By direct base composition analysis it has been shown that most of the DNA molecules isolated from Chinese hamster nuclei band in CsCl at a density linearly related to their G + C content. A small fraction (less than 1 %) of the molecules band at a higher buoyant density and have anomalous properties. Some of the high density fractions have properties that suggest that they are single-stranded: their buoyant density does not increase after heating and fast cooling, they bind to nitrocellulose filters, their buoyant density corresponds to that of single-stranded DNA of average base composition, and they are degraded by mung bean nuclease which preferentially attacks single-stranded DNA. The nitrocellulose filter technique was used to examine DNA-DNA homologies among these CsCl gradient fractions. As expected, but not previously demonstrated, fractions of low G + C content and fractions of high G + C content are not homologous to each other as measured by the filter technique. Hybridization studies also indicate that the fractions of differing G + C content do not appear to be significantly enriched in any of four morphologically distinct size classes of metaphase chromosomes. The plateau value is higher when the filters containing total nuclear DNA are saturated with A + T-rich DNA fractions than with other fractions.
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