Rat liver nucleoli were isolated from nuclear rat liver preparations. The nucleolar DNA and extranucleolar DNA, i.e. chromosomal DNA were examined. After deproteinization, nucleolar and chromosomal DNA were separated from RNA on 6 M CsCl isopycnic gradient. Nucleolar and chromosomal DNA were of the same density (1.703 g/cm 3 ) in CsCl. The specific activity of nucleolar DNA from normal adult rat liver was significantly higher than chromosomal DNA after 32 P incorporation in vivo , independent of the time after the label. Nucleolar DNA is metabolically distinct from chromosomal DNA. Results are discussed and a possible role of nucleolar DNA replication in ribosomal RNA synthesis is suggested.
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Ittel et al. (1970) studied this question.
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