III. BACTERIOPHAGE DNA AND BACTERIAL DNA 137 



Phage bearing 6 markers were crossed with wild type, and the mottled 

 plaques were resuspended and replated. The r-type plaques were then 

 tested to learn exactly which of the 6 r's are represented on the plate, 

 for these r's must have been originally found in the heterozygous phage. 

 They found that these heterozygous particles must have contained all 

 6 r+ markers (otherwise they would not have been detected as mottled 

 plaques), and in general 1, 2, 3, 4, 5, or 6 r's usually in contiguous 

 groups. For example, if a, b, and c were present and d, e, and /, were 

 absent it was concluded that the heterozygous segment must have 

 terminated in the region between c and d. By further examination of 

 these heterozygotes which had a single termination in the region covered 

 by the 6 r markers, Doermann and Boehner demonstrated that r markers 

 near the termination of a string of r's were not present in the mottled 

 plaques as frequently as markers near the extremes of the r region. This 

 would only be expected if the existence of a termination somehow 

 caused the markers near the termination to be copied less frequently (in 

 preference to the wild type alleles). From these experiments Doermann 

 and Boehner conclude that the physical structure of these heterozygotes 

 is probably not a simple duplex DNA molecule, but rather a more 

 complicated structure which actually has more DNA than a non- 

 heterozygous particle. Their proposal is shown in Fig. 10. 



iiiiiiiii I iii^iiiiiiiiiiii \ 



t - )iiiiiiiiiiiiiiiiiiiiiiiiiiim 



' "" ' ^ < i , 



< iii ii iMM ii q i n i m i ' ^^-^^^ 



Fig. 10. Proposed structures for heterozygous DNA molecules. (From Doermann 

 and Boehner, 1962.) 



This model receives some support from the findings of Doermann 

 (1961). When progeny phage containing recognizable heterozygotes are 

 banded in CsCl and fractions taken, the heterozygous particles are 15 

 times as numerous in the denser edge of the band, indicating that they 

 could indeed contain more DNA than the average phage particle, or 

 that there is some other special reason for these particles to be denser. 



The termination of the genetic strand in the heterozygous region 

 and the finding that HETS have an increased density in CsCl suggests 

 that this discontinuity should be recognizable by physicochemical ex- 



