Abstract
The observation of Thomas et al. (1970) that certain double-stranded DNA fragments from eukaryotic chromosomes can form circular structures after treatment with exonucleases (Fig. 1, first two steps) provides a method of considerable potential for the topographical analysis of repetitive sequences. The formation of such “Thomas circles” is expected for fragments derived from either of two topographical classes of repetitive sequences—the tandem repetition and the intermittent repetition. In the tandem repetition, a given sequence is serially repeated without contamination with other sequences. This appears to be the case for the small, highly repetitive sequences in satellite DNAs (Peacock et al., this volume; Gall, 1973; Gall et al., this volume), and for the repeated gene-spacer sequences found in DNAs specifying the 5S and 18–28 S rRNAs (Brown and Sugimoto, this volume). In the intermittent repetition, the repeated sequence is interspersed among sequences that are not repeated, at least within the DNA...
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