Figure 1.

DAPI staining shows that DNA is released from formaldehyde treated nuclei by DNase I digestion. CaSki cells were grown on coverslips and processed as described. (Upper) Cells were permeabilized with 0.5% Triton X-100 in CSK buffer before formaldehyde cross-linking and DAPI staining. (Left) Phase contrast image. (Right) Fluorescent image. (Lower) Cells were treated in the same way and then digested with 400 μg/ml DNase I for 50 min at 32°C before staining. The micrograph exposures and magnifications are identical to those in the Upper panels. Almost all detectable DNA was removed by the DNase I digestion despite the prior, extensive cross-linking of the structure.