Pyrimidnie Moieties in Animals, Plants, and Bacteria 85 



Since the molecular weight of Shope papilloma virus is about 

 4x 10" (78) and the molecular weight of an average nucleotide 

 base pair is about 600, it is obvious that Shope papilloma virus 

 has a total of about 4 x 10V6 x 10", or 6600 nucleotide base pairs 

 in its DNA. Vaccinia virus has roughly 300,000 base pairs; bac- 

 teria have roughly 20x10*^ base pairs, and mammalian cells a 

 total of about 7x10^ base pairs. If there are no restrictions as to 

 the proportion in which base pairs occur or as to the sequence in 

 which they occur, the number of different DNA molecules possible 

 is 4", where n is the number of base pairs. Thus it is clear that 

 DNA provides an adequate basis for gene specificity. 



The increase in the relative amount of DNA from the lowest to 

 the highest forms of life reflects the need for an increasing number 

 of genetic units for embryogenesis and differentiation and for 

 various regulatory mechanisms. 



How Large are DNA Molecules? 



The molecular weight of Shope papilloma virus is about 4x10'' 

 (78). The weight average molecular weights of most of the DNA 

 preparations which have been studied are about 5-14 x 10^ How- 

 ever, the molecular weight of DNA may actually be much greater 

 than this. Very high molecular weight DNA has been isolated 

 from the T-even bacteriophages and it is possible that the entire 

 genome of the T-even bacteriophages consists of one long DNA 

 chain having a molecular weight of 90x10'' to 150x10" (15). 

 There is reason to believe that very long DNA molecules are 

 partially fragmented to smaller pieces when they are isolated from 

 tissues and viruses. 



Average Composition of DNA 



The average nucleotide base composition of DNA molecules 

 may be measured by hydrolyzing the DNA and then measuring 

 the nucleotide bases after they have been resolved by paper 

 chromatography, ion exchange chromatography, or paper elec- 

 trophoresis. To the extent that a mixture of DNA molecules can 

 be partially separated, the distribution of base compositions among 

 the molecules of the mixture can also be estimated. 



Two other important methods are available for measuring the 

 molar nucleotide composition of DNA: 1) equilibrium sedimenta- 



