. Cytology. Cytology. details). In some viruses, for example the tobacco mosaic virus, RNA apparently also plays a major genetic role (Figure 2-4). The molecular organization of nucleic acids, especially DNA, has received a good deal of attention in recent years. In view of the fact that DNA appears to represent the basis of organization of genetic in- formation, the question of structure is vital. The classic notion, primarily that of P. A. Levene, was that the nucleic acid molecule is made up of. m-'mnims-aw- - Figure 2-4. Electron Micrograph of Tobacco Mosaic Virus Protein Rods. Approximate
. Cytology. Cytology. details). In some viruses, for example the tobacco mosaic virus, RNA apparently also plays a major genetic role (Figure 2-4). The molecular organization of nucleic acids, especially DNA, has received a good deal of attention in recent years. In view of the fact that DNA appears to represent the basis of organization of genetic in- formation, the question of structure is vital. The classic notion, primarily that of P. A. Levene, was that the nucleic acid molecule is made up of. m-'mnims-aw- - Figure 2-4. Electron Micrograph of Tobacco Mosaic Virus Protein Rods. Approximately 70,000x. (From WyckofY, R. W. G., 1958. "The World of the Electron Microscope," Yale University Press, New Haven. Conn., Fig. a, Plate X. Courtesy of Dr. R. W. G. Wyckoff, University of Arizona.) repeating units of the four bases in equal amounts. This has been re- ferred to as the tetranucleotide hypothesis. One expectation from this hypothesis would be that the four bases should show a 1:1:1:1 corre- spondence. Biochemical work over the past twenty years has shown quite clearly that this expectation is not realized (Davidson, 1957). Most analyses of DNA show that there is a 1:1 relationship between adenine and thymine and between guanine and cytosine but the sum of the last two may be greater or less than the sum of the first two. This fact alone suggests intimate association of specific pairs of bases. From the biochemical data and from X-ray diffraction studies, Watson and 18 / CHAPTER 2. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Wilson, G. B. (George Bernard), 1914-; Morrison, John H. (John Herbert), 1927-. New York, Reinhold
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