Abstract
When high concentrations of deoxyribonucleic acid (DNA) were included in the homogenization medium, it was possible to isolate a microsomal fraction from HeLa cells which contained a major part of the newly synthesized DNA of the cells. Kinetic studies with very short exposure times suggested that dTR-H3 incorporation into this fraction actually preceded any incorporation into the nuclei. An examination of the distribution of DNA polymerase activity in the cells showed that 90% of this enzyme was in the soluble fraction, 10% on the microsomes, and essentially none in the nuclei.