294 FACTORS MODIFYING THE SENSITIVITY OF CELLS 



13. Kelner, A., Effect of visible light on the recovery of Strepiomyces griseus conidia 

 from ultraviolet radiation, Proc. Natl. Acad. Sci., 35: 73, 1949. 



14. Kelner, A., Photoreactivation of ultraviolet-irradiated E. coli, with special refer- 

 ence to the dose reduction principles and to ultraviolet induced mutation, J. 

 Bad., 58:511, 1949. 



15. Kelner, A., Action spectra for photoreactivation, Bact. Proc, 1950: 53, 1950. 



16. Knowles, T., and A. H. Taylor, Spectral radiation involved in photoreactivation 

 of ultraviolet-irradiated cultures of microorganisms, Bact. Proc, 1950: 49, 1950. 



17. Lea, D. E., Actions of Radiations on Living Cells, Cambridge, 1946. 



18. Stapleton, G. E., and F. L. Martin, Comparative lethal and mutagenic effects 

 of ionizing radiations on Aspergillus terreus (abstract). Am. J. Botany 36: 816, 

 1949. 



19. Swanson, C. P., and A. Hollaender, The frequency of X-ray-induced chromatid 

 breaks in Tradescantia as modified by near infrared radiation, Proc Natl. Acad. 

 Sci., 32:295, 1946. 



20. Suskind, S. R., Resuscitation of heat-inactivated seeds by X radiation, /. He- 

 redity, 41:97, 1950. 



21. Wyss, O., W. S. Stone, and J. B. Clark, Production of mutations in Staphylococ- 

 cus aureus by chemical treatment of substrate, J. Bad., 54: 767, 1947. 



DISCUSSION 

 Taylor : 



From the standpoint of radiation chemistry, an interesting feature of HoUaen- 

 der's and Giles' papers is that there is no mention of a marked effect of hydrogen. 

 This would seem to indicate a small importance for H2O2, since its concentration 

 is repressed in irradiated aqueous systems by the presence of hydrogen. Definite, 

 quantitative conclusions are made difficult by the relatively small concentration 

 of hydrogen (less than 10"^ molar at 1 atm) relative to other solutes, and by the 

 laclc of detailed knowledge of radiation chemical effects in solutions (that is, 

 individual radical concentrations, individual reaction rates). One gets, however, 

 the qualitative impression that much of the biological effect occurs through 

 immediate coUision of freshly formed oxidizing radicals (for example, OH) with 

 the biological material. 



Sparrow : 



In cooperation with M. J. Moses we have obtained some information con- 

 cerning the effect of x-radiation on chemical changes in fixed nuclei. The pre- 

 liminary results indicate that the desoxypentose nucleic acid of the fixed cells is 

 more susceptible to mild acid hydrolysis after x-irradiation than before. The 

 interpretation is that modifications induced by the irradiation permit hydrolysis 

 to proceed at a faster rate. It is not known whether the changes induced are 

 chemical or physical or a combination of both. 



I should also like to mention that we now have data indicating that sensitivity 

 of chromosomes to x-ray breakage may be related to the synthesis of desoxy- 

 pentose nucleic acid (DNA). Sensitivity increases as DNA synthesis progresses, 

 reaching a maximum where synthesis stops and decreasing to a minimum to the 

 point where synthesis again begins. 



