Chapter *33 



CHEMICAL NATURE OF GENES 



WHILE many of the preceding 

 Chapters dealt with defining 

 the genetic material in terms 

 of its capacity to recombine, to mutate, and 

 to act by performing a single primary func- 

 tion, no mention was made of the possibility 

 of studying the chemical nature of the genetic 

 material. Our attention is now drawn to 

 the nature of the genetic material as revealed 

 by studies utilizing the operation of chemi- 

 cal analysis. Let us use the knowledge we 

 already have to decide which of the cell's 

 chemical components are, and which are not, 

 suitable candidates for genetic material. 

 Since we know that the nucleus contains 

 genetic material in its chromosomes, any 

 chemical substance which is located exclu- 

 sively in the cytoplasm can be eliminated from 

 consideration as the chemical basis for nuclear 

 genetic material. In view of the fact that the 

 properties which we know the genetic ma- 

 terial possesses are complex, we should ex- 

 pect the genetic substance to be of suitable 

 complexity chemically. On this basis, we 

 can eliminate from consideration all inor- 

 ganic compounds (compounds not containing 

 carbon), since such compounds do not pro- 

 vide evidence of being suitably versatile in 

 their chemical reactions. Elimination of in- 

 organic compounds as genetic material, on 

 this basis, is particularly reasonable with 

 respect to those inorganic components which 

 are present in greater abundance in the cyto- 

 plasm than in the nucleus. 



We have already noted (p. 274) that the 

 unique feature of protoplasm is the speed and 

 293 



orderliness of its chemical activities. This 

 we have attributed to the presence of pro- 

 teins, in the form of enzymes and cellular 

 structures. We have found, in Chapters 31 

 and 32, that the primary action of some 

 cistrons is to specify the amino acids in a 

 polypeptide chain. This would require that 

 cistrons be capable of providing at least 20 

 different kinds of meanings or specifications 

 (one for each of the 20 different kinds of 

 amino acids found in protein), and suggests 

 that intra- or intercistronic arrangement is 

 somehow capable of manifesting the same 

 degree of complexity as is exhibited by the 

 polypeptide chain this arrangement specifies. 

 It is entirely reasonable, therefore, to enter- 

 tain the idea that the genetic material is itself 

 protein, in which case it would clearly possess 

 the correct amount of complexity. 



If the gene is protein in nature, we would 

 expect to find protein in the chromosomes. 

 Moreover, we might hope to find that the 

 chromosomes contain a unique type of pro- 

 tein, one not found in the cytoplasm. Chemi- 

 cal analyses of nuclei and chromosomes fulfill 

 these expectations in the form of the protein 

 histone. Histone is a complex basic protein 

 that is found only in chromosomes. How- 

 ever, while it is found in the chromosomes of 

 many organisms, it is not found in all chro- 

 mosomes. Thus, for example, though it is 

 present in the somatic nuclei of fish, it is 

 replaced in the sperm of trout, salmon, 

 sturgeon, and herring by a basic protein, 

 protamine, of simpler composition. 



Assume protamine is genetic material. If the 

 protamine in fish sperm is replaced by histone, 

 in the somatic cells produced mitotically after 

 fertilization, then the genetic specifications or 

 information must be transferred from prota- 

 mine to histone, which in turn is capable of 

 acting as genetic material. In these organisms, 

 then, the same genetic specifications would 

 have to be carried in two chemical forms, 

 protamine and histone. There is nothing in 

 our previous knowledge to prevent us from 



