Abstract
Fragments of various lengths derived from the DNA of Escherichia coli were fractionated with respect to the buoyant density of their mercury complexes. Fragments 10 5 nucleotide pairs long proved to be uniform in density and composition (51% guanine plus cytosine) except for a small fraction ranging down to 49%. Fragments 1900 nucleotide pairs long varied between 39 and 56% guanine plus cytosine, with a standard deviation of ±3.8 per cent. The distribution was little affected by variation in length of fragments between 1200 and 2300 nucleotide pairs. At all lengths examined, fragments rich in adenine and thymine were more numerous than fragments rich in guanine and cytosine. Molecular segments of the extreme composition 39% guanine plus cytosine were found in lengths up to 30,000 nucleotide pairs. They comprised 3.3% of the DNA when resolved as fragments 1200 nucleotide pairs long.
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