Determination of linking number of pBR322 DNA

Do the two chains of the DNA molecule coil round one another plectonemically? If so, the linking number of relaxed circular DNA containing N base pairs, expected from the classical double helix, should be roughly equal to N/10. pBR322 DNA is a covalently closed circular DNA duplex containing 4362 base pairs. When it is in supercoiled state, its linking number calculated from B-DNA should be of the order of 4 X 10(2). We have determined the linking number or relaxed pBR322 DNA I' with electron microcopy. With the help of gel electrophoresis. It is proved that when DNA I' has the writhing number Wr equal 2 + 2, its linking number L equal 1, 2, 3, 4. A further deduction is that the linking number of native pBR322, Lk less than 4362/10. The difference between our experimental results and the estimates from the classical double helix structure is tremendous. The phenomena cannot be explained if there is only one form of DNA structure. Probably the tertiary structure of DNA is intrinsically determined by its primary structure, influenced by various factors such as temperature, pH, solvent, positive ions and the concentration of salt. In fact, right-handed, left-handed and presumably certain intermediate conformations may coexist in the same DNA duplex. In native DNA, the left-handed form is probably not a rare structure and Z-DNA may be only one example of the left-handed family. If the two strands really exist as we described above, it would be convenient for the elucidation of the mechanism of DNA replication and RNA transcription.