Using the LKB-2277 flow microcalorimeter (Bioactivity Monitor) and the LKB-2107 batch microcalorimeter, the scission of calf thymus ds DNA by the mixture of (1, 10-phenanthroline) copper (Ⅱ) [(phen) ~2Cu^2^+], mercaptoethanol and O~2 has been studied. The overall reaction of this DNA cleavage has been established as a faster, exothermic and enthalpy-driven reaction, but the earlier stage of this cleavage is a fast endothermic process. The overall molar reaction enthalpy and the molar reaction enthalpy of the earlier stage of this cleavage have been measured as Δ~rH~m~,~3= -35.1kJ· mol^-^1 and Δ~rH~m~,~1=7.29kJ·mol^-^1, respectively, at 310.15K and pH 7.0. DNA backbone-cleavage has also been observed in agarose gel electrophoresis.

Key words: MICROCALORIMETRIC METHOD, DEOXYRIBONUCLEIC ACID, CUTTING, PHENANTHROLINE P, COPPER COMPLEX, MERCAPTO COMPOUNDS, ETHANOL P, AGAROSE COAGNLATION, GEL ELECTROPHORESIS, THERMODYNAMICAL MODEL