III. BACTERIOPHAGE DNA AND BACTERIAL DNA 145 



stood in tenns of a biologically produced hybrid molecule made of a 

 single duplex consisting of one new and one old polynucleotide chain. 



C. THE STRUCTURE OF DNA LIBERATED FROM BACTERIA 



The great sensitivity of bacteriophage DNA to mechanical breakage 

 by shear means that one should proceed with caution in the interpreta- 

 tion of any molecular weight determined by whatever method as repre- 

 senting a structural or functional subunit of the bacterial genome. The 

 inadvertent fragmentation by stirring or shaking already has proved to 

 be a stumbling block in the extraction of phage DNA ; it will be a more 

 acute problem with the extraction of bacterial DNA's which in prin- 

 ciple at least could be 50 to 100 times longer than T2 phage DNA 

 molecule. Among the recent investigators who have been studying the 

 extraction of high molecular weight DNA from bacteria one has obtained 

 preparations with sedimentation coefficients of 22 to 29 S corresponding 

 to molecular weights greater than 8 million (Marmur, 1961), while others 

 find 1.2 to 2.4 million, which is thought to be the molecular weight of 

 DNA subunits in vivo (Cavalieri et al., 1961). 



An interesting kind of DNase has been reported by Bernardi et al. 

 (1960, 1961), Bernardi and Sadron, 1961) which reduces the apparent 

 light-scattering molecular weight to about 500,000 and no further. If 

 this proves to be the enzymatic cleavage of unique regions of the DNA 

 molecule, it will prove a useful tool for the examination of DNA 

 organization. 



D. THE GENETIC STRUCTURE OF BACTERIA 



The study of bacterial conjugation has led to two major ideas: (1) 

 that there does exist a physical structure called the bacterial chromosome 

 which can be transferred from a donor to a recipient cell and which 

 contains all of the genes that have so far been recognized, an {£) that 

 there can exist certain strains that contain genetic elements (which 

 contain DNA), which are not physically united with the chromosome; 

 these independent elements, which can exist in a number of different 

 forms, are called episomes. Jacob and Wollman (1957, 1958a, b, 1961) 

 have suggested that the bacterial chromosome is originally a circular 

 structure which is broken by the introduction of the fertility factor 

 (F^^) to produce the various Hfr strains which are capable of trans- 

 ferring genes (by conjugation) with high frequency to the recipient cell. 

 By examining the order and time sequence of the entiy of a number of 

 genes, it was found that each Hfr strain had the same gene order, but 

 a diiferent starting point. Such a situation might arise if these Hfr 

 strains were formed by the fracture of the circular chromosome at 



