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The mitotic spindle is assembled during prophase, which is the stage where the chromatin condenses into visible chromosomes. The spindle apparatus is crucial for chromosome movement and separation during cell division. In prophase, the centrosomes, which are the organizing centers for the microtubules that make up the spindle, move to opposite poles of the cell. As the centrosomes migrate, they begin to form microtubules that extend towards the chromosomes, creating the mitotic spindle. This process is essential for ensuring that each daughter cell receives the correct number of chromosomes during mitosis.

In contrast, during telophase, the chromosomes begin to de-condense, and the spindle apparatus disassembles. In metaphase, the spindle is already fully formed and the chromosomes align at the metaphase plate, but assembly has already occurred in prophase. The S phase refers to the period when DNA is replicated, and while it is crucial for preparing cells for division, it is not the stage where the spindle itself is assembled.