t2 and other bacterial viruses 471 



electrons leave a visible track which can be observed directly under the micro- 

 scope. Using P^^-labeled T2, Levinthal has followed individual phage par- 

 ticles through three infectious cycles involvmg phage adsorption, multiplica- 

 tion, and liberation. Since he could detect significantly labeled DNA in the 

 progeny, despite the formation of more than a thousand viral progeny, he 

 concluded that a large fragment of viral DNA (about 40 % of the size of the 

 parental DNA) was maintained and transmitted in an intact state. In the 

 second generation, derived from the progeny of the first replication, individual 

 virus particles contained 20 % of the original parental radioactivity; how- 

 ever, in subsequent generations, the 20 % fragment was maintained as a 

 unit. Levinthal has suggested that the original 40 % fragment is the duplicate 

 viral chromosome, dividing into its component parts on replication and is 

 the agent responsible for the transmission of genetic information. 



When P32.ia]3gig(j DNA from osmotically shocked T2 was examined by the 

 method of Brown and Martin (Brown and Simons, 1957), and the two 

 fractions examined by the radioautographic technique of Levinthal, it ap- 

 peared that fraction 1 (adenine + thymine/guanine + 5-hydroxymethyl 

 cytosine =1.9) was identical with Levinthal's large DNA fragment. How- 

 ever, Cohen (1957) reports that physical examination of the DNA isolated 

 by the urea method does not reveal two grossly different classes of polymer, 

 and the experiments of Stent, Sato, and Jerme (see Delbriick and Stent, 

 1957) are in conflict with the view that the big piece of DNA is the phage 

 chromosome. 



V. Other Viral Specific Products 



Inasmuch as viral replication involves a redirection of the metabolic 

 activities of the host cell with the synthesis of characteristic protein and, in 

 the case of the T-even phages, the synthesis of 5-HMC, one might expect to 

 find, in the infected cell, specific viral compounds that are intermediate or 

 corollary to the synthesis of phage. The presence of particles, presumably 

 representing empty phage heads, tails, and tail cores in lysates from infected 

 cells or from infected cells treated with proflavin has been described (see 

 Section III, B). Burton (1955) has presented evidence for the necessity of a 

 brief period of protein synthesis prior to the synthesis of viral DNA. This 

 early formed protein may represent the special enzymes needed to synthesize 

 5-IIMC, but definite information with respect to either the chemical composi- 

 tion or function of this material is lacking. While there are promising hints 

 with respect to the biospithetic paths leading to 5-HMC in infected cells 

 (see Cohen and Barner, 1957), here, also, the final answer awaits further 

 investigation. 



