398 



CHAPTER 43 



case we would also be correct to call a cistron 

 the much shorter linear sequence of nucleo- 

 tides required to specify a single amino acid. 

 (Recall that it has been suggested on p. 370 

 that heterochromatin is composed of short 

 cistrons that specify, not polypeptides, but a 

 much simpler kind of chemical product.) It 

 will be necessary, therefore, to realize that 

 although a cistron is recognized as a linear 

 sequence of nucleotides which produces a 

 single primary phenotypic effect, the particu- 

 lar primary function under consideration will 

 determine the complexity of the product and 

 therefore the length of the cistron involved. 



Seymour Benzer, in 1961. 



SUMMARY AND CONCLUSIONS 



Genetic recombination can occur between temperate phages of different, but related, types, 

 but not between their prophage regions. The prophage stage of a phage is (1) associated 

 with a unique chromosomal site, (2) the cause of immunity, (3) essential for the subsequent 

 production of infective phage. The first two properties of prophage are identified with the 

 Ci region of the phage genetic map. However, this region does not necessarily contain all 

 the genetic material necessary for the production of complete infective phage. Not all 

 of the parental phage DNA is conserved in subsequent phage progeny. 



The genetic fine structure of the rll region of 0T4 is revealed by studies of mutation, 

 complementation, and genetic recombination. The data from this study and others suggest 

 the hypothesis that one recon equals one nucleotide. 



REFERENCES 



Benzer, S., "Fine Structure of a Genetic Region in Bacteriophage," Proc. Nat. Acad. Sci., 

 U.S., 41:344-354, 1955; reprinted in Papers on Bacterial Viruses, Stent, G. S. (Ed.), 

 Boston, Little, Brown, 1960, pp. 209-219. 



Benzer, S., "The Elementary Units of Heredity," pp. 70-93 in A Symposium on the Cliemical 

 Basis of Heredity, McElroy, W. D., and Glass, B. (Eds.), Baltimore, Johns Hopkins 

 Press, 1957. 



Benzer, S., "On the Topography of the Genetic Fine Structure," Proc. Nat. Acad. Sci., 

 U.S., 47:403-415, 1961. 



Benzer, S., "The Fine Structure of the Gene," Scient. Amer., 206 (No. l):70-84, 1962. 



Davidson, P. F., Freifelder, D., Hede, R., and Levinthal, C, "The Structural Unity of the 

 DNA of T2 Bacteriophage," Proc. Nat. Acad. Sci., U.S., 47:1123-1129, 1961. 



