The repeated DNA sequences of microtinae: II. Localization and interdispersion of repeated sequences in Microtus agrestis

Abstract

The repetitive DNA of Microtus agrestis which reassociates at a C 0 t of less than 5 is composed of two highly repetitive satellite-like components, comprising about 8 % of the nuclear DNA, and a fast-intermediate repetitive DNA component comprising about 17 %. In this investigation, labelled complementary RNA (c-RNA) to these components was synthesized and hybridized with a vast excess of nuclear DNA fragments at short, single-stranded terminal regions, mechanically generated by mild shear. Following density gradient centrifugation in neutral CsCl, the UV absorbance and amount of labelled hybrid within each fraction were determined. The resulting radioactive labelling patterns of the hybrids, after superimposition upon the UV absorbance patterns of the DNA, served as markers for the determination of the buoyant densities of the repetitive DNA components, as well as the study of their organization within nuclear DNA. The results suggest that the three repetitive DNA components are interdispersed since they cannot be isolated totally free from each other and their buoyant densities are dependent upon the molecular weight of the nuclear DNA. In situ hybridization of c-RNA to the three repetitive DNAs with metaphase chromosomes showed a preferential labelling of the giant and heterochromatic sex chromosomes, and a small amount of labelling over the centromeric regions and arms of the autosomes.