Chemical Nature of Genes 305 



SUMMARY AND CONCLUSIONS 



When restricted to substances unique to the nucleus, the search for chemical substances 

 which are genie, or intimately connected with the recombination, mutation, and function 

 of genetic material, led to a consideration of protein as a possible candidate. The evidence 

 available does not support protein as having such a primary role. 



In view of the localization of DNA, its quantity and distribution in mitosis, meiosis, fertiliza- 

 tion, and polyploid and polytene chromosomes, the parallelism between DNA absorption 

 and the mutability of ultraviolet light, the molecular integrity of DNA as revealed by turn- 

 over studies, and its long, linear, unbranched arrangement, it is hypothesized that DNA is 

 the genetic material in chromosomes or is at least intimately associated with the genetic 

 material therein. It is also hypothesized that RNA may assume the genetic role of DNA 

 in certain DNA-free viruses. 



Accordingly, some details of the chemical nature of RNA and DNA were presented. 



This Chapter initiates our attempt to discover the chemical units of the genetic material 

 corresponding to the recon and cistron, and the chemical basis for single gene mutation. 



REFERENCES 



Chargaff, E., and Davidson, J. N. (Eds.), The Nucleic Acids, 2 Vols., New York, Academic 

 Press, 1955. 



Loeb, T., and Zinder, N. D., "A Bacteriophage Containing RNA," Proc. Nat. Acad. Sci., 

 U.S., 47:282-289, 1961. 



Potter, V. R., Nucleic Acid Outlines, Vol. 1, Minneapolis, Burgess Publ. Co., 1960. 



QUESTIONS FOR DISCUSSION 



33.1. Do you think it is simpler to postulate that DNA rather than protein is genetic 

 material? Why? 



33.2. What is the chemical distinction between: 



a. a mononucleotide and a polynucleotide? 



b. a nucleotide and a nucleoside? 



c. a pyrimidine and a purine? 



d. a ribose and a deoxyribose sugar? 



33.3. Draw the detailed chemical structure of a polyribonucleotide having the base se- 

 quence adenine, uracil, guanine, cytosine. 



33.4. Express thymine as a derivative of uracil. What part of the term deoxythymidine is 

 superfluous? Why? 



33.5. What evidence can you provide, from your own knowledge, to support the view 

 that viruses possess genie properties? 



33.6. How would you proceed to measure the absorbency of ultraviolet light by chro- 

 mosomal DNA? chromosomal RNA? 



33.7. Do you believe that the evidence so far presented provides conclusive proof that 

 DNA is genetic material in chromosomes? Why? 



33.8. What is your opinion of the hypothesis that DNA is the chemical basis for recons, 

 but that protein is the chemical basis for cistrons? 



33.9. Is DNA complex enough to serve as the chemical basis of cistrons? Explain. 

 33.10. Do you think the term chemon could be defined usefully? Justify your opinion. 



