80 D. SHUGAR 



that DNA is much more sensitive to degradation in vivo, a fact which, 

 although perhaps not general, yet derives support from other observations. 



Loofbourow et al. lb9 have claimed, on the basis of optical observations, 

 that ultraviolet-injured yeast cells exhibit an increased production of 

 nucleic acid-like material, and have actually extracted twice as much crude 

 RNA from irradiated yeast cells, as compared to nonirradiated controls. 

 However, the extraction procedure used was not an efficient one, the maxi- 

 mum yield of crude RNA from nonirradiated cells being only 1 % ; further- 

 more, the extinction coefficient of the RNA from irradiated yeast was 

 higher than that from nonirradiated, indicating that it was partially de- 

 graded (see Section IV, 4) and hence easier to extract. 



Ultraviolet treated DNA also exhibits a decreased affinity for methyl 

 green 160 which, while it does not necessarily indicate degradation of the 

 nucleotide chains, does suggest a separation of the twin strands of the 

 two-stranded Watson-Crick structure. 161 A similar loss in staining affinity 

 for methyl green is exhibited by irradiated cell nuclei; rough calculations 

 indicated that this process requires 50 times less energy than for isolated 

 DNA. 162 



This surprising result may be linked to the state of DNA in cellular 

 material. Possibly related to this are the studies of Setlow and Doyle 163 

 on the irradiation of dry DNA films, as a result of which the DNA forms 

 a gel insoluble in aqueous solution with a quantum yield of about 10~ 2 , or 

 about 4 orders of magnitude greater than that for loss in viscosity. The 

 phenomenon has been examined in greater detail by Kaplan 164 in connec- 

 tion with the effect of humidity on the ultraviolet radiation sensitivity of 

 microorganisms. The rate of gelation of DNA films was found to increase 

 by a factor of 22 as the humidity was decreased from 97.5 to 33%. A 

 quantitatively similar relationship prevailed for the s-mutation and kill- 

 ing rates of Serratia marcescens and it was suggested that a common mech- 

 anism accounts for all three processes, involving hydrogen-bond rupture 

 in the DNA molecule. 



Irradiation of RNA from TMV with fairly high doses results in com- 

 plete destruction of viscosity and intrinsic viscosity, as well as a loss in 

 ability to stain metachromatically with safranin, which is reasonable evi- 

 dence for molecular degradation. 98 



The radiation sensitivity of DNA was found to be unaltered at low 

 temperatures while that of proteins is decreased, and it has been sug- 



159 J. R. Loofbourow, Growth 12, Suppl., p. 77 (1948). 



160 S. Devreux, M. Johannson, and M. Errera, Bull. soc. chim. biol. 33, 800 (1951). 



161 H. S. Rosenkranz and A. Bendich, J. Biophys. Biochem. Cytol. 4, 663 (1958). 



162 M. Errera, Ann. soc. roy. sci. med. et nat. Bruxelles 5, 65 (1952). 



163 R. Setlow and B. Doyle, Biochim. et Biophys. Acta 12, 508 (1953); 15, 117 (1954). 



164 R. W. Kaplan, Naturwissenschaften 42, 184, 466 (1955). 



