When a mixture of two DNA samples is denatured and then renatured, single strands from the two types of DNA will specifically recombine if the base sequences are essentially the same. Such hybrid DNA molecules can be identified in density gradient ultracentrifugation if one of the samples has been made heavy. Using 5-bromouracil substitution or N15 and D replacement, heavy DNA from T4 and T7 bacteriophages was prepared and used with normal DNA from T2, T3, and T6 bacteriophages in a test of sequence homology. A high degree of homology was found between pairs of T-even phage DNA and between the DNA from T3 and T7. No indication of homology was found between DNA from T-even and T-odd phages, nor between these and Escherichia coli DNA. These results are in general agreement with genetic evidence except that genetic exchange between T3 and T7 bacteriophages has not yet been reported.
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