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Dividing amoebae were manually selected from the culture of Amoeba proteus, and so groups of synchronously dividing (synchronized) amoebae were obtained. These synchronized amoebae were maintained without food. In spite of starvation, individual amoebae in some particular groups were seen to divide, whereas in other groups of amoebae there was no division at all. The starving amoebae died not earlier than 2 weeks after the last division. A relative DNA content in isolated nuclei has been determined cytofluorometrically for each of 6 groups of synchronized starving amoebae, unable to divide. The nuclei were isolated in different intervals after division (after the feeding was ceased): 1.0-1.5 h, 1 day and up to 13 days with 1-2 day intervals. In the all groups of amoebae DNA synthesis occurred on the first 1-2 days after division. The nuclear DNA content in amoebae of 3 groups increased more than two-fold as compared with the 1 h level, in other 3 groups the nuclear DNA content did not exceed the doubled 1 h level, but probably exceeded the doubled postmitotic level. Later on, the nuclear DNA content in starving amoebae of each group was seen to decrease by 16-20%. Amoebae of 3 of the 6 groups were given the food organisms (Tetrahymena pyriformis) 8 days after division (after cessation of feeding). 2-3 days after refeeding some of these amoebae divided, and the nuclear DNA content of the refed amoebae proved to be higher than that in amoebae that continued to starve. It is suggested that the decrease of DNA content in the nuclei of starving amoebae and the increase of DNA quantity in the nuclei of refed amoebae may result from degradation and induction of synthesis of specific extra DNA synthesized in amoeba nuclei during each cell cycle. |
