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 m elting point (similar to 73.5 degrees C) and is resistant to DNase I-d igestion, has successfully been isolated from lambda DNA Hind III trea ted by cooling down quickly following thermal denaturation. Transmissi on electron microscope (TEM) and atomic force microscope (AFM) visuali zations of the DNA molecule have shown that their lenghts fail mainly in three regions (300-500 nm, 750-1 000 nm and similar to 1 500 nm), i ndicating that longer fragments of lambda-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 images of a few longer molec ules of the new structure of DNA is relatively thinner, which may be r easonably deduced from the inhibition of DNase I (an endonuclease) bin ding to the new structure of DNA. CD spectrum of the new structure con tains a broad positive: band centred at 245 nm and a weak negative ban d at 220 nm, indicating that the DNA molecule is distinctly different from all the known structures of DNA.