THE BIOCHEMISTRY OF PLANT VIRUSES 55 



virus nucleic acid with reasonable precision (Fig. 3) (J. D. Smith and 

 Markham, 1950; Markham and J. D. Smith, 1951b), For the analysis it is 

 unnecessary to remove the protein, because ribonucleic acids hydrolyze 

 much more readily than does protein, and in any case the absorption of the 

 protein constituents in the ultraviolet is quite minor compared with that of 

 nucleotides or of purines. 



Another method of hydrolysis which has been used for ribonucleic acids 

 is by means of strong (about 11 iV) HCIO4 (Marshak and Vogel, 1950). 

 This method has much to commend it for the analysis of small polynucleo- 

 tides, but it is not reliable enough for accurate analysis. 



The analyses performed on the plant viruses and summarized in Table II 



TABLE II 

 The Composition of Some Plant Virus Ribonucleic Acids* 



*Tlie above figures are obtained from Ivnight (1954) and Markham and J. D. Smith 

 (1954), which give data obtained from several sources. The values are rounded off; in 

 general, the t-no sources referred to agree to within 5 %. The data on turnip crinkle 

 \ irus are merely included for interest. 



have shown that the ribonucleic acids do not have equivalent amounts of 

 the four bases and also that, in general, related viruses have very similar 

 general nucleic acid compositions. Of course, as these substances are very 

 complex, it is not possible on the basis of estimating their nucleotide (or base) 

 ratios to decide whether two viruses are related. Indeed, there is at least one 

 instance known to the writer where this was not the case, and, all in all, 

 the simple estimation of four substances, all present in about equal amounts, 

 is not likely to give more than a small amount of information about the 

 composition of substances as complex as nucleic acids. In fact it is surprising 

 that the differences found are so great. 



C. The Finer Structure of the Ribonucleic Acids 

 The types of bonds present in the ribonucleic acids which link the various 

 nucleotides together are now fairly well recognized. The determination of 



