Synchronized mouse strain L-cells were pulse-labeled with [14C]thymidine at various stages of the DNA synthetic (S) period. Satellite DNA was then separated from main band DNA by buoyant density gradient centrifugation in Ag+-Cs2SO4 and by CsCl gradient centrifugation of heated and annealed DNA. Centrifugation in analytical alkaline CsCl density gradients showed that the satellite was free of contamination by main band DNA. Changes in the relative specific activities of the main band and pure satellite DNA fractions during the S period confirmed our earlier report that most satellite DNA synthesis occurs after main band DNA has reached its maximum rate of synthesis and did not support the interpretation that the pattern we observed was due to contaminating main band DNA of low G+C content in the satellite DNA fractions. There was no preferential synthesis of satellite DNA in populations of mitotic L-cells.
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