148 CllAKLKS A. THOMAS, .IR. 



E. CONCLUSIONS 



At the pro!>ent time we can say very little al)Out the j)hysieal struc- 

 ture of the DNA in bacteria. Genetically the j^enonie ajipear.s circular, 

 hut this inference is based on the finding of many different Hfr strains 

 which themselves luive a linear genetic structure. The genetic evidence, 

 which has led to the idea of indei)endent genetic elements called epi- 

 soines, seems to be confirmed by tiie first density gradient equilibrium 

 ex]K'riments. 



Acknowledgments 



The author has j^rofited by many holijful conversations with members of the 

 Biophysics Department, Johns Hopkin.s University. He is particularly appreciative 

 of the help and criticism of Drs. Irwin Rubenstein and Michael Beer in the prepara- 

 tion of this manuscript. 



References 



Ames, B. X., Dubin, D. T., and Rosenthal, S. M. (1958). Science 127, 814. 

 Anderson, E. S., Armstrong, J. A., and Niven, J. S. F. (1959). 9lh Symposium Soc. 



Gen. Microbiol, p. 224. 

 Arber, W. (1958). Arch. sci. (Geneva) 11, 259. 

 .\urisicchio, S., Chiozzotto, A., Coppo, A., Donini, P., Frontali, C, and Graziosi, F. 



(1960). Nuovo cimento 18, Suppl. 2, 197. 

 Beer, M. (1961). /. Mol. Biol. 3, 263. 

 Beer, M. (1962). Personal communication. 

 Beer, M., and Zobel, R. (1961). J. Mol. Biol. 3, 717-726. 

 Bendet, I., Goldstein, D. A., and Lauffer, M. A. (1960). Nature 187, 781. 

 Bendet, I., Schachter, E., and Lauffer, M. A. (1962). J. Mol. Biol. 5, 76. 

 Bernal, J. D.. and Fankuchen, I. J. (1941). J. Gen. Phyxiol. 25, 111. 

 Bernardi, G., Champagne, M., and Sadron, C. (1960). Nature 188, 228. 

 Bernardi, G., Champagne, M., and Sadron, C. (1961). Biochim. et Biophys. Acta 



49, 1. 

 Bernardi, G., and Sadron, C. (1961). Nature 191, 809. 

 Berns, K. I., and Thomas, C. A., Jr. (1961). J. Mol. Biol. 3, 289. 

 Burgi, E., and Hershey, A. D. (1961). J. Mol. Biol. 3, 458. 

 Cairns, J. (1962). J. Mol. Biol. 3, 756. 

 Campbell, A. (1962). Adranccs in Genet. 11, 101. 



Carey, W., Spilman, W., and Baron, L. S. (1960). Bacteriol. Proc. p. 73. 

 Cavaheri, L. F., and Ro.senberg, B. H. (1961a). Biophys. J. 1, 317. 

 Cavalieri, L. F., and Rosenberg, B. H. (1961b). Biophy.-^. J. 1, 323. 

 Cavalieri, L. F., and Rosenberg, B. H. (1961c). Biophys. J. 1, 337. 

 Cavalieri, L. F., Deutsch, F., and Rosenberg, B. H. (1961). Biophys. J. 1, 301. 

 Celano, A., Aurisicchio, S., Coppo, A., Donini, P., and Graziosi, F. (1960). Nuovo 



cimento 18, Suppl. 2, 190. 

 Cohen, S., and Barner, H. D. (1954). Proc. Natl. Acad. Sci. U.S. 40, 885. 

 Cordes, S., Epstein, H. T., and Marmur, J. (1961). Nature 191, 1097. 

 Davi.son, P. F. (1959). Proc. Natl. Acad. Sci U.S. 45, 1560. 

 Davison, P. F. (1962). Personal communication. 



