III. BACTERIOPHAGE DNA AND BACTERIAL DNA 



147 



episome (Carey et al., 1960). Wlien Serratia cells hearing the E. coli 

 F-lac episome are cultured and the DNA is examined in the CsCl density 

 gradient, a small amount of DNA which has a density corresponding to 

 E. coli DNA now appears as a small peak close to the peak which is the 

 majority of the Serratia DNA. This small amount of DNA whicli has 

 the density and presumably the composition of E. coli DNA is absent 

 in uninfected Serratia DNA (Fig. 12A) (Marmur et a/., 1961 I. 



A 



S marcescens 



EPISOME INFECTED 

 S. marcescens 



E coh F 



■* DENSITY 



Fig. 12. Baad profiles of unfractionated DNA from parental and recombinant 

 strains. Microdensitometer tracings of ultraviolet absorption photographs of samples 

 at equilibrium in a CsCl density gradient at 44,770 rpm. The ordinate represents 

 DNA concentration as a function of the distance from the axis of rotation. The 

 band of buoyant density 1.742 gm/cm^ (N'Mabeled Pseudomonas aeruginosa DNA) 

 is used as a reference standard. (From Marmur et al., 1961.) 



