When cells are lysed in solutions containing high concentrations of salt and a non-ionic detergent, structures are released which retain many of the morphological features of nuclei. These nucleoids contain superhelical DNA but are depleted of nuclear protein. We have analysed DNA conformation in nucleoids derived from HeLa cells synchronized at different stages in the cell cycle. The gross differences in nuclear morphology seen during the cell cycle are reflected in the morphology of the nuc...