88 Information Storage and Neural Control 



tion in CsCl density gradients; and 2) the estimation of the melting" 

 temperature (T,„) of DNA by a study of the change in DNA 

 absorption as a function of temperature (16). The latter two 

 methods have the advantages that less material is required to 

 analyze the DNA and that an estimate of the variation from the 

 mean of nucleotide base composition can be made. Of the three 

 methods, density gradient centrifugation has the highest accuracy, 

 requires the least DNA per experiment, and permits the detection 

 of DNA molecules of unusual base composition even where the 

 latter comprise less than 5 per cent of the total DNA. A further 

 discussion of density gradient centrifugation will be presented later. 



Since DNA is double stranded* and the guanine and adenine 

 of each chain are paired, respectively, with the cytosine and 

 thymine of the complementary chains, the total purine bases 

 (A+G) are equal to the total pyrimidine bases (C+T) and the 

 total 6-amino bases (C+A) are equal to the total 6-keto bases 

 (G+T). However, the ratios of guanine plus cytosine to adenine 

 plus thymine (G+C)/A+T) are not the same in different or- 

 ganisms and provide a parameter by which organisms can be 

 characterized. 



The molar (G + C) content of the DNA of seventy-two different 

 bacterial species, thirteen species of higher plants, ten species of 

 algae, four species of fungi, two species of protozoa, sixteen species 

 of invertebrates, twenty-three species of animals, twelve bacterial 

 viruses, six animal viruses and rickettsiae, and twelve insect 

 viruses have been measured and are shown in Tables II through IX. 



The molar per cent (G+C) varies from 26.5 per cent in the 

 protozoan, Tetrahymena, to 73 molar per cent (G + C) for the 

 bacterium, Mycobacterium phlei. Of the 170 species listed in Tables 

 II through IX, 112 have DNA molecules whose average molar 

 (G + C) contents are 40 to 60 per cent. This is not surprising. It is 

 probable that Kutagenic transitions of adenine-thymine to qua- 

 nine-cystosine base pairs in one part of a DNA molecule are 

 compensated for by transitions from quanine-cystosine to adenine- 

 thymine base pairs in other parts of the molecule so that the molar 

 per cent (G + C) remains, on the average, close to 50 per cent. 



*Exceptions to this statement are the DNA of the bacteriophages, 0X174 and 

 SI 3, which are single stranded (65, 66). 



