Nucleolus and nuclear membrane always disappears
during mitosis and meiosis. At the beginning of mitosis, the
chromosomes condense, the nucleolus disappears, and the
nuclear envelope breaks down, resulting in the release of
most of the contents of the nucleus into the cytoplasm. At
the end of mitosis, the process is reversed. In diakinesis stage
of prophase I of meiosis I, the nucleolus and nuclear membrane
disappear and chromosomes set free in the cytoplasm.