100 Information Storage and Neural Control 



any one bacterial species covers less than one tenth of the range 

 over which the mean guanine-cytosine content varies among the 

 various species. 



Doty and co-workers (1^) have sliown that the total variance 

 of the DNxA. bands equals the sum of the variance due to molecular 

 weight {(tmw'^) and that due to density heterogeneity (or density)- 



[5] C'r = (J''mW + 0-" density 



Since the variance due to molecular weight can be estimated, 

 <^^deiisity can be calculated. The latter value can be expressed in 

 units of molar per cent (G+C); that is, in terms of ctqc. 



For bacterial DNA, Doty, et al. (16) have shown that age is 

 actually equal to about ±1.7 molar per cent (G + C). 



The values for animal tissues are considerably greater (Table VI). 

 The standard deviation in units of density (o-density) ranges from 

 0.0037 to 0.0047 gcm"~^ The latter value is equivalent to a standard 

 deviation corresponding to the molecular content of guanine plus 

 cytosine of about 3 molar per cent (G + C). 



DNA obtained from various adult tissues of mice and from mouse 

 tumors do not differ significantly in their effective buoyant den- 

 sities or in the standard deviations of the density gradient bands. 

 Although all differentiated tissues of a given organism are pre- 

 sumed to have identical genomes, there is evidence that normal 

 tissues and tumors differ genetically. The fact that no significant 

 differences between the DNA of normal and of malignant tissues 

 have been found does not necessarily contradict the latter concept. 

 Instead, it reflects the fact that existing techniques are insufficiently 

 sensitive to detect such differences. It is quite possible that thou- 

 sands of point mutations exist in the genomes of cancer cells. 

 These could not be detected by the relatively gross physical 

 methods so far employed. 



Although significant differences between the DNA of adult 

 animal tissues and the DNA of tumors have not been found, it 

 has been possible to recognize specific differences between the 

 DNA of various species of higher animals (37). As shown in Table 

 VI, frog, turtle, and alligator DNA are slightly heavier than other 

 vertebrate DNA's. Chinese hamster and frog DNA have relatively 

 low standard deviations for the DNA bands. Also, mouse and 

 guinea pig DNA manifest bimodal distributions. 



