Dissociation of lipid and protein from cell-DNA by direct alkaline sucrose gradient analysis

Hela cells and normal human embryo cells were analyzed in alkaline sucrose gradients after labeling with 14C-thymidine and 3H-choline chloride or 3H-amino acid mixture. Radioactivities from both choline and amino acids were observed in accord with DNA peak when the labeled cells were analyzed after 1 or 2 hour lysis in the lysing solution at 25 degrees C; i.e., a sharp and high peak observed with HeLa cells and a peak with more reduced sedimentation rate observed with normal human embryo cells could be considered to indicate the association of DNA with lipid and protein components, probably the nuclear membrane. The peak which was obtained after 4 hour lysis at 25 degrees C or after 1 hour lysis at 37 degrees C was not associated with label from choline and amino acids and, thus, proved to have been released from lipid and protein fractions. When metaphase HeLa cells were collected after labeling with choline or amino acids and then analyzed, the radioactivity was demonstrated in the sharp and high peak after 1 or 2 hour lysis at 25 degrees C; that is, the lipid and protein portions might be somehow associated with the mammalian metaphase chromosomes.