Inhibition of nucleic acid synthesis caused by X-irradiation of the nucleolus

Living cells grown in tissue culture have been irradiated with a narrow beam of soft X-rays of effective diameter 2$\cdot $5 $\mu $. The method has been used in conjunction with quantitative ultra-violet photomicrography to investigate the role of the nucleolus in nucleic acid synthesis. The results show that X-irradiation of the nucleolus reduces the amount of nucleic acid synthesized in the nucleus when measured 3 to 7 h after irradiation, whereas X-irradiation of the nuclear sap with an equal dose does not show this effect. The significance of this is discussed.