eneral Interspersion of Repetitive with Non-repetitive Sequence Elements in the DNA of Xemggts

,The arrangement of repetitive and non-repetitive sequences was studied in the DNA of Xenopus. Labeled DNA sheared to various fragment lengths was reassociated to C,t 50 (SO’C, O-18 M-Na+) with excess 450 nucleotide fragments of unlabeled DNA, and binding of the labeled DNA to hydroxyapatite was measured. Repetitive sequences monitored in this way are present on about 45% of the 450 nucleotide fragments. As DNA fragment length is increased, larger fractions of the DNA are found to contain repetitive elements. Up to 80% of the DNA binds at an average fragment length of 3700 nucleotides. Analysis of the data shows that a little more than 50% of the genome consists of closely interspersed repetitive and non-repetitive sequences. The average length of the repetitive sequence elements is 300+ 100 nucleotides, while the non-repetitive sequences separating adjacent repetitive sequence elements average 800 & 200 nueleotides. The remainder of the DNA is mainly non-repetitive, though most of it contains rare interspersed repetitive elements spaced at a minimum of 4000 nueleotides”apart. It is concluded that a high degree of order exists in the arrangement of DNA sequences in the Xenopus genome.