Final answer:
During prophase, the initial phase of mitosis, chromosomes become visible as chromatin condenses, the nuclear envelope disappears, and the spindle apparatus forms to prepare for chromosome segregation.
Step-by-step explanation:
During prophase, the first phase of mitosis, a few critical events take place to prepare the cell for division. The loosely packed chromatin coils tightly and condenses to form visible chromosomes. This condensation allows chromosomes to be visible under a microscope. Simultaneously, the nuclear envelope breaks down and disappears, removing the barrier between the nucleus and the cytoplasm.
Concurrently, spindle fibers emerge from the centrosomes, with the spindle apparatus forming and attaching to the kinetochores on the sister chromatids. The centrosomes move to opposite poles of the cell, and the nucleolus also disappears during this phase, indicating that the cell is gearing up to ensure proper chromosome segregation to the daughter cells.