44 READINGS IN BIOLOGICAL SCIENCE 



expected in regard to their nature. A few crystals were dissolved in a 

 neutral liquid a hundred million or more times their bulk. Healthy leaves 

 were again inoculated, and all the symptoms of the mosaic virus disease 

 resulted. The conclusion of this experiment was that the crystals were 

 made of many protein molecules, and each molecule of this cluster of 

 crystals is a single virus of the tobacco mosaic disease. 



Chemical analysis proves that the virus molecule is very large, a macro- 

 molecule. Carbon, nitrogen, hydrogen and chlorine have been found in 

 these molecules, but how many atoms of each and their arrangement is 

 not known. That is, there is no chemical formula for a virus as yet. 



In addition to the above chemical methods the ultracentrifuge clarifies 

 the evidence as to the nature of these macro-molecules. The ultracentri- 

 fuge gives us a knowledge of the protein itself, degree of purity and the 

 extent of its concentration at each step in its isolation. A pure protein in 

 true solution is made up of molecules of the same size and shape, and it 

 will sediment at a constant rate in an intense uniform centrifugal field. The 

 heavier the molecule the greater the rate of sedimentation. The sedimentary 

 boundaries that arise between protein and solvent are determined by 

 photographing. The molecular weight of the mosaic virus was found to 

 be seventeen million times as heavy as a hydrogen atom. We may now 

 think of this virus as a "macro-molecule" with a structure that must con- 

 sist of hundreds of thousands of atoms, and may be more. 



Is this virus living or non-living? Remember that it can't be cultivated 

 in a test-tube, but bacteria which seem to be their nearest living relatives 

 can assimilate, grow and reproduce in this non-living medium. Yet the 

 only way this virus can grow and reproduce itself is when it is stimulated 

 by contact with the tobacco plant tissues. An infinitesimal particle will 

 infect a normal plant and in a few days the whole tobacco crop will be 

 diseased and producing the original amount of virus a million times over, 

 is not this ability to propagate itself a property of living things? 



Maybe this virus lives a Dr. Jekyll and Air. Hyde sort of life, a dual 

 personality, alive in certain phases of its existence and raising havoc in a 

 tobacco field, and under another set of conditions not alive and harmless 

 as sterile water. It is alive and has the attributes of living things when in 

 the presence of tobacco protoplasm and non-living in other environments. 



This crystalline protein causing tobacco mosaic has many points in com- 

 mon with a gene. They are about the same size. They both reproduce 

 under certain specific conditions and can refrain from reproducing for 

 long periods of time without losing this property when favorable condi- 

 tions exist. Quite a human characteristic! This characteristic can be il- 

 lustrated by the inactiveness of the genes in an unfertilized egg of a hu- 

 man, thank goodness, or in the resting seed of a daisy or the inactiveness of 

 a virus in dried tobacco leaves or in a spittoon. 



Furthermore, the gene and the virus have another similarity in common 



