Early and late helix-coil transitions in closed circular DNA. The number of superhelical turns in polyoma DNA.

摘要: Abstract The titration of closed circular polyoma DNA with small amounts hydroxyl ions causes an unwinding the duplex and a simultaneous quantitatively equivalent superhelices. buoyant densities both intact, I, singly nicked, II, were measured as function pH, fraction base pairs titrated was calculated from density shifts. A but significant increase in occurred I prior to onset II. This early or helix-coil transition completed when 3.2% titrated. We have foregoing result molecular weight that molecule contains −15 ± 1 superhelical turns cesium chloride at neutral pH. Each turn consists approximately 1000 length about 300 pairs. remaining 97% alkaline occurs pH values are higher than required completely titrate nicked late curve is broad, has midpoint 12.28, not until 12.37 20 °C. stabilization caused by inability unwind correspond elevation melting temperatures standard saline citrate 17 °C 22 completion titration. An absorbance study thermal confirmed temperature exceeded 104 °C, while II melted 89 Thermal 7.2 m -NaClO4 studied methods. similar for T7 DNA. broad 73 compared 48 It estimated results should be 107