860 



ADVEXTURES IX RADIOISOTOPE RESEARCH 



but rather interferes at some later stage in the biosynthesis of desoxy- 

 ribopolyniicleotides (Errera, 1954). 



The DNA molecule has the largest weight, about 6x10^, of all com- 

 pounds found in the mammalian organism. Its length can be estimated 

 to be about 3x10"^ cm. A molecule of such length has a very much 

 larger probability of picking up energy in the exposed organism and of 

 leading it to a radiosensitive spot than a small one. Watson and Crick 

 (1953) have given arguments for assuming that the DNA molecule 



75- 



c50- 

 o 



25- 



1 

 25 



"1 \ r 



500 100 200 500 1000 

 Exposure (roentgen) 



3200 



Fig. 7. Incorporation of ^^P into rat thymus DNA after X-irradiation. 



consists of two polynucleotide chains wound helically in a large number 

 of turns around a common axis and tied together by hydrogen bonds 

 between corresponding purines and pyrimidines. To explain the dupli- 

 cation process of the desoxyribosenucleic acid molecule, Watson and 

 Crick suggested that the two polynucleotide chains may be separated 

 and each chain catalyses the synthesis of a complementary chain. 



Different modifications of this scheme were put forward, all of which 

 suggested that the multiplication of the DNA molecule takes place in 

 a template fashion and necessitates the presence of undisturbed DNA 

 molecules. By accepting this view^ we arrive at the conclusion that an 

 interference with the DNA molecules present may block new formation 

 of such molecules and that the observed effect of irradiation on DNA 

 formation may be a consequence of the disturbance of the DNA mole- 

 cules or of the protein moieties of the nucleoprotein which have to serve 

 as a template in the process of additional formation of such molecules. 



