General interspersion of repetitive with non-repetitive sequence elements in the DNA of Xenopus

Abstract

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_ot 50 (60 °C, 0.18 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 nueleotides. 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 nueleotides, 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.