[Effect of ultraviolet irradiation on infrastructure and mitotic activity of Tetrahymena pyriformis]

Effects of U.V. irradiation, 3,600 erg/mm2, administrated at a low dose rate, over 75 mn at the very beginning of the cell cycle, were investigated in Tetrahymena pyriformis. Irradiation results in mitotic activity changes, followed either by cell death or cell survival with subsequent normal division activity. Nucleolar fusion, possible intracytoplasmic extrusion of nucleolar aggregates and particular pictures of nucleologenesis are described. Simultaneous cytoplasmic lesions can contribute to the delayed death of Tetrahymena after U.V. irradiation.