Chapter *31 



BIOCHEMICAL GENETICS (I) 



li 



"t was stated earlier (p. 16) that 

 the nucleus is known to be very 



.active chemically. While this con- 

 clusion can be justified by abundant evidence, 

 let us consider one particular line of support 

 at this time. It is possible ^ to remove the 

 zygotic nucleus of a fertilized frog egg by 

 microsurgery. The enucleated cell, so pro- 

 duced, cannot perform normally the func- 

 tions of maintenance, growth, cell division, 

 and differentiation, and eventually undergoes 

 degeneration because of the failure of normal 

 metabolism — the normal chemical reactions 

 necessary to carry on such functions. That 

 the metabolic failure is attributable to the loss 

 of the nucleus, rather than being an effect of 

 the operation, can be proven by the fact that 

 zygotes undergoing similar operations with- 

 out being enucleated subsequently show 

 normal behavior. Most important, more- 

 over, is the fact that the same or a similar 

 nucleus can be replaced in a second opera- 

 tion, which is then followed by normal zygotic 

 activity. We may conclude, therefore, that 

 the nucleus is essential for normal metabo- 

 Hsm, that is, for the cell's normal chemical 

 activity, which has as its consequences cell 

 maintenance, growth, multiplication, and 

 differentiation. 



Let us make the simplest assumption, 

 namely, that the nuclear components which 

 are essential for normal metabolism are the 

 genes in the chromosomes. Let us presume 

 also, as the simplest explanation, that all of 

 the features of metabolism which are unique 

 ^ Based upon work of R. Briggs and J. T. King. 

 271 



to cells are the direct or indirect consequence 

 of genie action. On this basis, then, all as- 

 pects of the phenotype having a genetic origin 

 are founded upon the biochemical effects of 

 genes. We would predict, because of the 

 presence of numerous chemical substances 

 in a cell, that one gene-initiated biochemical 

 reaction would usually lead to others, which 

 in turn would lead to still others, to form a 

 kind of tree, whose successive branchings 

 represent successive chemical reactions. Since 

 all the branches would have been affected by 

 the initial gene-caused biochemical change, 

 one should find many different chemical, 

 and/or physiological, and/or morphological 

 consequences of it in the fully developed cell 

 or individual. It would not be surprising to 

 find, therefore, that a given genetic change has 

 many different effects upon the phenotype, 

 and that most, if not all, mutants have mani- 

 fold or pleiotropic effects (see Chapters 10 

 and 30). It would also be expected, when 

 these pleiotropic effects are traced back 

 toward their origin, that the many different 

 end effects would be found to be the conse- 

 quence of fewer earlier-produced effects. 

 Moreover, in tracing this pedigree of causes 

 back toward its genie origin, we would also 

 expect the more primary causes to be based 

 upon metabolic changes, changes sometimes 

 identifiable with modifications of particular 

 chemical substances (such as hemoglobin in 

 Chapter 10, or pituitary hormone in Chap- 

 ter 30). 



With this orientation in mind, let us at- 

 tempt a study of the biochemical basis of 

 gene action, an area of investigation which 

 we can label biochemical genetics. Where 

 shall we look for information regarding the 

 biochemical basis of gene action? It might 

 be fruitful to study a trait like eye color, which 

 itself is describable in terms of chemical sub- 

 stances, pigments, for there might be, in this 

 case, a relatively short series of steps to trace 

 back before arriving at, or near, the primary 

 gene-caused biochemical changes. 



