In which mitotic phase do the nuclear envelope and nucleoli disappear?

During prophase, the first stage of mitosis, significant changes occur that are critical for the cell's division process. One of the key events of prophase is the breakdown of the nuclear envelope, which encases the nucleus, and the disintegration of the nucleoli, the structures within the nucleus responsible for ribosome synthesis. This breakdown allows the spindle fibers, which are essential for separating the chromosomes later in mitosis, to access the chromosomes that are becoming visible as they condense. The loss of the nuclear envelope and nucleoli marks the transition into the prophase stage, setting the stage for chromosome alignment in the subsequent phase of mitosis. This is a crucial step that prepares the cell for the accurate distribution of genetic material into the two daughter cells.