Direct visualization of the formation of a new structure of lambda DNA in vitro

摘要

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 Ⅲtreated by cooling down quickly following thermal denaturation.Transmission electron microscope(TEM)andatomic force microscope(AFM)visualizations of the DNA molecule have shown that their lenghts fall mainly inthree regions(300 — 500 nm,750 — 1 000 nm and ～ 1 500 nm),indicating that longer fragments of λ-DNA HindⅢ are apt to form the new structure of DNA under experimental conditions;moreover,the apparent width inthe 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 anda weak negative band at 220 nm,indicating that the DNA molecule is distinctly different from all the known struc-tures of DNA.