade the nuclear space as the nuclear membrane disassembles. This allows the microtubules to reach for the chromosomes and attach to their kinecticores.

Most fungi and many protists, such as algae, undergo a variation called "closed mitosis" where the microtubules are able to penetrate an intact nuclear envelope through a spindle pole body.

Metaphase, the next step in mitosis, is where the chromosomes align.

There are two types of microtubules...

The microtubules are composed of two types, kinetochore microtubules and non-kinetochore microtubules.

• Kinetochore microtubules begin searching for kinetochores to attach to.
• A number of non-kinetochore microtubules or polar microtubules find and interact with corresponding nonkinetochore microtubules from the opposite centrosome to form the mitotic spindle.

Transition from prometaphase to metaphase

The role of prometaphase is completed when all of the kinetochore microtubules have attached to their kinetochores, upon which metaphase begins. An unattached kinetochore, and thus a non-aligned chromosome, even when most of the other chromosomes have lined up, will trigger the spindle checkpoint signal. This prevents premature progression into anaphase by inhibiting the anaphase-promoting complex until all kinetochores are attached and all the chromosomes aligned.

Early events of metaphase can coincide with the later events of prometaphase, as chromosomes with connected kinetochores will start the events of metaphase individually before other chromosomes with unconnected kinetochores that are still lingering in the events of prometaphase.