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

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 micron. 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.