66 J. A. V. Butler 



know what the function of this part of the nucleoprotein 

 system is. 



The influence of this protein on radiation effects is not 

 easily assessed. It might be expected a priori that histone 

 would have a protective effect on DNA by mopping up 

 radicals, which would be prevented from reaching the DNA. 

 No accurate experiments on this have been made as it is 

 difficult to be sure that DNA has been recovered quantita- 

 tively from the nucleoprotein, and slow degradative changes 

 occur in the nucleoprotein on standing. 



It may be useful to estimate the degree of damage in 

 the chromosome particle by different processes. If we take 

 the molecular weight of DNA as 6 X 10^, it can be estimated 

 that 10^ r will cause about 7 ionizations within each molecule, 

 and we have seen that 2-3 ionizations will cause a considera- 

 able amount of damage, which can be expressed as sufficient 

 to reduce the viscosity in aqueous solution by half. In the 

 hydrated state in the presence of water it may be expected 

 that this will be increased by several times at least, i.e. about 

 10^ r will cause the same degree of damage (expressed as 0*5 

 of the original viscosity). As there are 10^ particles in the 

 chromosome, this means that the probability of any one DNA 

 particle in the chromosome being damaged to this extent by 

 exposure to 1 r is of the order of 10~^. 



It is estimated that (in mice) the probability of mutation 

 in any one locus is between 3 and 200 X 10"^ per r unit 

 (Russell, 1952). If we take 10"'^ or 10~^ as possible values, we 

 see that the probability of producing a specific mutation is 

 about 10"^ or 10"^ of that of damaging, to the extent mentioned, 

 the DNA particle which carried the gene in question. This 

 means that a considerable amount of damage can be done to 

 a DNA particle without affecting a specific gene carried by it. 

 This can be expressed in another way by saying that the locus 

 in question is not greater than the order of 10 ~^ or 10 ~^ of the 

 size of an average DNA particle. There are, of course, great 

 uncertainties in many of the quantities entering into this 

 calculation. 



