Heterogeneity of Deoxyribonucleie 

 Aeid (DNA) 



AARON BENDICH 



A great many explorations have led to the view, which few people 

 now question, that the assertion of biological characters and their 

 transmission from one generation of cells or organisms to the next 

 require the intervention of DNA. In fact, there is a growing convic- 

 tion that the actual genetic determinants of the cell are composed of 

 DNA. This conclusion has received its most impressive support from 

 the knowledge that DNA preparations from many microorganisms can 

 carry out heritable transformations. There are, of course, a great 

 many heritable factors in the total genetic complement of any cell. 

 It would seem necessary, for the gene: DNA relationship to be valid, 

 to postulate that the total DNA of the cell consists of a great many 

 DNA molecules each concerned with one phenotypic expression and 

 differentiated by a particular chemical structure. Or, alternatively, 

 these macromolecules may be few in number but possessing structures 

 that are heterogeneous along the chain. According to this idea, bio- 

 logical characteristics are associated with specific regions on a DNA 

 molecule; hence, a given molecule may be polyfunctional. It is not 

 the purpose here to choose between these alternatives; indeed, both 

 may be involved. Rather, this essay will deal with several lines of 

 evidence which have revealed the heterogeneous nature of DNA. 



Perhaps the earliest published account x that is suggestive of this 

 idea is the report that the great bulk of the DNA of isolated "chromo- 

 somes" is soluble in M NaCl, but a little DNA was still detected in the 

 insoluble "residual chromosomes." This residual DNA was thought 1 

 to be a contaminant, but, in the light of more recent developments, 

 it would be good to re-examine this finding since it may reflect hetero- 

 geneity in distribution of DNA within the cell. The same applies to 



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