Abstract
When human genome was fractionated by thermal elution chromatography, repetitious DNA was found in every arbitrary temperature segment. When these repetitious DNA families were used for in situ hybridization, the following conclusions were evident: (a) Unlike the case of mouse, where essentially all centromeric heterochromatins appear to be composed of one DNA family, human heterochromatin is composed of various DNA families. (b) Some human heterochromatin pieces, (e.g., that of chromosome 9) appear to have more heterogeneous composition than others (e.g., that of chromosome 1). (c) The highly repetitious human DNA fractions are located primarily at the centromeric and telomeric regions, but the interstitial regions also contain these fractions. (d) The more slowly reassociating repetitious sequences are distributed over the length of the chromatid, with a slight bias in favor of the telomeric regions. (e) The repetitious DNA fractions of higher (guanine + cytosine) content have less affinity for the centromeric regions.