title: A study of the reversibility of helix-coil transition in DNA.
abstract: 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 (pBR322, a fragment of T7 phage DNA, a fragment of phage Lambda DNA, a fragment of phiX174 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.
mesh_terms: D003090:Coliphages / Q000378:metabolism; D004278:DNA, Superhelical / Q000378:metabolism*; D004279:DNA, Viral / Q000378:metabolism; D006358:Hot Temperature; D008956:Models, Chemical; D009691:Nucleic Acid Denaturation; D009695:Nucleic Acid Renaturation; D009994:Osmolar Concentration
chemical_list: D004278:DNA, Superhelical; D004279:DNA, Viral