Increased3H-thymidine Incorporation into DNA of Irradiated Slime Mould

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Abstract
Synchronous cultures of the slime mould Physarum polycephalum were incubated in H3-thymidine. Some cultures were exposed to 25 kr of X-rays. The specific activity of DNA from irradiated cultures was 40% greater than that from non-irradiated preparations. Feulgen spectrophotometric measurements indicated the same amount of DNA per nucleus in both cases. After exposure of samples to ultra-violet light the proportion of thymine to thymine dimers was the same in irradiated and non- irradiated samples, indicating there had been no large addition of thymine to DNA. Heating and cooling in 1%, formaldehyde did not release any pieces of low molecular weight that were separable from DNA on Sephadex columns, a finding that argues aganist loose association of precursor and DNA. The possibility that specific activity of the precursor pool may be changed by irradiation, or that replacement of damaged sites on DNA may occur, are offered as possible explanations.