During which phase of mitosis does the mitotic spindle form?

The mitotic spindle begins to form during prophase, which is the first stage of mitosis. In prophase, the chromatin condenses into visible chromosomes, and each chromosome is made up of two sister chromatids joined at the centromere.

As prophase progresses, the nucleolus fades and the nuclear envelope begins to break down. This is also the stage where the centrosomes, which are the microtubule-organizing centers, move to opposite poles of the cell. They begin to produce spindle fibers, which are formed from microtubules. These fibers emanate from the centrosomes and extend across the cell, preparing to attach to the chromosomes' kinetochores at their centromeres.

This formation of the mitotic spindle is essential for the proper segregation of chromosomes later in mitosis, as it helps to ensure that each daughter cell will receive an accurate number of chromosomes. Thus, prophase is critical in setting up the infrastructure for dynamic changes that occur in subsequent phases of mitosis.