IS DNA REALLY A DOUBLE HELIX

Do the two chains of the DNA molecule coil round one another plectonemically ? If so, what is the approximate value of Lk (the linking number) for any closed, circular DNA molecule? Experiments using gel electrophoresis have shown that supercoiled DNA molecules usually migrate in a series of discrete bands. The only tenable explanation for this quantized behavior is that the molecules in one band all have the same value of Lk and that this value differs by unity from that of the adjacent bands. Various experiments in which circular DNA is unwound by known amounts show that (given this assumption) Lk for relaxed DNA is very roughly equal to N 10 (where N is the number of base-pairs), as expected from the classical double helix. The original model for the double helix was right-handed. The experimental evidence for this feature is suggestive but not yet completely compelling. © 1979.