Prometaphase

Prometaphase is a stage of mitosis that follows prophase and precedes metaphase. During this phase, the nuclear envelope breaks down, releasing condensed chromosomes into the cytoplasm and permitting spindle microtubules to interact with kinetochores. Chromosomes, each consisting of two sister chromatids held at the centromere, become more distinct under light microscopy as they attach to spindle fibers.

Kinetochore microtubules capture the chromosomes by attaching at kinetochores. Attachments form between microtubule ends and kinetochores from opposite poles to establish bi-orientation. The dynamic instability of microtubules, together with motor proteins such as dynein and various kinesins, promotes movement of chromosomes toward the center of the cell and toward opposite poles. Some microtubules not attached to kinetochores interact with the cell cortex or with each other, contributing to spindle organization. The result is chromosome congression to the metaphase plate, and the maturation of a bipolar spindle with astral, kinetochore, and polar microtubules.

By the end of prometaphase, most or all chromosomes are attached to spindle microtubules and aligned near the center, and the cell proceeds into metaphase. If improper attachments persist, the spindle assembly checkpoint delays anaphase until errors are corrected.