Bacteriophage: Recombination and Genetic Maps 351 



tribution of labeled parental DNA is de- phages. The simplest explanation for this 



termined both among the parental and the result is that recombinants are formed after 



recombinant genotypes. In such experi- breakage and rejoining of the parental DNA 



ments, 1 - discrete amounts of the original strands. Such results do not support — but 



parental DNA are found in the recombinant do not exclude — phage recombinations oc- 

 curring via a copy-choice mechanism accord- 



ing to which recombinant phage DNA is 

 expected to be 

 (1961). labeled material. 



12 See M. Meselson and J. J. Weisle (1961), and 



G. Kellenberger, M. L. Zichichi. and J. J. Weigle expected to be made of entirely new, un 



424 

 Lac, Gal b 82 X 434 361 21 !466 



---I 1 I I I I I H- 



figure 26-10. Part of the E. coli linkage map showing the location of certain inducible 

 prophages. 



SUMMARY AND CONCLUSIONS 



The morphology and lytic cycle of the virulent T-even phages of E. coli are discussed, 

 and their genetic material identified chemically as DNA. After multiple infection 

 with T4 phages carrying different genetic markers, genetic recombinants are found 

 among the progeny. 



From the results of such phage crosses, a recombination map can be constructed 

 for T4 in which the genes are arranged linearly in a single circle. Recombinant phages 

 are often diploid for a short region between the recombinant markers. 



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

 complementation, and genetic recombination. Their data suggest the hypothesis that 

 the smallest recombinational unit in phage is one nucleotide. 



Genetic recombination also occurs between mutants of a temperate phage. The 

 single linkage group of lambda is not circular. Immunity to superinfection is deter- 

 mined by the c x region of the phage genetic map. Sometimes, if not always, phage 

 recombination involves parental strands which have broken and rejoined. 



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 

 Chemical 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. 



Brenner, S., Streisinger, G., Home, R. W., Champe, S. P., Barnett, L., Benzer, S., and 

 Rees, M. W., "Structural Components of Bacteriophage," J. Mol. Biol., 1:281- 

 282, 1959. 



