Prometaphase is a phase of mitosis that follows prophase and precedes metaphase in eukaryotic somatic cells. During prometaphase, the nuclear envelope breaks apart into numerous "membrane vesicles," exposing nuclear structures and its contents to the cytoplasm. The chromosomes inside form protein structures called kinetochores. Other key events that occur during prometaphase include:

• Chromosome attachment: The chromosomes attach to the mitotic spindle.

• Formation of the spindle: 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.

• Spindle checkpoint: The cell checks to make sure that all the chromosomes are at the metaphase plate with their kinetochores correctly attached to microtubules. This is called the spindle checkpoint and helps ensure that the sister chromatids will split evenly between the two daughter cells when they separate in the next step.

• Transition to metaphase: The role of prometaphase is completed when all of the kinetochore microtubules have attached to their kinetochores, upon which metaphase begins.

In summary, during prometaphase, the nuclear envelope breaks down, the chromosomes attach to the mitotic spindle, and the spindle checkpoint ensures that all the chromosomes are properly aligned before proceeding to metaphase.