Final answer:
The fragmentation of the nuclear envelope occurs only in eukaryotic cells during prophase, as prokaryotic bacteria do not have a nuclear envelope.
Step-by-step explanation:
The fragmentation of the nuclear envelope, which allows for chromosome separation, occurs in eukaryotic cells only, particularly during the prophase stage of mitosis. Prokaryotic cells, such as bacteria, do not undergo this process because they lack a nucleus and instead carry out cell division through a simpler process known as binary fission. In eukaryotes, once the nuclear envelope fragments, the spindle fibers can engage the kinetochores on the sister chromatids, facilitating their separation into daughter cells. The nucleolus also disappears, and the centrosomes move to opposite poles of the cell. This process is specific to eukaryotes only.