The Chemical Basis of Heredity Determinants 333 



The Resistance to Physical and Chemical Agents 



The literature on the effect of physical and chemical agents on DNA 

 is rather voluminous. In most cases the starting material for these 

 studies was prepared by the methods which are now known to give 

 denatured and therefore less resistant DNA. The following discussion 

 will be limited to studies in which the starting material had full trans- 

 forming activity. 



1. Heat and pH. Quantitative study of the effect of these factors 

 has been made for the transforming principle of H. influenzae. 21 Both 

 the viscosity and the activity of the purified preparation remain un- 

 changed after 1 hour of heating at temperatures up to 81° (in citrate 

 buffer) or after incubation at 23° between pH 5 and 10. The remark- 

 able stability to heat, which is higher than for most known proteins, 

 is similar to the stability of human DNA and calf-thymus DNA when 

 they are prepared by a similar method ; 25 this stability is much higher 

 than the values reported in the literature for DNA prepared by the 

 previous, somewhat injurious methods. However, it should be pointed 

 out that the inactivation at the low range of pH could be due to a 

 removal of purines and/or hypothetical hydrogen bonds to these 

 purines. A study 21 of the amount of such "depurination" of the trans- 

 forming principle reveals that a 100-fold inactivation occurs with the 

 removal of less than two purines per thousand; thus, practically every 

 purine may be necessary for the activity.* 



2. Deoxyribonuclease . Mammalian and bacterial DNAases in mi- 

 nute amounts destroy the activity of pneumococcal transforming prin- 

 ciple. 3 A quantitative study on the transforming principle of H. in- 

 fluenzae 21 reveals that crystalline pancreatic DNAase in concentra- 

 tions of less than 10 -4 /xg./ml. causes a 10-fold decrease of activity 

 within 28 minutes and complete inactivation within 140 minutes. On 

 the other hand, the drop of viscosity at the beginning of inactivation 

 is insignificant. The reason for this discrepancy is still not clear, but 

 the initial change might involve the breaking of a few phosphate bonds, 

 sufficient to destroy the activity but insufficient to cause any decrease 

 in the size of the molecule still held together by hydrogen bonds. 



3. Ionic strength. A quantitative study of the effects of exposure 

 to various ionic strengths on activity and viscosity has been made for 

 the transforming principle of H. influenzae. 21 Previous exposure to 

 lower or higher ionic strengths did not affect the viscosity (as measured 



*This may be true only on the assumption that the active molecule behaves 

 like an average molecule of DNA. 



