CHAPTER XI 



STAGES IN PHAGE MULTIPLICATION 



An over-all view of the lytic cycle of phage reproduction was 

 given in Chapter II. In this chapter we shall examine in more 

 detail some of the biological and physical methods used to 

 elucidate the sequence of events. Strictly chemical experiments 

 will be discussed in Chapter XIV. 



1. Initiation of Infection 



a. Penetration 



The electron microscope furnished convincing evidence that 

 phage multiplies within the host cell and is liberated by rupture 

 of the cell membrane (DeMars, Luria, Fisher, and Levinthal, 

 1953). A section of an infected bacterium illustrating in- 

 tracellular particles of phage T2 is shown in Figure 2 (Chapter 

 TV). Evidently the infecting phage particle must penetrate 

 through the cell wall to reach the interior. Chemical evidence 

 of penetration was furnished by the observation that phosphorus 

 and nitrogen from the infecting phage were contained in the 

 phage progeny (Chapter XIII). However, these observations 

 did not at first yield information about the chemical composition 

 or biological organization of the phage material which pene- 

 trated the host cell wall. 



A direct attack on this problem was made by Hershey and 

 Chase (1952) by using isotopic labels. The proteins of phage 

 T2 were labeled with S^^ and the nucleic acid was labeled with 

 P^2. The labeled phage was purified by differential centrifu- 

 gation and its properties studied by following its radioactivity. 

 Neither the phosphorus nor sulfur was acid-soluble and neither 

 became soluble on exposure of the phage particles to deoxy- 



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