ISOLATION AND COMPOSITION OF PENTOSE NUCLEIC ACIDS 407 



the quantitative recovery of PNA as mononucleotides. In this way changes 

 in nucleotide composition due to the great susceptibility of nucleic acid to 

 enzymic and other degradations during isolation are avoided. These authors 

 were able to show that all pentose nucleic acids analyzed in their labora- 

 tory, including specimens from bacteria, yeast, and sea urchin and starfish 

 eggs, as well as from the cytoplasm and the nuclei of vertebrate cells, 

 possessed an ecjual number of 6-keto (guanine and uracil) and 6-amino 

 (adenine and cytosine) groups. The preparations from animal cytoplasm, 

 marine eggs, and yeast showed the additional regularity of a purine-to- 

 pyrimidine ratio of 1. These relations are well illustrated in the examples 

 taken from the work of these authors as well as from that of others pre- 

 sented in Tables VII, VIII, and IX (preparations 7-10). It is of interest to 

 compare the composition of PNA and DNA of animal cells. PNA is com- 

 posed of equal amounts of guanylie acid and cytidylic acid, and of adenylic 

 acid and uridylic acid, with the first pair predominating.^^ DNA is com- 

 posed of ecjual amounts of guanylie acid and cytidylic acid, and of adenylic 

 acid and thymidylic acid, but the second pair predominates.^** The com- 

 position of DNA is compatible with a double-stranded helical structure, 

 proposed by Watson and Crick on the basis of X-ray diffraction measure- 

 ments.''^ A similar correlation of physical and chemical properties of PNA 

 has not yet been made. 



These are the conclusions which can be drawn from the work of the last 

 seven years, during which modern, accurate methods of nucleotide analysis 

 were used. Further progress will not depend on the development of more 

 accurate and efficient methods of analysis, but rather on new approaches 

 to the problem of the isolation and physical characterization of PNA. 

 Until homogeneous PNA preparations can be isolated, the knowledge of 

 the nucleotide composition of individual PNA specimens will not greatly 

 further the study of the structure and biological role of PNA. The most 

 fruitful approach to the problem of PNA structure has been the study of 

 the degradation products produced enzymically from PNA. The results of 

 such studies, which are discussed in Chapter 15, have shown that animal, 

 yeast, and viral PNA preparations are broken down by ribonuclease into 

 qualitatively very similar mixtures of mono-, di-, and polynucleotides. 

 For example, the largest nucleotide fragments are in all cases extremely 

 rich in guanylie acid. It would thus appear that these PNA preparations, 

 although differing in composition, are constructed according to very similar 

 patterns. The definition of these patterns remains a task for future research. 



'" J. D. Watson and F. H. C. Crick, Nature 171, 737 (1953). 



