[State of DNA structural alkalinity in tissue of rats of different ages]

The single-strand breaks in DNA of cell nuclei of the normal and regenerating liver and kidneys are studied depending on the rat age by the sedimentation analysis within the gradient of alkaline sucrose without previous purification and isolation of the preparations. It is found that DNA of tissues under study in the six-month rats sedimentates, forming one fraction of high-molecular DNA (molecular mass 1.3-1.5 x 10(8) Daltons). In sedimentograms of DNA of the same tissues of 24-month rats there appear additional peaks which correspond to the fractions with the low molecular mass (1.6 x 10(7) and 1.2 x 10(5) Daltons), which evidences for the presence of single-strand breaks in DNA of old animals.