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Information Storage and Neural Control 



with heavy isotopes and the second is of normal density. Upon 

 renaturation, hybrid molecules of intermediate density will be 

 formed: 



[6] 



Normal Density 

 DNA 



II, , II 



Heavy Density 

 DNA (n'^, c'^) 



Upon Renaturation Gives 



Heated Gives 



Single Strands 



Hybrid DNA of 

 Intermediate Density 



The normal density DNA, "heavy" density DNA, and "inter- 

 mediate" density DNA can be resolved as discrete bands by 

 density gradient centrifugation. Hybrid formation, then, is de- 

 tected by the presence of a new DNA band of intermediate density. 

 It should be emphasized that hybrid formation can take place 

 only when long regions of the nucleotide sequences of DNA 

 molecules are identical or very nearly so. Molecules having the 

 same average (G+C) content but differing in the sequence of 

 G, C, T, and A along the polynucleotide chain will not form 

 hybrids. 



The possibility that renaturation and hybrid formation might 

 take place between the DNA of bacterial strains with close taxo- 

 nomic, physiological, and genetic relationships was investigated 

 by Schildkraut et al. (61). Hybrid formation was readily demon- 

 strated between the DNA of E. coli and of six other E. coli strains. 



Interspecies hybridization of DNA was also demonstrated in 

 certain instances for bacteria having the same nucleotide content 

 of (G + C). Thus, the DNA from B. subtilis and B. natto formed 

 hybrids, and in addition, the DNA from E. coli K-12 formed 

 hybrids with those from E. coli B. and Shigella clysenterioe. Sig- 

 nificantly, these are instances where genetic exchange has been 



