INFLUENCE OF THE NUCLEOLUS ON MITOSIS AS REVEALED BY ULTRAVIOLET MICROBEAM IRRADIATION

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Abstract
Exposure of one nucleolus in a grasshopper neuroblast to a microspot of high-intensity ultraviolet radiation for 3 seconds permanently stops mitosis of cells irradiated in stages from late telophase to the middle of middle prophaae. During the latter half of middle prophase, the cell becomes mitotically less sensitive to nucleolar irradiation and by late prophase is completely insensitive to it. Microspot irradiation of a non-nucleolar region of the nucleus causes mitotic delay but does not usually prevent cell division. These observations support the hypothesis that the nucleolus has an intimate functional relation to the mitotic process.