Viruses — Cell Parasites 69 



It was shown by Pine that they are not simple proteins, but nucleo- 

 proteins, i.e. compounds of a nucleic acid with proteins. The nucleic 

 acid is sometimes similar to the dna of the chromosomes (but the 

 bases present are sometimes different, e.g. T2 bacteriophage contains 

 hydroxymethylcytosine instead of the usual cytosine); in others the 

 nucleic acid is similar to that present in the cytoplasm (rna). Plant 

 viruses frequently contain rna and many animal viruses dna. 



The infectivity is usually highly specific in that a strain of the virus 

 will only live and multiply in one particular host species, e.g. tobacco 

 mosaic virus only attacks tobacco plants, and the common polio- 

 myelitis virus attacks very few animal species. Sometimes the virus 

 can multiply only in some strains of a species and not in others, e.g. 

 some varieties of potato such as the King Edward are resistant to 

 potato mosaic virus. They cannot usually be made to multiply in an 

 ^artificial' medium, although some will grown when introduced into 

 living eggs. Either their nutritional requirements are so exacting that 

 proper mixtures which will support tliem have not yet been found, 

 or they require to make use of the whole apparatus for providing 

 energy and growth substances, which is present in living cells. 



Are we to class these substances as Hving or non-living? They have 

 the basic properties of living things — the ability to reproduce them- 

 selves in a suitable environment. But the size and crystallizability 

 of their molecules brings them close to the inanimate world. They 

 seem to be on the dividing line between the two worlds. Dr Stanley 

 has said : 'As we go from the admittedly non-living to the admittedly 

 living, I think there must be a transition stage where there are entities 

 that may possess some properties of the non-living and of living 

 things. What could fill this place more simply and logically than the 

 high molecular weight virus proteins that are intermediate in com- 

 plexity betwen the protein enzymes and hormones, the wonderful 

 properties of which we already recognize, and the system of proteins 

 that we call protoplasm and which constitutes life? There is evidence 

 that even within the virus group there is a gradual increase of com- 

 plexity of structure from the small nucleo-proteins to the more 

 "elementary-body" type of virus.^ There is, however, no sharp 



break. . . .' 



It seems that a certain degree of complexity is needed before an 

 organism can maintain itself as an independent living thing with the 

 ability to live on its environment and grow and multiply. As we have 

 seen, this ability involves a most complicated apparatus of reactions 

 which will extract raw materials and energy sources from the food 

 and use them not only to maintain life but to make all the enzymes 

 1 Large round virus particles, about 175 mju across. 



