Final answer:
The phase of mitosis where sister chromatids condense, the nuclear envelope breaks down, and spindle formation begins is called prophase.
Step-by-step explanation:
The stage of mitosis during which the sister chromatids condense, the nuclear membrane dissociates, and the spindle microtubules begin to form is known as prophase. At this stage, the nuclear envelope breaks down into small vesicles, and the organelles like the Golgi apparatus and endoplasmic reticulum disperse toward the cell's periphery.
The centrosomes migrate to the opposite poles of the cell, and microtubules extend between them, lengthening and pushing the centrosomes further apart. The condensation of the sister chromatids makes them visible under a light microscope.