NUCLEOTIDES I4I 



has been found except in those of the more complex animal 

 viruses. Interest has therefore centred around the mechan- 

 ism whereby a virus assumes control of the metabolic 

 activities of the host cell and reorganizes them for the syn- 

 thesis of identical virus particles. The virus-host-cell system 

 investigated in most detail is that concerned with the infec- 

 tion of Esch. coli B with a phage designated as the T phage, 

 of which seven types are known. Electron microscopy has 

 revealed that Ti and T5 are more or less spherical, whereas 

 the other five T phages are club-shaped and have a distinct 

 head, containing most of the DPNA, and a tail. Phage repro- 

 duction involves three phases: (i) adsorption on to and 

 invasion of the susceptible cell; (ii) multiplication in the host 

 cell, and (iii) liberation from the host cell, a process usually 

 accompanied by lysis, though liberation and lysis are not 

 always coincident. Unless stated to the contrary, the follow- 

 ing description applies to phage T2 and the details are not 

 necessarily the same for other viruses. 



The adsorption of T2 by Esch. coli B is a rapid and 

 reversible process dependent on the presence of tryptophan 

 in the medium. It is soon followed by an irreversible process 

 in which part of the virus, mainly DPNA and a little pro- 

 tein, enters the cell, an event which immediately results in 

 a marked disturbance of the host cell's nucleic acid meta- 

 bolism, the first visible signs being degradation of the 

 chromatinic bodies. The inability of virus-infected cells to 

 grow and divide is presumably an outw^ard expression of 

 this destruction of nuclear material. Nucleic acid synthesis 

 is at first completely halted, but after a little while the 

 synthesis of purines and pyrimidines commences and is soon 

 followed by the formation of DPNA. Pentose nucleic acid 

 synthesis is completely suppressed in Esch. coli B infected 

 with T2, but in other virus-host-cell systems, the synthesis 

 of both types of nucleic acid may take place. The amount 

 of DPNA synthesized is equivalent to the total amount of 

 both types of nucleic acid synthesized by uninfected cells, 

 and this indicates that in the infected cells the units 

 normally incorporated into PNA are being diverted to the 

 synthesis of DPNA. In attempting to discover the reason 



