Title: Principles of modern biology
Identifier: cu31924001154115 (find matches)
Year: 1964 (1960s)
Authors: Marsland, Douglas, 1899-
Subjects: Biology
Publisher: New York, Holt, Rinehart and Winston
Contributing Library: Cornell University Library
Digitizing Sponsor: MSN

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10 - The Cell
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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

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