What occurs during the prophase of mitosis?

During prophase of mitosis, several key processes occur that are crucial for the successful division of the cell. One of the primary events is that chromosomes become visible as they condense and coalesce into distinct structures, which can be observed under a microscope. This visibility is due to the tightly packed chromatin fibers which form into recognizable chromosomes.

Additionally, centrioles, which are important for organizing the mitotic spindle, begin to separate and move towards opposite poles of the cell. This movement is critical as it helps facilitate the formation of the spindle fibers that will later assist in pulling the chromatids apart during the later stages of mitosis.

The other options refer to events that occur at different stages of mitosis. For instance, lining up at the cell's equator happens during metaphase, separating chromatids occurs in anaphase, and the reformation of nuclei around chromatids takes place in telophase. Each of these processes is essential to ensure that genetic material is correctly divided and each daughter cell receives the appropriate number of chromosomes.