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CHAPTER 33 



FIGURE 33-1, Whole mount of a larval salivary 

 gland of Drosophila. DMA stain is restricted to 

 the nuclei. (Courtesy of J. Schultz.) 



accepting the view that the chemistry of the 

 genetic substance might change in this way. 

 We merely require, if there are alternative 

 chemical compositions for the genetic ma- 

 terial, that these be capable of performing a 

 variety of activities in accordance with the 

 principles already established. Nevertheless, 

 the hypothesis that protamine and histone are 

 both genetic material, in the same organism, 

 is complicated, at least in the respect that it 

 requires two chemical substances to account 

 for a single genotype. It would be more 

 satisfactory, because of its economy of as- 

 sumptions, to discover a single nuclear chemi- 



cal which could serve as a candidate for the 

 genetic material. 



There are other proteins found in chro- 

 mosomes. Unfortunately, the quantity of 

 these proteins changes according to the type 

 and rate of metabolic activities performed by 

 the cell. There is, therefore, no simple one- 

 to-one relationship between their quantity 

 and gene quantity. Accordingly, as in the 

 protamine-histone case, additional hypoth- 

 eses would be required in order to explain 

 genetic behavior. We may conclude that de- 

 spite the initial attractiveness of the hypoth- 

 esis that the genetic material is proteinaceous, 

 the types and amounts of nuclear protein ac- 

 tually found do not offer any clear support 

 for this view. 



There does remain another chemical sub- 

 stance found in chromosomes, which is 

 routinely absent in the cytoplasm (Figure 

 33-1). This is a type of nucleic acid called 

 deoxyribonucleic acid ov DNA, which is found 

 combined with basic proteins like protamine 

 and histone, by means of a chemical linkage 

 whose nature is not completely understood, 

 to form deoxyribonucleoproteins. Before dis- 

 cussing DNA as a candidate for being genetic 

 material, let us first proceed to study the 

 chemical composition of this material as it is 

 found in chromosomes. 



Chemical Composition of DNA 



When DNA from chromosomes is analyzed 

 chemically, it is found to contain organic ring 

 compounds of which nitrogen is an integral 

 part. The basic N-containing ring is six- 

 membered like benzene, CeHe. Figure 33-2a 

 shows the complete structural arrangement 

 of benzene. Figure 33-2a' abbreviates this, 

 by omitting the carbon atoms in the ring, and 

 Figure 33-2a" also eliminates showing the 

 hydrogen atoms attached to ring carbon 

 atoms. The basic N-containing ring in DNA 

 is called a pyrimidine. This has N substituted 

 for the CH group at position 1, as well as at 

 position 3, in benzene (Figure 33-2b). 



