Prometaphase is the phase of mitosis following prophase and preceding metaphase, in eukaryotic somatic cells.The nuclear envelope breaks into fragments and disappears. 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 microtubles 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 envelope disassembles. This allows the microtubules to reach for the chromosomes to align them at the metaphase plate to ensure proper segregation of the genetic material.

Some protists, such as algae, undergo a variation called "closed mitosis" where the microtubules are able to penetrate an intact nuclear envelope.

Formation of kinetochore

Following nuclear envelope diassembly, kinetochores form at the centromeres of the chromosomes, with one for each chromatid.

A kinetochore is a complex protein structure where microtubules interact with the chromosome. Although kinetochore structure and function are not fully understood, it is known that it contains a molecular motor.

When a microtubule connects with the kinetochore, the motor activates, using energy from ATP to "crawl" up the tube toward the originating centrosome. The kinetochore provides the pulling force necessary to later separate the chromosome's two chromatids.

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" 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 is 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.

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