III. BACTERIOPHAGE DNA AND BACTERIAL DNA 



135 



of the sedimentation constant estimated before and after stirring 

 indicates that the molecular weight is lower by a factor of 2. The bio- 

 logical assay of the stirred DNA now shows that most of the phage 

 progeny contain the i^ marker, but very few now contain the h marker; 

 there is no longer the simultaneous transfer of all markers. By replating 

 the resulting phage, it is possible to examine all the markers shown in 

 the map in Fig. 8. It turns out that both the s and mi markers are 



"1 I I i I — I — I — I — TTi — I — I — r 

 /lo 



SHEAR 



20 



Fig. 8. The map of lambda. Tlie re.sult of hydrodynamic shear is to ehminate 

 the simultaneous transformation of both h and i genes. The simplest interpretation 

 of these experiments is depicted by the arrow labeled "shear." 



transferred with essentially the same frequency as is the i gene. 

 Thus it appears that stirring, a treatment which breaks the molecule 

 near its mid-point, actually causes a separation of the map between the 

 h and s markers. This would be expected if the map sequence and the 

 nucleotide sequence were equivalent. The fact that both the map and 

 the molecule are broken near their midpoints suggests that the available 

 markers are distributed over most of the length of the molecule. 



8.0 9.0 10.0 1 1.0 12.0 13.0 



DISTANCE FROM AXIS OF ROTATION (cm) 



Fig. 9. The separation of DNA fragments bearing the i^ gene (O) from unbroken 

 molecules bearing the pair hi^ (%) by sucrose gradient centrifugation. (From Kaiser, 

 1962.) 



