Discrete length classes of DNA depend on mode of dehydration.

The length of double-stranded coliphage lambda DNA, as determined by electron microscopy using the benzyldimethylalkyl ammonium chloride technique, depends on the mode of dehydration. The freeze-dried DNA form is the longest (16.5 micron), whereas dehydration in methanol (15.9 micron) or in ethanol (three forms: 15.2 micron, 13.9 micron, and 12.4 micron) results in progressively shorter molecules. These measured lengths of the freeze-dried, methanol-dehydrated, and shortest ethanol-dehydrated forms correspond to the axial rise per nucleotide pair in the B, C, and A forms of DNA, respectively. The remaining forms of ethanol-dehydrated DNA seem to represent novel intermediary conformations of DNA. In agreement with the predicted increment, DNA exposed to ethidium bromide and freeze-dried is elongated by 39% (22.9 micron). All size classes show the same relative distribution pattern of bound Escherichia coli RNA polymerase molecules (nucleoside triphosphate:RNA nucleotidyltransferase, EC2.7.7.6), used as intramolecular markers, indicating that the dehydration-caused transitions are uniform.