The fine structure of the melting curve for the linear ColE1 DNA has been obtained. To find the ColE1 DNA regions corresponding to peaks in the melting curve's fine structure, we fixed the melted DNA regions with glyoxal /12/. Electron-microscopic denaturation maps were obtained for nine temperature points within the melting range. Thereby the whole process of ColE1 DNA melting was reconstructed in detail. Spectrophotometric and electron microscopic data were used for mapping the distribution of GC-pairs over the DNA. molecule. The most AT-rich DNA regions (28 and 37% of GC-pairs), 380 and 660 bp long resp., are located on both sides of the site of ColE1 DNA's cleavage by EcoR1 endonuclease. The equilibrium denaturation maps are compared with maps obtained by the method of Inman /20/ for eight points of the kinetic curve of ColE1 DNA unwinding by formaldehyde.

The reversibility of DNA melting has been thoroughly investigated at different ionic strengths. We concentrated on those stages of the process that do not involve a complete separation of the strands of the double helix. The differential melting curves of pBR 322 DNA and a fragment of T7 phage DNA in a buffer containing 0.02M Na⁺ have been shown to differ substantially from the differential curves of renaturation. Electron-microscopic mapping of pBR 322 DNA at different degrees of unwinding (by a previously elaborated technique) has shown that the irreversibility of melting under real experimental conditions is connected with the stage of forming new helical regions during renaturation. In a buffer containing 0.2M Na⁺ the melting curves of the DNAs used (pBR 322, a fragment of T7 phage DHA, a fragment of phage λ DNA, a fragment of ØX174 phage DNA) coincide with the renaturation curves, i.e. the process is equilibrium. We have carried out a semi-quantitative analysis of the emergence of irreversibility in the melting of a double helix. The problem of comparing theoretical and experimental melting curves is discussed.

A previously elaborated technique for fixing a chosen partially melted state of DHA with glyoxal was used in a study of the melting process of the replicative form (RF III) of φX174DNA. Electron-microscopic maps corresponding to five points of the melting curve of RF III were obtained and compared with the theoretical melting maps obtained in (4) and (6). Ibis comparison clearly shows that only rigorous calculations (4) and not the ones proposed by Azbel (6,7) correctly predict the course of RP III melting.