As the chromosomes condense, the nuclear envelope begins to disassemble (in most eukaryotes). Dissolution of the nuclear envelope signifies further progress in mitosis, and this landmark is used to identify a cell that has successfully finished prophase.
The transition into metaphase represents an intricate cellular event whereby all of the chromosomes move into a line (the metaphase plate ).
It is not fully understood how the cell "knows" when the chromosomes are precisely lined up. However, it is known that this is an important "checkpoint" and that the cell will not proceed to the next step until all of the chromosomes are properly positioned. The jostling of chromosomes can take a good deal of time. Therefore, metaphase is the stage of mitosis that takes the longest time in most cells.