TRACER EXPERIMENTS 101 



ably less than the total number that actually occur. As a result, activity 

 has to be corrected for the efficiency of the counter (by using a standard 

 source of radioactivity), or else the results are given as counts per min- 

 ute per microgram, which is entirely valid so long as only relative radio- 

 activity levels are necessary for the conclusions of the study. 



1 . Tracer experiments 



(a) One of the most significant metabolic studies using the tracer 

 aspects of isotopes has been the determination of the pathway of uptake 

 of C0 2 by plants. In these experiments, C 14 2 was bubbled into chambers 

 containing photosynthesizing algae, and the algal metabolism was 

 stopped after various times by taking aliquots and placing them in 

 metabolic poisons. The algae were then fractionated by standard bio- 

 chemical techniques, and the amount of radioactive carbon in the vari- 

 ous compounds was determined. In this fashion it was found that the 

 carbon first appears in the 3-carbon molecule of phosphoglyceraldehyde. 

 The subsequent metabolic journey could be deduced by noting which 

 compounds contained the tracer in the next aliquot, etc. Finally, a sub- 

 stantial proportion of the metabolic pathway was inferred. The pathway, 

 of course, then had to be checked by establishing the existence of the 

 requisite enzymes and of energy available in the proper amounts and in 

 the proper forms (i.e., in the proper energy-rich compounds) to effect the 

 chemical conversions indicated by the tracer study. 



(b) An equally significant genetic study was carried out using sulfur 

 and phosphorus labeling of bacteriophages. At the time of this study 

 (1952) it was not known with compelling certainty that the nucleic 

 acids carried the genetic information for the entire organism. Hershey 

 and Chase grew two batches of phages, one in S 35 , the other in P 32 . They 

 infected unlabeled bacteria with the S 35 phages, and at various times 

 after infection, aliquots were taken to be placed in a blender for study. 

 Operation of the blender was shown to strip phages from the bacterial 

 surface. In the experiment, about 95% of the sulfur-labeled protein 

 could be stripped at all times during the latent period. If the blenderiz- 

 ing was done just after adsorption of the phages (within 2 or 3 minutes) 

 the number of cells yielding progeny bacteriophages was very small. By 

 about 3 minutes after adsorption was initiated, the stripping had no 

 effect on the infective process; all cells yielded progeny phages. This 

 experiment showed that the protein is almost irrelevant (to within about 

 5% of the total protein), since normal phage synthesis could ensue with- 

 out the presence of about 95% of the protein. 



When this experiment was repeated with the P 32 phages, the amount 

 of strippable P 32 decreased steadily from 100% when the phages were 



