470 E. A. EVANS, JR. 



cytidyKc acid is resistant to pliosphatase action while the unsubstituted 

 mononucleotide is readily split suggests that the presence of the glucose 

 substituent may be responsible for the lack of complete enzymic hydrolysis 

 of viral DNA in the T-even phages. 



B. Heterogeneity of Viral DNA 



The principal chemical evidence for the heterogeneity of viral DNA is 

 derived from the work of Brown and Martin (1955). These workers prepared 

 DNA from purified preparations of T2 after precipitation with 95 % ethanol, 

 resuspending the precipitate in 0.2 M NaCl, and agitating with chloroform 

 in a blendor and centrifuging to remove denatured protein. After three such 

 treatments with chloroform, the process was repeated with a 9 : 1 chloroform- 

 octanol mixture until no farther denatured protein precipitate was formed. 

 The DNA was then precipitated in 95 % alcohol, washed in 75 % alcohol, and 

 stored m 75 % alcohol at 4°C. until required. By combining fractionated 

 histone with cellulose by diazotization, adsorbing the nucleic acid on the 

 cellulose histone, and then elutmg with NaCl, they were able to obtain 

 elution patterns showmg two large peaks. One of these contained about 30 % 

 of the total DNA phosphorus, with the ratio adenine + thymine/guanine + 

 5-HMC being approximately 1.9; the remaining 70 % had a ratio of about 2.15. 

 Measurement of the glucose content of the two fractions by anthrone indi- 

 cated that the quantity of glucose in the first fraction was molecularly 

 equivalent to 65 % of the 5-HMC, while with the second fraction there was 

 complete equivalence between 5-HMC and glucose. As it is unhkely that these 

 distinct fractions would be produced by degradation of a larger single mole- 

 cule by the preparative methods used, it appears that the T2r phage particle 

 contains at least two different species of DNA molecule which differ in their 

 ease of dissociation from protein and in their content of purine and pyrimi- 

 dine bases. 



These chemical observations are of interest in comiection with the findings 

 of a number of investigators over the last six years, which show that about 

 half of the parental DNA of an infecting virus particle is transferred to the 

 viral progeny in big pieces, the remainder being more widely distributed. 

 (See Stent, Chapter III under Bacterial Viruses in Volume II for a full 

 discussion.) In experiments in which the distribution has been traced into 

 the second generation, the large pieces do not midergo any further breakdown. 

 Levinthal (Levinthal, 1955, 1956; Levinthal and Thomas, 1957) has recently 

 developed a sensitive radioautogTaphic technique that allows him to measure 

 the radioactivity of a single virus particle labeled with P^-, if its atoms emit 

 more than 10 ^ particles per month. This involves surrounding the particle 

 mider study with a pliotographic emulsion sufficiently sensitive so that fast 



