Ultrastructural organization of double minute chromosomes and HSR regions in human colon carcinoma cells

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Abstract
The ultrastructural organization of double minute chromosomes (DMs) and marker chromosomes displaying homogeneously staining regions (HSRs) have been investigated in two cell lines, COLO-320 DM and COLO-320 HSR, derived from a human colon carcinoma. Double minute chromosomes in the COLO-320 DM cell line vary in number from several to several hundred and in diameter from 0.5 to 2 µm. These chromosomes lack kinetochore structures which appear as discrete electron dense plates on the centromeric chromosomes. The DMs are composed of regularly arranged nucleosome-containing fibers. These are folded to form 30 nm fibers which extend as loops from the central portion of the chromosome. In COLO-320 HSR, a majority of cells contain a single submetacentric HSR marker chromosome with HSR regions in both chromosome arms. The HSR chromosome appears structurally analogous in EM preparations to the other centromeric chromosomes of the karyotype.