232 CELL HEREDITY 



sponses are not Fouik] when fonvard mutation from the wild-type alleles is 

 studied, presumably because so many modifications within each of them 

 will yield an auxotroph. Each auxotroph arising by point mutation carries, 

 on the contrary, a specific modification at a specific site, and only when 

 this is restored to the wild-type condition is there a back mutation. 

 Although the average base composition of genes may turn out to be 

 similar, we have not yet the necessary evidence to decide whether all 

 genes are of similar size. Clearly the total number of nucleotide pairs 

 should have something to do with stability and apparently so does the 

 particular pattern of nucleotides within the gene, for a particular site 

 does not mutate at a rate independent of neighboring nucleotides. The 

 base sequences of different genes may soon be known; we must hope 

 that, in the course of their determination, something will be learned 

 which will enable us to circumvent the common denominators which 

 make them so similar in stability. Otherwise the elective mutation of 

 chromosomal genes seems a hopeless ambition. 



It is otherwise with extrachromosomal genes; mutants for some of 

 these have already been induced electively. Hereditary respiratory 

 deficiency can be induced at will in yeast by the use of acridine dyes; the 

 same is true for the conversion of F"^ to F~ E. coli, and for the loss of 

 the basal granule in trypanosomes. By treatment with streptomycin, 

 Euglena can be permanently freed of its chloroplasts. But these muta- 

 tions may involve only the loss of an extrachromosomal particle by 

 destruction or the inhibition of its replication. Chapter 9 will deal 

 with this question. 



BIBLIOGRAPHY 



Free.se, E., 1959. On the molecular explanation of spontaneous and induced 

 mutations. Bruokluwen Symp. Biology, 12:63-75. 



Freese, E., 1959. The difference between spontaneous and base-analogue in- 

 duced mutations of phage T4. Proc. Natl. Acad. Set. Wash., 45:622-633. 



Freese, E., 1959. The specific mutagenic effect of base analogues on phage 

 T4. ;. Molecular Biol.. 1:87-105. 



Giles, N. H., 1959. Mutations at specific loci in Neurospora. Proc. X Inter. 

 Congress oj Genetics, Montreal, 1958, 1:261-279. 



Hollaender, A., 1954. Radiation Biology. New York, McGraw-Hill, 3 vols. 



Lea, D. E., 1955. Action of Radiations on Living Cells. Cambridge, The 

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Martin, J. H. (ed.), 1959. Radiation Biology. Proc. 2nd Australian Confer- 

 ence, Melbourne, 1958. New York, Academic Press, Inc., 305 pp. 



