BIOLOGICAL ROLE OF DEOXYPENTOSE NUCLEIC ACIDS 461 



ing a mass of free or loosely bound DNA. This is not to be taken as a 

 complete description of T phage structure, and by no means as a picture 

 necessarily applicable to other viruses. However, it does make possible 

 a differentiation of the functions of the protein and DNA components. 



b. Life Cycle of Virulent Phage — Role of the Protein and DNA 



When coliphages are added to E. coli cells in a suitable environment, 

 the phage particles are rapidly adsorbed in random distribution upon the 

 bacteria. They adhere to the cells through their "tails," and antisera which 

 react with the tail protein can, if added in advance, prevent attachment. 

 Adsorption soon becomes irreversible, and the bacterium is now killed, i.e., 

 no longer able to produce more bacteria, becoming instead a producer of 

 phage. After a latent period, which in part is determined by the growth 

 medium and the temperature, the cell lyses and liberates a large number 

 of new phage particles. These phenomena are indicated schematically in 

 Fig. 2. 



The surface of the phage (probably the tail) is the part which makes 

 contact with the bacterial cell, and it is in fact possible to obtain an abor- 

 tive "infection" with the protein ghosts alone. ^^^ If no DXA is present, the 

 ghost kills the cell and no phage is formed. This finding suggested that 

 DNA was directly or indirectly required for new phage production. It 

 turns out that DXA plays a major role as soon as infection has been es- 

 tablished. 



Hershey and Chase have made a classic demonstration of the different 

 roles of DNA and protein of both damaged and intact phage. ^" Taking 

 advantage of isotopic marking as already mentioned, these investigators 

 clearly showed that the DNA (P^-) of infecting T2 or Te phages enters into 

 the infected cell, whereas the protein (S^*) does not. When infected cell 

 suspensions were subjected to the vigorous shearing force of a mechanical 

 blendor, phage protein was stripped off, leaving an infected cell containing 

 all the phage DNA and only a small part (never over 20 %) of the protein. 

 Even that part of the protein which remained attached made no isotopic 

 contribution to the new phage that was synthesized, but could be recovered 

 completely among the bacterial debris after premature or ultimate normal 

 lysis. Since the stripped cells still produced the normal amount of virus 

 progeny, it appeared that phage DNA by itself is capable of carrying for- 

 ward the process of infection, and that the protein is no longer needed 

 after the DNA has been introduced into a host cell. Phage DNA has, how- 

 ever, never been successfully used in separated form to initiate infection. 

 The impression has grown that phage protein is a protective membrane 

 shielding phage DNA from deoxyribonuclease and other influences and 

 serving to insure its introduction into the cell. It has been suggested that 



