Prometaphase is the phase of mitosis following prophase and preceding metaphase, in eukaryotic somatic cells. Prometaphase is sometimes simply included as part of the end of prophase and early metaphase.
In prometaphase, the nuclear membrane breaks apart into numerous "membrane vesicles", and the chromosomes inside form protein structures called kinetochores. Kinetochore microtubules emerging from the centrosomes at the poles (ends) of the spindle reach the chromosomes and 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 kinetichores.
Metaphase, the next step in mitosis, is where the chromosomes align.
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.
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