Viruses — Cell Parasites 71 



appears to be filled with new particles which escape through the dis- 

 rupted cell membrane. 



It has been found that only the nucleic acid of the bacteriophage 

 passes into the bacterial cell, the protein sack in which it was con- 

 tained remaining outside. (Hershey and Chase, 1952.) 



Once inside the bacterium, a very extraordinary series of changes 

 occurs. The bacteriophage takes over, as it were, the whole chemistry 

 of the cell for its own purpose. The cell begins to synthesize 

 substances to make new bacteriophage. In some cases this involves 

 making compounds which were not even present in the normal cell; 

 thus T2 bacteriophage contains the base hydroxymethylcytosine 

 instead of cytosine which is normally present in cells. 



With the bacteriophage called T2, it has been shown that, for the 

 first seven minutes after the entry of the agent into the bacterium, 

 little can be observed to happen. The bacteriophage material appears 

 to 'dissolve' and for a short time cannot be detected in the cell. After 

 nine minutes the synthesis of new bacteriophage particles begins 

 and continues until about the thirteenth minute, when all the avail- 

 able material in the cells has been used up and an average of 130 new 

 particles of bacteriophage are formed in each bacterium. The bac- 

 teriophage particle has succeeded in imposing its pattern on the cell 

 substances and has used them for its own purposes. 



In the case of tobacco mosaic virus, it has been found by Fraenkel- 

 Conrat and by G. Schramm that, if the protein is stripped off the 

 virus particle, the nucleic acid (rna) is still capable of causing the 

 disease. It is true that the infectivity of the nucleic acid is a good deal 

 less than that of the intact virus, but it is possible that the rna is 

 damaged in the preparative processes. 



This experiment shows that the nucleic acid alone is capable of 

 causing the synthesis of the whole virus in the infected plant. How- 

 ever, it has not yet been possible to cause infection of bacteria by 

 using the nucleic acids prepared from bacteriophages. The probable 

 reason for this is that the nucleic acid by itself is unable to enter the 

 bacteria. The tail of the bacteriophage has a 'tip' which attaches itself 

 to the bacterium which is being attacked and the nucleic acid is then 

 injected into the bacterium through the tube. This is itself a very 

 remarkable process and it is not understood how the nucleic acid 

 from the head of the bacteriophage is pushed through the narrow 

 tube which constitutes the tail (Fig. 16). It has recently been found by 

 Dean Eraser and others, however, that if the bacteriophage dna is 

 added to broken bacterial protoplasts (i.e. bacteria without a cell 

 wall) synthesis of new bacteriophage particles occurs. 



This shows that the nucleic acid (dna) of bacteriophage is capable 



