Chapter 19 



CHEMICAL NATURE OF GENES 



I 



n the preceding chapters the pri- 

 mary concern was with the defi- 

 nition of the genetic material on 

 the basis of its capacity to recombine and 

 mutate; our concern here will be with the 

 chemical nature of the genetic material as 

 revealed through chemical analyses. Let us 

 try to determine which of the cell's chemical 

 components are and which are not suitable 

 to serve as genetic material. Since the nu- 

 cleus contains genetic material in its chro- 

 mosomes, any chemical substances located 

 exclusively in the cytoplasm can, of course, 

 be eliminated from consideration as the basis 

 for nuclear genetic material. Because the 

 genetic material seems to possess complex 

 properties, one would expect that its chemical 

 properties were also complex. On this basis, 

 we can eliminate from consideration all in- 

 organic compounds (compounds not con- 

 taining carbon), since no class of inorganic 

 compound enters into a sufficient variety of 

 chemical reactions. 



One unique feature of protoplasm is the 

 speed and orderliness of its chemical activi- 

 ties. These two characteristics are due to 

 the presence of proteins in the form of en- 

 zymes and cellular structures. Different 

 kinds of proteins contain different numbers 

 of amino acids. Since twenty or so different 

 kinds of amino acids are found in the pro- 

 tein of organisms, the total number of dif- 

 ferent combinations is, for all practical pur- 

 poses, infinite. Protein clearly possesses 

 adequate complexity, so it is not unreason- 

 252 



able to hypothesize that the genetic material 

 is composed of protein. 



It' the gene were protein in nature, one 

 would expect to find protein in the chromo- 

 somes but not, perhaps, of a type usually 

 found in the cytoplasm. Chemical analyses 

 of nuclei and chromosomes confirm both ex- 

 pectations by revealing the existence of 

 histories — complex proteins which act as 

 bases and are found primarily in chromo- 

 somes. Although the chromosomes of many 

 cells contain histones, they are not found 

 in the chromosomes of all cells. For ex- 

 ample, histones are usually present in the 

 somatic nuclei of fish; however, the sperm 

 of trout, salmon, sturgeon, and herring in- 

 stead contains protamine, a basic protein of 

 simpler composition. The protamine in fish 

 sperm is in turn replaced by histone in the 

 somatic cells produced mitotically after fer- 

 tilization. If genetic material is protein, the 

 genetic specifications or information must be 

 transferred from protamine to histone to 

 protamine. At least in some organisms, 

 then, the same genetic specifications would 

 have to be carried in two chemical forms, 

 protamine and histone. 



Present knowledge does not prevent us 

 from entertaining the view that alternative 

 chemical compositions are possible for the 

 genetic material, but any alternative must 

 be capable of performing a number of activi- 

 ties in accordance with the principles already 

 established. Nevertheless, the hypothesis 

 that protamine and histone are both genetic 

 material in the same organism is rather com- 

 plicated, at least when one considers that 

 there would be two chemical formulae for 

 a single genotype. For the sake of sim- 

 plicity, it would be more satisfactory if a 

 single nuclear chemical substance were the 

 genetic material. 



Other proteins are found in chromosomes. 

 Their quantity changes, however, according 

 to the type and rate of metabolic activity 



