Rat ascites nuclear DNA has been characterized by its reassociation profile [(Na+) = 0.18 at 62 °] as judged by chromatography on hydroxyapatite, : Single copy DNA (C0t1/2 obed=1.5×103) comprises 65% of the genome. Sequences repeated an average 1800 times (middle repetitive DNA, C0t1/2 obed=1) comprise 19% of the genome while 9% of the genome (highly repetitive DNA) reassociates faster than is measured in these experiments (C0t1/2 obed <2 ×10-2). Middle repetitive and single copy DNA have been isolated and characterized with respect to their reassociation kinetics and melting profiles. Both isolated classes reassociate with the kinetics describing these components in total nuclear DNA. Reassociated single copy DNA has high thermal stability indicative of fidelity in base pairing; reassociated middle repetitive DNA has a lower thermal stability that may be attributed, in part, to base-pair mismatch, and, in part, to the shortness of the duplex regions. Rat ascites heterogeneous nuclear RNA (HnRNA, 5-10 × 106 daltons) has been sheared and hybridized in excess to isolated single copy or middle repetitive DNA [(Na+) = 0.18 at 62°]. HnRNA hybridizes to a minimum of 4.5 % of the single copy and a minimum of 7.8% of the middle repetitive DNA. The Tm of single copy or middle repetitive hybrids is 1-2° lower than that of the reassociated homologous DNA. DNA isolated from single copy or middle repetitive hybrids reassúciates with kinetics similar to the input DNA. Sheared HnRNA has been hybridized to an excess of total nuclear DNA. 38 % of the HnRNA hybridizes with kinetics (C0t1/2 = 2×103) similar to single copy DNA and 12% hybridizes with kinetics (C0t1/2 = 5.6) a little more slowly than the major reassociating component of middle repetitive DNA. The remaining 50% of the HnRNA failed to hybridize.
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