414 R. M. S. SMELLIE 



a fashion similar to the phosphorus of the nucleotides. In normal liver, 

 however, all the PNA's exhibit a greater activity with respect to phos- 

 phorus uptake than to carbon uptake. 



5. It has been postulated that nuclear PNA serves as a precursor of 

 cytoplasmic PNA (footnotes 41, 94 and 96 and Chapter 28); the results 

 discussed here do not detract from this possibility, but they do suggest 

 that the PNA of the nucleus is not the immediate precursor of that in the 

 cytoplasm. 



6. For most purposes it would seem that the PNA of the cell may be 

 divided into two main metabolic classes : (a) the PNA of the nucleus which 

 is metabolically highly active; (b) the PNA's of the cytoplasm which are 

 qualitatively very similar but which may be slightly different quantitatively 

 and which are much less active metabolically than the PNA of the nucleus. 



The possible significance of all these observations is further discussed 

 in Chapter 28. 



V. Factors Affecting Nucleic Acid Metabolism 

 1. The Effect of X-Irradiation on Nucleic Acid Metabolism 

 a. General 



It is well known that exposure of rapidly growing tissues to X-irradiation 

 causes a marked inhibition of growth. In constrast, most adult tissues are 

 considerably less sensitive to such exposure, and the explanation of this 

 difference seems to lie in the relative proportion of dividing cells in the two 

 types of tissue. Irradiation causes marked disturbances in the normal 

 processes of cell division, and, since the nucleic acids (particularly those 

 of the nucleus) are intimately concerned in this mechanism, it can be ex- 

 pected that metabolic changes in the nucleic acids will occur simultaneously 

 with the disturbances in mitosis as a result of treatment with X-rays. 



Some indications of disturbances in nucleic acid metabolism have been 

 observed by Mitchell,'" ■>*'* who used microspectrophotometric techniques 

 in his studies on irradiated and non-irradiated tumor tissue. Mitchell found 

 in the cytoplasm of the irradiated tumor cells an accumulation of ultra- 

 violet-absorbing material which appeared to be of nucleotide nature. The 

 effect of X-irradiation on the nucleus and cytoplasm of Amoeba proteus 

 has been studied by Harriss et aL'"^ using the technique of nuclear transfer, 

 while Sparrow et a/."" have investigated the effects of X-rays on chromo- 

 some breakage and other abnormalities in the nuclei of Trillium erectum 

 and Tradescantia paluctosa. 



'" J. S. Mitchell, Brit. J. Exptl. Pathol. 23, 285, 296, 309 (1942). 

 "« J. S. Mitchell, Brit. J. Radiol. 16, 339 (1943). 



'09 E. B. Harriss. L. F. Lamerton, M. J. Ord, and J. F. Danielli, Nature 170, 922 (1952) . 

 "" A. H. Sparrow, M. J. Moses, andR. J. DuBow, Exptl. Cell Research Suppl. 2, 245 

 (1952). 



