[Site of nuclear DNA replication in embryonic cells of the sea urchin Strongylocentrotus intermedius].

In the sea urchin embryonic cells, all newly synthesized nuclear DNA (n-DNA) pulse-labeled by 3H-thymidine was found within DNA-membrane complex (DNA-mc) isolated by centrifugation of lysates of nuclei after their treatment with Sarkosyl, Brij-35, or sodium dodecylsulfate through neutral sucrose (10--30%) gradients. This attachment has been shown not to be an artifact due to the unspecific effect of the detergents or the destabilization of the secondary structure of n-DNA because the association of the exogenous 14C-DNA with nuclear membrane and chromatin did not occur during the isolation of the DNA-mc. n-DNA was not replaced from DNA-mc when the latter was isolated in the excess of unlabeled denatured DNA. n-DNA associated with DNA-mc behaved as a precursor of chromosomal DNA. It is suggested that in sea urchin embryonic cells the synthesis of nuclear DNA is carried out by the replicative complex attached to the nuclear membrane.