| Summary: | Evidence is cited to support the assumption that the time of mitosis and DNA synthesis in Physarum polycephalum is controlled to maintain a constant DNA: mass ratio. Five possible model mechanisms for the control are discussed and analysed to obtain predictions concerning the response to experimentally induced changes in the DNA: mass ratio. Experiments designed to test these predictions showed the following: 1. If mitosis is delayed either by FUdR and uridine treatment or by puromycin treatment the next intermitotic period is equivalently shortened. 2. If a proportion of DNA is destroyed by UV irradiation then, as shown by other workers, subsequent intermitotic periods are shortened. However, it was shown that an intermitotic period cannot be shortened beyond a minimum length and that this minimum length is independent of the growth rate. 3. If the DNA: mass ratio is lowered by UV irradiation by an amount which is not sufficient to cause the next period to be shortened to the minimum length, then only one shortened intermitotic period is required to restore the steady-state DNA: mass ratio. 4. Comparison of the length of the shortened period following UV irradiation with the amount of DNA destroyed agree with the predictions of two of the models considered. 5. If more than 50% of the DNA is destroyed, two successive mitoses can occur in the complete absence of mass increase. It is argued that these results are inconsistent with three of the models which may be eliminated from consideration. Consideration of experimental evidence from other cell types suggest there are difficulties in applying one of the two remaining models to the more general situation.
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