Direct Visualization of the Formation of a New Structure of Lambda DNA in Vitro

A DNA component(referred to as new DNA structure),which has higher melting point (～ 73.5℃)and is resistant to DNase I-digestion,has suocessfully been isolated from lambda DNA Hind Ⅲ treated by cooling down quickly following thermal denaturation.Transmission electron microscope(TEM)and atomic force microscope(AFM)visualizations of the DNA molecule have shown that their lenghts fall mainly in three regions(300 — 500 nm,750 — 1 000 nm and ～ 1 500 nm),indicating that longer fragments of λ-DNA Hind Ⅲ are apt to form the new structure of DNA under experimental conditions;moreover,the apparent width in the middle part of TEM imagesof a few longer molecules of the new structure of DNA is relatively thinner, wwhich may be reasonably deduced from the inhibition of DNase I(an endonuclease)binding to the new struc- ture of DNA.CD spectrum of the new structure contains a broad positive band centred at 245um and a weak negative band at 220 nm,indicating that the DNA molecule is distinctly different from all the known struc- tures of DNA.