30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 103 



cross-linking in DNA films, 163 while extensive cross-linking of DNA in 

 sperm heads has been produced by electron irradiation. 240 Since a DNA film 

 more nearly resembles nuclear DNA than does a solution, cross-linking by 

 ultraviolet and ionizing radiations may be involved in some cellular effects. 

 It would be of interest to examine the photochemistry of DNA films in 

 the vacuum ultraviolet, as has been done for proteins, 241 in order to obtain a 

 better comparison of the effects of ultraviolet and ionizing radiations. 



The apparent temperature independence of quantum yield for cross- 

 linking of dry DNA was proposed 165 as a tool for distinguishing between 

 protein and nucleic acid effects in action spectra (Sections VI, 1 and VIII). 

 This suggestion may require revision in view of the finding of Lerman and 

 Tolmach 237 that the quantum yield for inactivation of transforming DNA 

 at 77°K. is much lower than at 275°K. The latter authors calculate a chain 

 length of 1.6 X 10 3 nucleotide residues for the critical segment of trans- 

 forming DNA (cf. Guild and DeFilippes 242 ) and find a quantum yield at 

 253.7 m/x for this segment of about 3 X 10 -3 , or more than an order of 

 magnitude greater than that for infectious RNA from TMV (SectionVI,2,a). 



This chapter would hardly be complete without at least some reference 

 to the growing interest in the role of radiations in the origin of life on the 

 earth. 243 In a recent review of this subject Miller and Urey 244 point out that 

 the greatest source of energy for amino acid synthesis on the primitive 

 earth was ultraviolet. It would be of interest to examine the possibilities 

 of purine and pyrimidine synthesis in such a system, perhaps via dihydro, 

 hydroperoxy, or hydrated pyrimidines, all of which are more radiation 

 resistant than the pyrimidines themselves and, once formed, could under 

 more favorable conditions be transformed to nucleic acid building blocks. 



XII. General Bibliography 



Principles and techniques of photochemistry 



"Photochemistry and the Mechanism of Chemical Reactions" G. K. Rollefson and 



M. Burton, Prentice-Hall, New York, 1939. 

 "The Chemical Aspects of Light," E. Bowen, Oxford Univ. Press, London and New 



York, 1946. 

 "Heavy Metal Prosthetic Groups and Enzyme Action," O. Warburg, Oxford Univ. 



Press, London and New York, 1949. 

 "Radiation Biology" (A. Hollaender, ed.), Vols. II and III, McGraw-Hill, New 



York, 1955. 



239 S. Bitny-Szlachto and D. Shugar, Bull. acad. polon. sci., Classe II, 6, 129 (1958); 

 7, 293 (1959); D. Shugar, Proc. U. N. Intern. Conf. Peaceful Uses Atomic Energy, 

 2nd Geneva, 1958 24, 263 (1958). 



240 P. Alexander and K. A. Stacey, Nature 184, 958 (1959). 



241 R. Setlow, Radiation Research, in press (1960). 



242 W. R. Guild and F. M. DeFilippes, Biochim. et Biophys. Acta 26, 241 (1957). 



243 A. I. Oparin, "The Origin of Life," 2nd ed. Dover Publications, New York, 1953. 

 2 « S. L. Miller and H. C. Urey, Science 130, 245 (1959). 



