I »S CHAPTER 34 



34.6. What raw materials arc needed to make a mixed polyribonucleotide in the ab- 

 sence ol a primer'.' In the presence of a primer? 



34.7. Do you suppose the first genetic code was or was not degenerate? Explain. 



34.8. Work out the relative frequencies of the triplet code letters, UUU, UUA, AAU, 

 UAC, AAA, CCC, in the specific sequences given from a polymer synthesized 

 from ribotides of U. A, and C in the relative amounts of 6, L, and 1, respec- 

 tively. 



34.9. Using large quantities of riboside diphosphates of A, U, G, and C in the rela- 

 tive proportions of 4, 3, 2, 1, and polynucleotide phosphorylase to synthesize a 

 mixed polyribotide, give the proportion of sequences in the polyribotide for the 

 following types (all read in one direction only): 



(a) doublets AU; AC; CA 



(b) homotriplets; heterotriplets 



(c) quadruplet AUCG 



34.10. Make a list of the minimal requirements for the functioning and reproduction 

 of the simplest free-living organism you can imagine; estimate the minimum 

 number of nucleotides required to perform these functions assuming the genetic 

 material is RNA; assuming it is DNA. Compare your estimates with the num- 

 ber of nucleotides in TMV and </>X174. What are your conclusions? 



34.11. Devise experiments which permit the collection of essentially pure sRNA carry- 

 ing phenylalanine; sRNA carrying lysine. 



34.12. Cysteine, while still attached to its normal sRNA type, is converted to alanine 

 by reduction with Raney Nickel. Using synthetic polyribotides, design a di- 

 rect test of the hypothesis that sRNA functions as an adapter in specifying the 

 fit of amino acids on a template. 



34.13. How can you explain the observation by L. Grossman that ultraviolet irradia- 

 tion of polyuridylic acid not only results in a marked depression in incorpora- 

 tion of phenylalanine in an in vitro protein synthesis but is accompanied by an 

 increased incorporation of serine? What relation has your explanation to the 

 observation that polyuridylic acid can normally code not only for phenylalanine 

 but for leucine? 



34.14. How can you explain the observation by G. E. Magni (see reference on p. 389) 

 that normad mutations (reversions) for certain UV-induced mutants in yeast 

 occur six to twenty times more frequently during meiosis than mitosis? 



34.15. A number of rll point mutants can be classified as resulting from transitions 

 A:T-» G : C or G'-C -* A:T according to their reversibility after treatment with 

 various chemical mutagens. Addition of FU to the nutrient medium does not 

 produce the r+ phenotype in any of the mutants which supposedly carry the 

 (,:(' pair at the mutant DNA site. On the other hand, some r+ phenotype is 

 produced by FU in 17 of the 46 mutants presumed to carry A:T at the mutant 

 site. What conclusions can be drawn? 



34.16. What bearing does the observation that a hemoglobin chain is always synthe- 

 sized beginning at the N-terminus have upon the alternatives of one-complement 

 and two-complement transcriptions? 



34.17. Of the possible 64 unidirectionally-read triplets using AUGC, how many have 

 one or more U's? No U's? 



