2 NUCLEUS 241 



its propagating power. This proves beyond doubt that the mysterious auto- 

 reproduction of the crystalhzable viruses is determined by nucleoproteins. 



There is, however, a fundamental difference as compared w4th the nucleo- 

 proteins of the nuclei of the cell, the virus protein showing no nucleal reac- 

 tion. Thus the phosphoric compounds in the viruses are of the ribosenucleic 

 acid type, and not the thymonucleic acid found in the nuclei. The tobacco 

 mosaic virus molecules are threadlike, judging by their birefringence of flow 

 (Takahashi and Rawlins, 1933, 1935) and as demonstrated by the electron 

 microscope (Fig. 84c, p. 126). The thread molecules unite into bundles 

 liable to grow to microscopic dimensions and then appear as crystallized 

 virus protein. This, however, is not in a true crystalline, but rather in a 

 mesomorphous state, for the X-ray analysis of these "crystals" produces 

 only intramolecular interferences (Bernal, 1939) and does not reveal any 

 molecular lattice arrangement of the virus molecules (Wyckoff and Corey, 

 1936). Thus, like liquid crystals, the parallelized thread molecules are free 

 to revolve and shift individually. 



The structure of the mesomorphous virus rodlets, which is reminiscent of 

 that of the chromonema, favours their cleavability. On the other hand, the 

 reduplication of the chromomeres can hardly be understood as a mere split- 

 ting of bundles of parallelized molecules. The comparison is also prejudiced 

 by our complete ignorance as to how the nucleic acids are distributed in the 

 submicroscopically visible virus molecule. The analogy rests merely upon 

 the common filiform structure. 



It is the mysterious auto-reproduction of the virus protein which 

 encourages comparison with the chromonemata in the chromosomes. If 

 only a trace of the thread molecules of tobacco mosaic virus finds its way 

 into the cells of the tobacco leaf, they fill up completely, in an astonishingly 

 short time, with the pathogenic protein, which becomes visible as birefrin- 

 gent rodlets, whereas the protein proper to the cell diminishes. Thus, when 

 in contact with virus molecules, non-virus protein becomes virus. This 

 phenomenon has been termed autocataljtic reproduction. It is known in other 

 compounds; for example, small amounts of trypsin are liable to change a 

 larger amount of another compound, known as "protrypsin", into trypsin. 

 Energy is required for the spontaneous reproduction of the virus protein 

 and this is supplied by the living cell. There can, therefore, be no reproduc- 

 tion of virus outside the living cell. 



It is tempting to regard the duplication of the chromonemata in mitosis 

 likewise as autocatalytic reproduction; but we should not forget that we 

 have simply coined a term for what is at present an inexplicable process and 

 are still quite in the dark as to the nature of the "first step" which, through 

 contact with the specific nucleoproteins of the chromonema, has autocatalyt- 

 ically to be transmuted into identical nucleoproteins. 



The electron microscope shows that the rod-like shape of the tobacco 

 mosaic virus (Wyckoff, 1947a) is an exception. The majority of the virus 

 species photographed by Wyckoff (1947b) have a pronounced globular 



