The Superhelical Density of Nuclear DNA from Human Cells

Abstract

Structures resembling nuclei may be released by gently lysing human [cervical carcinoma HeLa] cells in solutions containing non-ionic detergents and high concentrations of salt. These structures, which are designated nucleoids, sediment in sucrose gradients containing the intercalating agent, actinomycin D, in the manner characteristic of superhelical DNA. The concentration of actinomycin that minimizes the rate of sedimentation of nucleoids was determined. At this concentration, the amount of drug bound per base pair of DNA was determined using a double-labeling procedure. Assuming that each molecule of actinomycin bound to nucleoid DNA unwinds the double helix by 26.degree., then there is 1 supercoil every 90-180 base pairs in nucleoid DNA. These values lie within the range found for the circular DNA molecules of plasmids and viruses.