Electron microscopy of membrane-associated folded chromosomes of Escherichia coli.

Membrane-associated folded chromosomes were purified from log-phase cultures of Escherichia coli 15 TAU-bar and prepared for electron microscopy by aqueous spreading techniques. A spectrum of structures was observed, ranging from condensed structures with no DNA fibers visible, to extended structures with DNA fibers. In the extended structures, loops of DNA radiated from residual envelope, the loops sometimes appeared supercoiled, and both their number and apparent contour length approximated previous estimates from physical and biochemical data. It is proposed that the structures with free DNA arose from the condensed structures.