Prophase is the beginning of mitosis (Fig. 3).
During interphase, the chromosomes look like a plate of spaghetti in the nucleus. It is difficult to pick out an individual chromosome because they are each so spread out. The chromosomes in the nucleus change from being loosely dispersed to becoming more condensed. This change in chromosome structure makes them easier to move around the cell, an important issue for what is about to happen. As the chromosomes condense, they get shorter and thicker and can be seen through the microscope as individual structures (Fig. 4). The chromosomes at prophase will consist of two identical parts called sister chromatids that stay connected at the centromere. It is now clear that the chromosomes have been replicated. Chromosome replication occurred during the S-phase of interphase (Fig. 1). Next, the nucleus dissolves. At the end of prophase, the spindle apparatus moves the replicated chromosomes to the center of the cell.