Metabolic DNA in heart and skeletal muscle and in the intestine of mice [41]

STUDIES of the incorporation of labelled thymidine into the DNA of heart muscle and skeletal muscle in adult animals have shown significant values which indicate regular renewal of some or all of the DNA in these non-dividing organs^1,2. Autoradiography has shown labelled muscle nuclei in smooth muscle and in heart muscle and therefore subsidiary cells, such as lymphocytes, cannot be held responsible for the labelling. The existence in plant materials of two separable fractions of DNA with different rates of turnover has been shown by Sampson et al.³, and Sampson and Davies⁴ have presented good evidence for the location of a fraction (DNA1) with a high rate of turnover in differentiating cells and of a stable fraction (DNA2) in meristematic cells of Vicia faba. The results of experiments designed to compare the specific activities of the DNA fractions with a molecular weight below 4 × 10⁶ with those above 4 × 10⁶ in heart, skeletal muscle and intestine are reported here.