254 BACTERIOPHAGES 



findings of Volkin and Astrachan (1956a, b). They demon- 

 strated that an amount of RNA small relative to the total cellular 

 content is synthesized beginning immediately following infection. 

 This RNA has a base composition unlike that of the RNA in un- 

 infected cells and more nearly like the analogous base composi- 

 tion of the infecting T2 DNA. 



c. Nucleic Acid Materials Assimilated before and after Infection 



Because of the importance of nucleic acids in heredity a major 

 effort has been expended in numerous laboratories on the study 

 of nucleic acid synthesis in phage-infected bacteria. As indi- 

 cated in Chapter VII chemical analyses of various phages shows 

 that those studied so far contain only protein and deoxyribose 

 nucleic acid. The work of Hershey and Chase (1952) demon- 

 strated that infection with phage T2 involved penetration of 

 phage nucleic acid into the host cell, leaving most if not all of the 

 phage protein outside of the host cell. This immediately gave 

 phage nucleic acid the dominant position in phage replication. 

 The equally startling discovery by Wyatt and Cohen (1953) of a 

 new pyrimidine, 5(hydroxymethyl)cytosine, in various strains of 

 the T2 species of phage furnished material for speculation as well 

 as an invaluable tool for distinguishing phage nucleic acid from 

 bacterial nucleic acid in bacteria infected with these phage 

 strains. Because of the very large number of papers related to 

 the problem of phage nucleic acid synthesis it would be difficult 

 as well as confusing to follow a chronological development of this 

 field. Instead we shall describe present concepts of the synthesis 

 of phage nucleic acid referring to those papers which relate 

 directly to the problem. Most work has been done with phage 

 strains T2, T4, and T6, and will be described first. 



Infection with phage T2 involves injection of phage nucleic 

 acid into the host cell (Hershey and Chase, 1952). Infection 

 prevents further host cell division and stops the synthesis of RNA 

 and of bacterial DNA (Cohen, 1947a) without interrupting bac- 

 terial respiration. Infection is soon followed by cytologically 

 evident disintegration of the bacterial nuclei (see Chapter XII). 



