Synthesis of Desoxyribonucleic Acid in Normal and Irradiated Cells and Its Relation to Chromosome Breakage

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Abstract
(1) The normal mitotic cycle in the meristem of main roots of Vicia faba seedlings is estimated to take 30 h, of which 4 are spent in division, 12 between the end of division and the beginning of 32P uptake into new DNA, 6 in DNA synthesis as judged by 32P uptake, and 8 between the end of synthesis and the prophase of the next division. (2) A dose of 13 × 103 erg/g (140r) of X-rays does not immediately stop DNA synthesis in cells which are in or near the period of synthesis at the time of irradiation, but causes these cells to be delayed in entering division for a period of about 4 hours. (3) The same dose of X-rays causes cells which are at other stages of the mitotic cycle at the time of irradiation to be delayed in entering synthesis for a period of 10 h or more. (4) Neutron doses of 1·1, 2·4 and 4·9 × 103 erg/g and 13 × 103 erg/g of X-rays have, within the standard deviations, the same effect on the number of cells synthesising DNA during 12 h following irradiation. (5) The estimated delay in entering division in cells irradiated with three different doses of neutrons indicates that this delay is dependent on dose, and that neutrons are not markedly more efficient than X-rays in causing it. (6) It is concluded that the immediate delay in entering division which results from irradiation cannot be attributed to delay in DNA synthesis, and that delay in entering division and chromosome breakage respond differently to irradiation and are therefore initially independent effects. (7) The question is discussed of the possible relationship between the stages of maximum sensitivity to chromosome breakage by radiations and by chemicals and the stage at which DNA is synthesized.