microscope, but they can be resolved by the 

 electron microscope (Fig. 1-6). Among vi- 

 ruses with spherical particles, the diameters 

 range from about 17 millimicrons (m^) for 

 the alfalfa mosaic virus, to about 225 milli- 

 microns (Fig. 1-6) for the vaccinia (cowpox) 

 virus (used for vaccinations against smallpox 

 in man). In some cases (for example, human 

 influenza virus), traces of lipid (p. 80) and 

 carbohydrate (p. 76) compounds are present, 

 in addition to the nucleoprotein; and gen- 

 erally speaking the protein fraction of the 

 particles is localized at the surface, forming 

 a sort of envelope surrounding the nucleic 

 acid. 



Virus particles are much smaller and sim- 

 pler than bacterial cells, which are perhaps 

 the smallest cells. The bacterial cell is several 

 thousand times larger and it contains a wide 

 variety of chemical components — water, in- 

 organic salts, lipids, and carbohydrates — in 

 addition to proteins and nucleoproteins. In 

 short, bacteria show a fairly typical proto- 

 plasmic structure. The simplest virus, on the 

 other hand, may be a single large molecule 

 of nucleoprotein or, at most, a complex of 

 relatively few molecules. Consequently, if it 

 ever is proved that viruses are truly alive, it 

 will have to be admitted that the simplest 

 forms of life are not much more complex 

 than certain inanimate kinds of matter. 



Are Viruses Alive? The crux of the ques- 

 tion as to whether the viruses are truly alive 

 lies in the fact that no virus has ever dis- 

 played any capacity for growth and replica- 

 tion, except when the virus is inside some 

 well-recognized kind of living cell. When a 

 virus particle makes contact with the proper 

 kind of living cell, penetration occurs, but 

 only the DNA fraction actually enters (p. 524). 

 In the cell, the viral DNA rapidly undergoes 

 self-replication and less than an hour later, 

 usually, the amount of viral DNA has in- 

 creased more than a hundredfold. Viral pro- 

 tein then begins to appear and the multipli- 

 cation of complete viral particles has been 

 achieved (p. 525). In other words, viral DNA, 

 inside the host cell, behaves like a foreign 



Life and Protoplasm - 1 1 



gene, usurping the material that ordinarily 

 would be used in the replication of the nor- 

 mal genes of the host cell itself (p. 522). The 

 virus appears to provide a pattern that ac- 

 tivates certain molecules present in the host 

 cell to assemble and unite in such a way as 

 to form new virus. Indeed, it sometimes hap- 

 pens that a certain virus may become incor- 

 porated into a chromosome of a host cell and 

 thus be carried from cell generation to gen- 

 eration quite indefinitely. 



The foregoing observations make it doubt- 

 ful that the growth and replication of viruses 

 represent a truly independent type of repro- 

 duction, such as is characteristic of living 

 things generally. On the other hand, certain 

 bacteria are obligatory parasites, which never 

 reproduce except within some living host; yet 

 no one is inclined to doubt the living status 

 of these forms. Certainly viral replication 

 bears a very close resemblance to genie repli- 

 cation (p. 522), which is an integral part of 

 reproduction in all organisms. Many unsuc- 

 cessful attempts 2 have been made to cultivate 

 viruses and to induce them to reproduce in 

 some medium in which no living cells are 

 present. But unless or until such cultivation 

 is achieved, we must continue to regard the 

 viruses as more or less transitional between 

 the living and nonliving forms of matter. 



THE PROGRESS OF SCIENCE 



A science is a systematically organized body 

 of knowledge, based upon precise unbiased 

 observation and integrated by logical reason- 

 ing. To the fullest possible extent, scientific 

 knowledge is checked and counterchecked by 

 carefully planned and strictly controlled ex- 

 periments. 



Without special training in scientific re- 

 search, man has always been quite helpless 

 in trying to understand the nature of his 

 universe. In very early times man depended 



2 A recent report indicates that viral growth and 

 replication has been observed in a cell-free medium. 

 However, preformed DNA, extracted from cells, was 

 provided in this medium. 



