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If there is a correlation between the site of a reaction in vivo and the localization of an enzyme, one would expect to recover DNA polymerase in the nuclear fraction. Assuming the usual relationship between the scale of operation in vivo and the amount of enzyme, one would expect nuclei from a source in which DNA synthesis is particularly rapid to provide maximum amounts of enzymes intimately concerned with DNA replication. During the early cleavage stages of sea urchin embryos, the number of nuclei doubles in two hours or less and the DNA, amounting to 1.8 X 10-12 gm per diploid nucleus, can double in about 10-12 min at 15'C. In an earlier report, it was shown that isolated nuclei from such embryos do have a high DNA polymerase activity and native DNA seemed to be preferentially used as a primer.1 In contrast, DNA polymerases obtained from homogenates of most eucaryotic tissues exhibit a marked preference for single-stranded DNA primers.2 Since priming by double-stranded DNA may be a characteristic of a replicative polymerase, this property has now been studied in detail.

PRIMING OF DNA POLYMERASE IN NUCLEI OF SEA URCHIN EMBRYOS BY NATIVE DNA