VARIATION AND ITS CHEMICAL CORRELATES 153 



appropriate hosts. It is assumed that the different biological properties of 

 strains are a reflection of different chemical structures and, conversely, that 

 their similar biological properties have a common chemical basis. Common 

 chemical characteristics have actuaUy been found for plant virus strains. 

 For example, strains of TMV have been found to have particles of the same 

 size and shape; they have been found to possess the same proportions of 

 protein and nucleic acid, the same kinds and proportions of nucleotides, the 

 same numbers of peptide chains (viral subunits), and the same kinds of 

 endings of the peptide chains. On the other hand, distinct chemical differ- 

 ences have also been found among the strains. In some cases, the strains 

 differ in proportions of amino acids present and, in some cases, even in kinds 

 of amino acids. This in turn leads to some different sequences of amino acid 

 residues in the peptide chains. In some instances the nucleic acids of strains 

 of TMV have been fomid to have differences in nucleotide sequence. Thus, 

 there are pronounced chemical similarities in plant virus strains and there 

 are distinctive chemical differences. 



It seems almost certain that these similarities and differences in chemical 

 structure are related to the similarities and differences in biological properties 

 exhibited by the strains. In other words, it can be suggested that a chemical 

 basis for the hereditary properties of these viruses is being established. A 

 vital question in this regard is whether or not the whole nucleoprotein is 

 involved in the genetic function. At present, evidence seems to favor a 

 genetic role for the nucleic acid alone. If this is true, then the protein com- 

 ponent of each virus is an even more direct expression of the genetic function 

 of the virus than the biological properties as expressed, for example, in symp- 

 tomatology. On this basis, it would be postulated that the various strains 

 of a virus possess unique nucleotide sequences in their RNA components. 

 These specific nucleotide sequences in turn furnish the patterns resulting in 

 the synthesis of characteristic protein components. Evidence for the exist- 

 ence of such a situation is now being sought experimentally by attempting 

 to determine the precise nucleotide sequences in the nucleic acids and the 

 amino acid sequences of the protein components of several strains. Another 

 obvious approach is the attempt to alter the nucleotide sequences in infec- 

 tious nucleic acid preparations and to note the results. In any case, it seems 

 that virus strains provide unique material with which to explore the chemical 

 basis of heredity, for they represent at present the only isolable, homogeneous 

 genetic substances available. 



References 



Anson, M. L., and Stanley, W. M. (1941). J. Gen. Physiol. 24, 679. 

 Bawden, F. C. (1950). "Plant Viruses and Virus Disease," Chronica Botanica, Waltham, 

 Massachusetts. 



