THE CHEMICAL BASIS OF HEREDITY 27 



somal proteins — the histories — is maintained with the same constancy 

 as DNA, but there is no evidence from other kinds of data that the 

 histones play a role as hereditary determinants. 



RNA is found in many places within the cell. In the nucleus, both 

 the chromosomes and the nucleolus contain RNA. At least two dis- 

 tinct RNA fractions have been isolated from the cytoplasm, and one of 

 them, located in the microsomal particles (ribosomes), has been shown 

 to vary greatly in amount from tissue to tisssue. There are probably 

 many other RNA fractions as yet unidentified. Indeed, most informa- 

 tion available about the distribution and function of RNA fractions 

 is very recent and fragmentary. 



With respect to a genetic role for RNA, as yet the only critical 

 evidence comes from virus infection experiments of the type just 

 described. There is genetic evidence for the existence of hereditary 

 determinants which are not chromosomal and possibly not DNA in com- 

 position. One may postulate RNA as the carrier of this type of hereditary 

 information, but there is no direct confirmation of this hypothesis as vet. 

 Genetic and chemical evidence about nonchromosomal determinants will 

 be taken up in detail in Chapter 9. 



Some indirect evidence of a genetic role of nucleic acids in all organ- 

 isms came with the demonstration that cells can be killed or genetically 

 altered by irradiation with ultraviolet light, and that the most effective 

 wave lengths are those at which the specific absorption of nucleic acids 

 is maximal. The results of some typical experiments, shown in Figure 

 8.13, do not distinguish between DNA, RNA, free nitrogen bases, 

 nucleosides, and nucleotides, since their UV spectra are all quite 

 similar. There is the further problem, always a diflficulty in the inter- 

 pretation of such action spectra, that the compounds which absorb the 

 energy may simply pass it on to other compounds which are the bio- 

 logically relevant ones. Thus, these experiments do not criticallv 

 demonstrate the genetic role of nucleic acids, but they are consistent 

 with such a view. 



FIGURE 1.10. Demonstration that RNA alone is the genetic material of the tobacco 

 mosaic virus. The RNA core can be dissociated from the protein coat and recombined 

 with the same or a related protein. Both the dissociated and reassociated forms are 

 infectious. Protein coats alone are not infectious, (ol and (b) show the different 

 lesions produced on the tobacco leaf by related forms of virus. In (c) and (d), the 

 RNA of one virus has been reassociated with the protein of the other. In both com- 

 binations, the RNA component determines the type of lesion, and thus the genetic 

 continuity of the virus. 



