SESSION IV. DISCUSSION 371 



normal can be infective. Could the earlier speakers tell us just how the infectivities of the 

 différent t3rpes of preparations compared ? 



A. A. Smorodintsev (U.S.S.R.): 



Since the classical work of Prof. Stanley and his school on the biochemical composition 

 and structure of viruses, published during the past 20 years, it has become indisputable 

 that, in its simplest form, life can exist as crystalline molecules of nucleoproteins while 

 retaining the essential characteristics of Uving material. 



In 1956, however, Fraenkel-Conrat and Schramm extended the limits of what could be 

 regarded as life, to include even simpler chemical structures than crystaUine molecular 

 nucleoproteins, namely nucleic acids. 



There is as yet, of course, no basis for generalizing from these important discoveries 

 to organisms other than viruses, bearing in mind that viruses probably followed a spe- 

 ciaUzed evolutionary path associated with their circumscribed adaptation to a parasitic 

 existence within the cells of bacteria, animals and plants. It is well known that parasitism 

 evokes a far-reaching simplification of morphological and biochemical organization such 

 as we do not see among free-living organisms anywhere on the Earth. 



Although the biochemical, morphological and biological aspects of the earliest stages of 

 viral multipUcation are still rather obscure, there can be doubt that they will be elucidated 

 by experiment. This would, in any case, seem to be a simpler task than the discovery of 

 the secret of the origin of Ufe on the Earth. 



To avoid any uncleamess in the fundamental concepts of viruses and life one must first 

 justify the assumption that viruses are living exogenous parasites in cells which are sus- 

 ceptible to them. This applies equally to viral nucleic acids when they enter the cell by 

 natural means or after their isolation in a biochemical laboratory. This important point 

 in Stanley's paper deserves to be fully recognized. 



However, Stanley's second point concerning the identity between viruses and genes — 

 i.e. the normal hereditary factors of the susceptible cells, requires reconsideration, as it 

 definitely contradicts the exogenous nature of viruses. This idea is supported by the 

 argument that genes are capable of multiplication and mutation and can also be introduced 

 into a cell from outside, e.g. the process of transformation or transduction from one group 

 of cells to another. These processes, however, always involve the introduction into the 

 cell of amounts of constitutional protein or nucleic acids which are quite adequate for the 

 species concerned. This process cannot be regarded as infection and is only comparable 

 with virus infection because, in both cases there is something introduced from outside 

 and the reproduction of similar new material. 



In virus infections, unUke the phenomena of transformation or transduction of nucleic 

 acids, a foreign and, usually, harmful exogenous factor enters the cell, having nothing in 

 common with any of the proteins or nucleic acids of the affected cell. The analogy between 

 virus infection and transformation or transduction is thus merely superficial or formal in 

 character and does not reflect the fundamental biological differences in the nature of the 

 phenomena. 



On the other hand the analogy between viruses and genes may give the illusion of the 

 existence of viral processes when they are not present (transformation and transduction) 

 while the parasitic process of reproduction of a virus will be removed from the category 

 of exogenous infections to be considered as an endogenous process in which there is 

 'production' of virus by the cell instead of autonomous multiplication of similar particles. 



From the biochemical point of view there may be no difference betv/een the introduction 

 of its own proper nucleic acid into a cell and the introduction of viral nucleic acid into it. 



From the microbiological or medical points of view, however, they are two completely 

 different phenomena. In the first case the cell receives an adequate hereditary factor and, 

 imder its influence, changes some of its characteristics, acquiring the ability to reproduce 

 the new nucleic acid. 



In the second case a completely foreign parasitic element enters the cell, leading, not 

 to construction, but to the actual or potential destruction of the life of the cell. Although 

 certain superficial analogies may be found between viruses and genes, nevertheless, the 

 biological concept of these two categories of life is completely different. 



