128 NUCLEOTIDE SEQUENCE IN DEOXYRIBONUCLEIC ACIDS 



from something like 12,000 to many millions^. Deoxyribonucleic 

 acid preparations appear, in contrast, to fall into two principal 

 groups of molecular weight: (a) of 6 to 8 million; (b) of 12 to 16 

 million^.* Such distinctions are probably not of great heuristic 

 value, since the cell, which is not simply a bag containing many 

 chemicals of different size and quality, must impress on its com- 

 ponents a pattern of multiple associations which it is not possible 

 even to define at present. 



Dissimilarity in size is, however, less of a problem for the 

 student of the chemistry of deoxyribonucleic acids than is the 

 possible heterogeneity of his specimens as regards their com- 

 position and sequence characteristics. It is, in fact, possible to 

 divide a preparation of total deoxyribonucleic acid into a series of 

 fractions of graded composition (starting with a, sometimes ex- 

 treme, GC type^ and going to a very marked AT type) which, 

 however, all still exhibit the pairing regularities^; but the real 

 meaning of such a fractionation is far from clear. It could be, 

 as was pointed out by us^, that these fractionated preparations 

 actually represent some form of sub-units of a large aggregate, 

 though it is not easy to describe the nature of the links that are 

 disrupted repeatably during the mild fractionation procedure. 

 In this view, in its extreme form, the nucleus of a given species 

 could conceivably contain only one type of deoxyribonucleic acid. 

 At the other extreme would have to be placed the view that a 

 given nucleic acid preparation may comprise an entire spectrum 

 of differently constituted individuals so that no two nucleic acid 

 molecules within the same nucleus would be entirely identicaP^. 

 That neither of these opposite extremes can as yet be disavowed 

 entirely demonstrates the deep cleft that still exists between 

 metabiological assertions and scientific facts. 



* An interesting exception — a polydeoxyribonucleotide of mol. wt. 10,000 

 to 12,000 — has been described recentlyf"' as a component of crystalline cyto- 

 chrome bey. It is noteworthy that this small polymer of only about 40 

 nucleotides still exhibits the pairing principlesi mentioned above. 



