104 ISOTOPE METHODS 



the same amount of irradiation, it was shown that this decrease was 

 due to the disintegration and not to the bombardment of the other 

 phages by the emitted beta particles. This kind of experiment is 

 called a reconstruction experiment, and is frequently used to test hy- 

 potheses. In the picturesque jargon appropriate to the experiment, it 

 was shown that very little "murder" occurred in the reconstruction 

 experiment, so that, by inference, the deaths of the P 32 -labeled phages 

 had to be due to "suicide" coming from the disintegration of P 32 which 

 had been incorporated into the phages. 



These P 32 phages were used to infect unlabeled cells in an unlabeled 

 medium, and at various times many aliquots were taken. The aliquots 

 were frozen rapidly, using liquid nitrogen as coolant, and stored at 

 liquid nitrogen temperature (about — 200°C). Each day an aliquot of 

 each infection time was warmed, and the number of cells yielding phage 

 progeny was determined. In this way, for instance, it was found that 

 after one minute of infection, the ability to yield phage progeny de- 

 creased at the same rate as the viability of the free phage particle. That 

 is, the injected nucleic acid had apparently not been altered at all, for 

 it suicided at the same rate as it would have suicided outside the cell. 

 After about one-third of the latent period, the cell was completely in- 

 sensitive to the suicide, in that there was no decrease at all in the number 

 of cells yielding viable phage progeny. The obvious interpretation is 

 that by this time in the latent period the nucleic acid has replicated 

 itself out of nonradioactive materials, as is necessarily the case since 

 there are only nonradioactive materials present. 



A complementary experiment was made using nonradioactive phages 

 to infect radioactive cells in a radioactive medium. In this case, the 

 cells were always stable to disintegrations, since the infecting phage 

 contained no radioactive atoms. The stability of the phage-yielding is 

 additional evidence that "murder" makes very little contribution to the 

 total effect, else the nonradioactive phage DNA would have been killed. 



It is of great interest that the designers of this experiment decided 

 to do a complete experiment, however foolish it may have seemed before 

 the results were known. They infected radioactive cells with radioactive 

 phages in a radioactive medium. Since all nucleic acids are now radio- 

 active, it is evident that there can never be any stabilization of the 

 cell's ability to yield phages. Nevertheless, when the experiment was 

 made, a stabilization very similar to that in the first of the experiments 

 was found: when about one-third of the latent period had passed, there 

 was no decrease in the number of cells yielding viable phage progeny! 

 This experiment remains to be satisfactorily interpreted. 



The suicide method has been used also in studies of bacterial growth 

 and function. In addition, suicide has been found to occur when atoms 



