Prophase is the first stage of mitosis, the process by which a cell divides to form two identical daughter cells. During prophase, the chromatin condenses into visible chromosomes, which are made up of two sister chromatids joined at a region called the centromere. The nuclear envelope begins to break down, allowing the chromosomes to move freely within the cell. Additionally, the mitotic spindle starts to form during prophase. This structure is made of microtubules and helps to organize and separate the chromosomes later in the process. The spindle fibers extend from the centrosomes, which move to opposite poles of the cell, preparing for the next phase of mitosis.