528 L. G. LAJTHA 



mutations, chromosome damage, and inhibition of mitosis are not primarily- 

 related to nucleic acid metabolism. While it is true that in most diploid cells 

 mitosis does not take place without previous DNA synthesis, it is known 

 that mitosis can be inhibited without affecting DNA synthesis. Also, while 

 it is increasingly evident that chromosome damage is a main cause of radia- 

 tion-induced cell death it is known that doses of radiations which will not 

 produce a measurable effect on DNA synthesis will cause a significant 

 amount of chromosome damage. 



It has been clear for some time that the study of radiation effects on 

 nucleic acid metabolism will not give the direct answers originally hoped 

 for, but it was thought to give some basic information on fundamental proc- 

 esses connected with nucleic acid synthesis. The purpose of this chapter 

 is to investigate the validity of this last assumption. 



II. Factors Influencing Deoxyribonucleic Acid 

 Specific Activity 



Even a brief survey of literature shows that a great variety of experi- 

 ments have been performed under the title "effects on nucleic acid (or DNA) 

 metabolism (or synthesis)." It is obvious from the apparent contradictions 

 of certain experimental findings that unless the term "nucleic acid metabo- 

 lism" is better defined it will remain so broad as to be meaningless. 1-4 



In the following, it will be attempted to define certain changes produced 

 by radiation, which may be reflected in nucleic acid content and specific 

 activity, although be essentially independent from the process of nucleic 

 synthesis. 



It may be mentioned here, that there is no good evidence which would 

 indicate that the specific biochemical process of ribonucleic acid (RNA) 

 synthesis is affected by doses of radiations which leave other biochemical 

 cell functions intact. Consequently the work surveyed in this chapter will 

 be concerned with DNA synthesis. 



1. Population Changes 



Differentiating cells have a finite life span or suffer random destruction 

 (or both), and in a steady state population cell production and cell removal 

 balance each other. Inhibition of mitosis, while not greatly affecting the 

 processes of differentiation, inhibits production thus resulting in a depopu- 



1 G. Hevesy, Revs. Modern Phys. 17, 102 (1945). 



2 A. Howard, in "Ciba Foundation Symposium on Ionising Radiations and Cell 

 Metabolism" (G. E. W. Wolstenholme and C. M. O'Connor, eds.), p. 196. Churchill, 

 London, 1956. 



3 L. S. Kelly, Progr. in Biophys. and Biophys. Chem. 8, 144 (1957). 



4 M. G. Ord and L. A. Stocken, in "Mechanisms in Radiobiology" (M. Errera and 

 A. Forssberg, eds.), Vol. 1. Academic Press, New York, 1960, in press. 



