REPOPULATION OF POSTMITOTIC NUCLEOLI BY PREFORMED RNA II . Ultrastructure

The reconstruction of the nucleolus after mitosis was analyzed by electron microscopy in cultured mammalian (L929) cells in which nucleolar RNA synthesis was inhibited for a 3 h period either after or before mitosis . When synchronized mitotic cells were plated into a concentration of actinomycin D sufficient to block nucleolar RNA synthesis preferentially, nucleoli were formed at telophase as usual . 3 h after mitosis, these nucleoli had fibrillar and particulate components and possessed the segregated appearance characteristic of nucleoli of actinomycin D-treated cells, Cells in which actinomycin D was present for the last 3 h preceding mitosis did not form nucleoli by 3 h after mitosis though small fibrillar prenucleolar bodies were detectable at this time . These bodies subsequently grew in size and eventually acquired a particulate component . It took about a full cell cycle before nucleoli of these cells were completely normal in appearance . Thus, nucleolar RNA synthesis after mitosis is not necessary for organization of nucleoli after mitosis . However, inhibition of nucleolar RNA synthesis before mitosis renders the cell incapable of forming nucleoli immediately after mitosis . If cells are permitted to resume RNA synthesis after mitosis, they eventually regain nucleoli of normal morphology .