536 - Heredity and Evolution 



substances needed in their self-templated 

 synthesis, and very probably the genes pro- 

 duce other substances as by-products of self- 

 synthesis. Every such removal and addition 

 must have a definite influence upon the 

 complexly interrelated reactions of metab- 

 olism, and consequently every gene must 

 have some metabolic importance in every 

 cell of the body at ever) 1 stage of develop- 

 ment. No doubt the importance of a given 

 gene is much greater in some parts of the 

 body as compared to others. If a gene is to 

 change the pigmentation of the eye, for ex- 

 ample, this elfect can appear only in such 

 cells as may contain the precursory sub- 

 stances from which the eye pigment is to be 

 formed. It seems probable that these pre- 

 cursory substrates serve not only as raw 

 materials for pigment synthesis, but also as 

 activators of the genes that produce the 

 proper enzymes. Thus, each gene may pro- 

 duce several or many effects in the different 

 cells of the organism, depending upon chem- 

 ical and physical conditions in the different 

 cells; and the destiny of a cell depends not 

 only on its genes, but also on the factors of 

 its environment, and, in addition, on the in- 

 trinsic composition and structure of its own 

 cytoplasm. 



GENIC DETERMINATION OF HORMONES 



Recent studies have made steady progress 

 in determining precisely how specific genes 

 may exert their developmental influence 

 upon the phenotypic characteristics of va- 

 rious organisms, but only one such study 

 will be mentioned. It has been shown, for 

 example, that dwarfism in mice, which is in- 

 herited as a single recessive factor, is medi- 

 ated through the pituitary gland. The pitu- 

 itary of a mouse that is homozygous for the 

 "dwarf gene" displays a deficiency in the 

 enzyme system of the particular cells that 

 are responsible for the production of the 

 growth hormone (p. 410). Thus if such 

 a genetically dwarfecf individual receives 

 growth hormone injections, it achieves full 

 stature and cannot be distinguished from a 

 genetically large mouse. 



In conclusion it may be said that our newly 

 acquired knowledge about the structure and 

 mode of replication oi DNA and about how 

 the genie materials influence synthetic me- 

 tabolism in every cell has given great im- 

 petus to the stud) 1 of biochemical genetics 

 and biochemical embryology, and one can 

 now expect a rapid "breakthrough" on these 

 scientific fronts. 



TEST QUESTIONS 



1. Summarize the main evidence that indicates 

 that DNA represents the essential genie ma- 

 terial. 



2. Differentiate between double- and single- 

 stranded DNA. How is this difference re- 

 lated to self-templated replication and to the 

 synthesis oi RNA? 



3. Distinguish between transfer RNA and mes- 

 senger RNA. What other terms are used in 

 this connection? 



4. What is the base pair rule? How is this rule 

 related to the triplet code? 



5. Explain why a doublet code would not be 

 adequate in determining protein synthesis. 



6. Mention the names of five men who have 

 made important contributions in biochemi- 

 cal genetics and explain the work of each. 



7. What is meant bv the term "bacterial trans- 



formation"? How do the transformation 

 experiments identify DNA with the genie 

 materials? 



8. Cite experimental evidence that indicates 

 that DNA is important in determining pro- 

 tein -enzyme synthesis. 



9. How does the bacteriophage virus achieve 

 self duplication? What seems to be the role 

 of DNA in this process? 



10. How and why do the experiments on the red 

 baker's mold (Xeurosjwtn) indicate that there 

 is a close relationship between genes, en- 

 zymes, and vitamins? 



1 I. Although all the cells of an embryo are equal 

 as to their genes, all the cells do not remain 

 alike as development proceeds. In general 

 terms, how can this differentiation of the 

 embryonic cells be accounted for? 



