Chemical Nature of Genes 



253 



performed by the cell. There is, therefore, 

 no simple one-to-one relationship between 

 their quantity and gene quantity. Conse- 

 quently, additional hypotheses are required 

 to explain genetic behavior. Despite the 

 initial attractiveness of the hypothesis that 

 the genetic material is proteinaceous, one 

 can conclude that the types and amounts of 

 nuclear protein actually found do not ade- 

 quately support this view. 



Chromosomes contain another chemical 

 substance which seems to be absent in the 

 cytoplasm (Figure 19-1). This chemical 

 is a type of nucleic acid 1 called deoxyribo- 

 nucleic acid, or DNA, a substance usually 

 found combined with basic proteins like 

 protamine and histone (by means of a chem- 

 ical linkage not completely understood) to 

 form deoxyribonucleoproteins. Before in- 

 vestigating the possibility that chromosomal 

 DNA is genetic material, let us consider first 

 the chemical composition of its molecule. 



Chemical Composition of DNA 



Organic bases. Chromosomal DNA con- 

 tains organic ring compounds of which nitro- 

 gen is an integral part. The fundamental 

 N-containing structure is a six-membered 

 ring, as found in benzene, C ( ;H (i . Figure 

 19-2a shows the complete structural ar- 

 rangement of benzene; Figure 19-2a' is an 

 abbreviated version with the carbon atoms 

 of the ring omitted; Figure 19-2a" is the 

 same model condensed further by eliminat- 

 ing the hydrogen atoms attached to ring 

 carbon atoms. The basic N-containing ring 

 in DNA is a pyrimidine . This molecule 

 (Figure 19-2b) has N substituted for the 

 CH group at position 1 as well as at position 

 3 in the benzene ring. Figure 19-2b' and 

 Figure 19-2b" show successive abbrevia- 

 tions of this formula corresponding to those 

 used for benzene. 



1 Discovered by F. Miescher (1869). 



The nitrogen found in DNA is also found 

 in a derivative of the basic pyrimidine ring, 

 called a purine. This molecule consists of 

 a pyrimidine ring — minus the H atoms at 

 positions 4 and 5 — to which an imidazole 

 ring (5-membered) is joined, so that the 

 carbons at these positions are shared by both 

 rings (Figures 19-2c, c', and c"). Hence- 

 forth, the most abbreviated structural repre- 

 sentation will be used for pyrimidines and 

 purines. Since all pyrimidines and purines 

 act chemically as bases, they are termed 

 organic bases. 



"•*y.i 



u * 



figure 19-1. Whole mount of a larval salivary 

 "land of Drosophila. DNA stain is restricted 

 to the nuclei. {Courtesy of J. Schultz.) 



