350 erwin chargaff 



2. Procedures 



The principal analytical methods make use of chromatography of the 

 nitrogenous constituents on filter paper; they are discussed in Chapter 7. 

 For the subsequent spectrophotometric operations Chapter 14 should also 

 be consulted. In addition, very occasional use has been made of the chroma- 

 tography of the free bases on a starch column"* and of the separation of 

 enzymically liberated mononucleotides on ion exchangers.^'**'' -^^ [Compare 

 Chapter 6.] In contrast to their usefulness in other aspects, color reactions 

 have found little direct application in studies on composition. [Compare 

 Chapter 9.] The isotope dilution method does not appear to have been 

 much used either."^ 



Requirements with respect to the purity of the specimens have been 

 discussed in Sections III and IV. The most troublesome impurity is pentose 

 nucleic acid. The analytical results on preparations containing more than 

 3 % of this contaminant command little confidence; for careful comparisons 

 this impurity should be reduced to less than 1.5%. Its removal has been 

 discussed in Section lU.2.h. Proteins and polysaccharides interfere less, 

 but must be eliminated to an extent permitting the nucleic acid specimen 

 to have a P content of more than 8%. 



The procedures used in the laboratory of this writer and some of the modi- 

 fications introduced in the course of time have been described in several 

 publications cited before." •"^■"*'2''8 



3. Distribution of Purines and Pyrimidines 



a. Grouping of Deoxypentose Nucleic Acids and Presentation of Results 



When the composition of many specimens of deoxypentose nucleic acid 

 from different cellular sources is compared, a very striking feature emerges, 

 as was pointed out some years ago: two principal groups can be distin- 

 guished, namely the "AT type," in which adenine and thymine predominate, 

 and the "GC type," in which guanine and cytosine are the major constitu- 

 uents.'^--'" In addition, an intermediate group was discovered in E. coli 

 which is characterized by the presence of almost equimolar proportions of 

 the four components^^^'^^^'^" All total deoxypentose nucleic acid prepara- 

 tions from animal sources described up to this time belong to the AT type, 

 which seems to be much more frequent .than the others. The GC type has 

 been encountered in several microorganisms and in some insect viruses. 

 Consequently, the molar ratios of adenine to guanine, of thymine to cyto- 

 sine, and of adenine + thymine to guanine + cytosine will be above 1 in 

 the nucleic acids of the AT type and below 1 in those of the GC type. This 

 will appear with particular clarity when the composition of microbial de- 



2" R. Abrams, Arch. Biochem. 30, 44 (1951). 



