24 CELL HEREDITY 



a site on the baeterial wall, the DNA enters the cell, but the coat remains 

 outside, attached to the wall. In the Waring Blendor, the coats are torn 

 off the cells by shearing forces and remain in the suspending liquid. The 

 bacteria are not harmed by the blendor treatment, and phage multiplica- 

 tion is normal in these cells. This interpretation has been fully sub- 

 stantiated by electron micrographs of the infection process. 



This experiment demonstrates beautifully that it is primarily the DNA 

 which enters the host cell, leaving most of the protein outside. None- 

 theless, a nonsulphur containing protein, amounting to some 1-2 per cent 

 of the total, has been found in association with the entering DNA. This 

 protein is a verv simple one, consisting mainly of the amino acids, lysine, 

 glutamic, and aspartic acids, and more likely represents a nonspecific 

 carrier of the DNA than hereditary material. 



Although this experiment itself does not provide a decisive demonstra- 

 tion of the postulated role of DNA as hereditary material, there are 

 manv supporting data from other experiments with the phage system 

 fully consistent with this hypothesis. For example, strong support comes 

 from the analysis of lethal effects of incorporated P^^ resulting from 

 atomic disintegrations within frozen, stored phage. 



Tobacco mosaic virus 



Another system which provides evidence of the nucleic acid composi- 

 tion of hereditary material is that of the tobacco mosaic virus (TMV). 

 TMV contains about 6 per cent ribonucleic acid (RNA), the rest being 

 protein. It has been shown in a number of experiments performed in- 

 dependently by Schramm in Germany and by Fraenkel-Conrat in Cali- 

 fornia that the TMV hereditary determinants are carried in the RNA. A 

 method was developed for separation of RNA from protein, and it was 

 demonstrated that the RNA alone is infectious; cells infected with this 

 naked RNA produce new virus particles, complete with their protein 

 envelopes. 



Although the RNA infection experiment demonstrates that hereditary 

 information can be carried by RNA alone, one may still ask whether the 

 protein moeity may also carry information. The following experiment 

 was designed to answer this question. 



In chemical studies it was observed that the RNA itself is highly 

 unstable, but that it could be stabilized by adding a protein, not neces- 

 sarily TMV protein. One protein which worked well was that extracted 

 from a related virus. Holmes rib-grass virus (HRV). Similarly, the 

 RNA of HRV is infectious, but more successfully so if stabilized with 

 a protein, such as that from TMV. Thus it is possible to make two 



