10 - The Cell 





Fig. 1-6. The unit particles of different viruses vary considerably as to size and shape; 

 and in some cases, more than one kind of molecule is represented in each virus particle. 

 Electronmicrographs: 1. vaccinia virus, from smallpox vaccine; 2. influenza virus; 3. 

 tobacco mosaic virus; 4. potato mosaic virus; 5. bacteriophage; 6. virus of the Shope 

 papilloma; 7. southern bean mosaic virus; 8. bushy stunt virus of tomato. All except 1 

 and 5 were shadow-cast with gold. This technique permits the depth of each particle to 

 be appreciated. (Courtesy of C. A. Knight, University of California.) 



that displayed a high degree of purity. Each 

 crystal, however, consists not of one virus 

 particle, but of many, arranged in the or- 

 derly pattern of a crystal structure. 



Unit Structure. Analysis of the isolated 

 tobacco mosaic virus crystals proved that the 

 disease is transmitted by extremely small unit 

 particles (Fig. 1-6) composed, essentially, of 

 a single kind of substance, nucleoprotein. 1 



1 Nucleoprotein molecules are formed by combina- 

 tion between nucleic acid and protein (p. 83). How- 

 ever, two general kinds of nucleic acid are known: (1) 

 DNA (deoxyribonucleic acid) and (2) RNA (ribo- 

 nucleic acid), each having a slightly different chem- 

 ical structure. The RNA proteins are present in the 

 cytoplasm as well as in the nuclei of cells generally; 

 but the DNA proteins, of which genes are composed, 

 are restricted (except for viruses) almost entirely to 

 nuclei. 



The nucleic acid fraction of most viruses 

 has proved to be of the DNA (deoxyribo- 

 nucleic acid) type. Throughout all nature, 

 such DNA proteins are recognized as the 

 most essential components represented in 

 the structure of the genes, the instruments 

 of hereditary transmission in all organisms. 

 The DNA proteins, indeed, show- two very 

 unique and important characteristics. Each 

 is capable of engineering self-synthesis by pro- 

 viding a template that guides the synthesis 

 of its own structure (p. 522). And in addition, 

 the DNA elements of the cell may provide 

 templates for the synthesis of other sub- 

 stances (Chap. 27). 



The unit particles of different viruses are 

 generally too small to be seen with any light 



