184 S. S, COHEN 



appendix in the presence of P^^ and ckromatographed, separated fractions 

 possess essentially identical specific activities, a result which also extends to 

 fragments of the DNA molecules. 



Finally, chromatographic procedures can separate biologically active 

 molecules of DNA possessing specific transforming ability for Pneumococcus 

 from DNA inactive in comparable tests (Bendich et al., 1956; Lerman, 1955). 

 Thus, the biological heterogeneity of the genetic material of cells reflects in 

 some part the physical and chemical heterogeneity of its constituent poly- 

 mers. The existence of this heterogeneity of cellular nucleic acid defines in 

 considerable measure the experimental advantages of chemical study of the 

 inheritance of a virus whose complement of nucleic acid may be of the order 

 of 1/25 to 1/500 that of a bacterium. Biological, physical, and chemical 

 heterogeneity are thereby effectively reduced by whole orders of magnitude. 



The particle sizes of the cellular nucleic acids have not yet been satisfac- 

 torily determmed. An RNA from calf Hver had an average molecular weight 

 of the order of 300,000 (Grinnan and Mosher, 1951; Magasanik, 1955); DNAs 

 from various ceU sources have been reported to have molecular weights of 

 from 1 X lO*' to 10 X 10® (Jordan, 1955). However, all of these cellular 

 nucleic acids are heterogeneous and the molecular weights in the Hterature 

 are averages of different kinds, depending on the methods of estimation. 

 The ranges of molecular weights for particles witliin each sample have not 

 been determined; this problem is only now becoming recognized as important. 



A few estimates of particle size are available for the biologically more 

 homogeneous viral nucleic acids; in general these appear to possess a greater 

 degree of physical homogeneity than do the cellular nucleic acids. Most 

 careful work has been done on plant virus RNA, starting with that of 

 tobacco mosaic virus (Cohen and Stanley, 1942; Hopkins and Sinsheimer, 

 1955; Schuster et al., 1956). The RNA of this virus may exist as a single 

 particle of about 1.9-2.1 X 10®, which degrades to a particle of 2.5-2.9 X 10^, 

 which in turn decomposes to units of weights 6 X 10^ and 1.5 X 10*. On the 

 other hand, the RNA of a spherical virus, turnip yellow mosaic virus, appears 

 to consist of many units (25 to 50) of 36,000 to 100,000 molecular weight, 

 which interact readily to form larger units (Cohen and Schachman, 1957). 

 Isolated DNA of T-even phages has an average molecular weight of 19 to 

 25 X 10® (Cohen, 1957; Meselson et al, 1957). Nevertheless, the physical 

 characterization of these substances can still be considered to be in a primi- 

 tive state. 



Analysis of the distribution of the component nucleotides along poly- 

 nucleotide chains has barely begun. As summarized by Markham (1956) and 

 most recently by Heppel et al. (1957a,b), techniques for the stepwise analysis 

 of RNA chains have been developed; hardly any comparable approaches 

 work for DNA. In both substances, the polynucleotide chains are recognized 



