The organization of repeated DNA sequences in the human genome

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Abstract
The arrangement of repetitive and non-repetitive DNA sequences was studied in the human genome. By Ag+-Cs2SO4 density gradient centrifugations of human DNA at different fragment size reannealed to different C0t values and c-RNA hybridization experiments, we have shown the presence of two repetitive DNA fractions, called fast and slow intermediate DNA, with different pattern of sequence organization. — The fast intermediate DNA sequences (6% of the genome; CsCl density in renatured form: 1.703 g/ml) are in part clustered in fragments greater then 24,000 nucleotide pairs and in part in fragments ranging from 1,800 to 600 nucleotide pairs spaced with longer more complex sequences. — The slow intermediate DNA sequences (30% of the genome; CsCl density in renatured form: 1.707 g/ml) appear to be finely interspersed with non-repetitive sequences. At a DNA fragment size of 600 nucleotide pairs only a third of the slow intermediate DNA sequences are free of unique sequences, while the other two thirds are still organized with unique sequences. — It has also been shown that a great amount of the repetitive DNA sequence transcripts in heterogeneous nuclear RNA of HeLa cells are complementary to slow intermediate DNA sequences.