Digestion of nuclear DNA using the restriction endonuclease II generates two components, distinguishable on the basis of their molecular size. The high-molecular-weight, II-resistant component, which accounts for 20% of the DNA, contains a fivefold greater concentration of 5-methylcytosine residues than the low-molecular-weight II-digested fraction. Segments of hypermethylated (M) DNA are largely composed of a single, long, highly repeated sequence, and this major element is sometimes associated with other less highly repetitive sequences in the M DNA fraction. Restriction mapping of cloned M DNA segments, and Southern blot analysis of genomic DNA using subcloned segments of M DNA as a probe, provide evidence for sequence variation within different copies of the dominant highly repeated element, and possibly the other associated repeats in M DNA, and additionally that almost complete tandemly repeated copies of the major repeat are found in some M DNA segments.


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