102 ISOTOPE METHODS 



first added to practically 0% after about 3 minutes of adsorption; the 

 phosphorus after that time was always found firmly attached to, or 

 within, the bacteria. Here again, after 3 minutes there was 100% in- 

 fection of the cells. But during the first 3 minutes, the percentage of 

 cells yielding progeny phages was strictly equal to the percentage of the 

 P 32 which could no longer be stripped. Thus, since the P 32 is contained 

 entirely in the DNA of the phages, it was deduced that the nucleic acid 

 was all injected into the cells within the first 3 minutes, and after that 

 time the S 35 -labeled protein was all that remained on the bacterial 

 surface. Thus the injected nucleic acid was able to replicate itself and 

 the normal protein coats of the phage. The genetic information and the 

 control of metabolic functioning were thus shown both to reside entirely 

 in the DNA (except for the possible 5% of the protein which was not 

 strippable). 



(c) The lifetime of human red blood cells has been determined through 

 the use of the nonradioactive isotope N 15 . The amino acid glycine was 

 synthesized with the isotope, and fed to human beings. This amino acid 

 is incorporated into the protein hemoglobin, so that within a few days, 

 there was a significant amount of N 15 in the red blood cells of the sub- 

 jects. The amount of N 15 in the red blood cells began to show a signifi- 

 cant decrease only after about 100 days, and reached half its peak value 

 after about 140 days. More refined calculations, taking into account 

 the time required to build the N 15 -containing glycine into the hemoglobin 

 molecule, yield a result of 127 days for the average life span of human 

 red blood cells. The fact that the content of N 15 drops, as expected from 

 the complete loss of label, leads to the conclusion that very little, if any, 

 of the hemoglobin molecule is re-utilized for the synthesis of new 

 erythrocytes. 



2. Localization experiments 



In these experiments, the information sought is generally of a strictly 

 qualitative nature, although some quantitative information is not in- 

 frequently obtained in addition. The most obvious experiments have to 

 do with cytological studies of localization within cells, for if the cell 

 is large enough, the information can be obtained by radioautography. 



Radioautography involves feeding the isotope to the organism and, at 

 suitable later times, stopping the metabolism by appropriate agents. The 

 intact cells are used, if possible, or else the cells are sectioned by the 

 now-standard techniques for preparation of specimens for light and 

 electron microscopy. The material is then placed in contact with a photo- 

 graphic emulsion sensitive to the radiation emitted by the isotope in use. 

 The material sits in contact with the film for a period found to be 



