BIOLOGICAL ASPECTS OF INTRACELLULAR STAGES OF VIRUS GROWTH 153 



of these extracts is due to viral nucleic acid. When we remember, too, that in 

 a number of animal viruses, as was shown for some plant viruses, the amount 

 of ribonucleic acid in viruses of very different sizes is surprisingly constant, 

 corresponding to a molecular weight of about two million per virus particle 

 (Frisch-Niggemeyer, 1956), it is difficult not to feel that viral nucleic acids 

 will all be found to play a similar role in other animal viruses. The difference 

 between viral ribonucleic and deoxyribonucleic acids and the information 

 contained in the nucleic acids which can induce the cell to synthesize viral 

 constituents and assemble them into mature virus particles are surely the 

 main challenges for future viral research. 



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