REPLICATION OF PURIFIED DNA INTRODUCED INTO LIVING EGG CYTOPLASM

The injection of purified DNA into unfertilized frogs' eggs stimulates a 20-fold or greater increase of [3H]dT incorporation into DNA. The labelled DNA has been identified and characterised by methylated serum albumin kieselguhr chromatography and by buoyant density centrifugation in CsCl. Incorporation is stimulated by native and denatured DNA of vertebrates but only by denatured DNA of bacteria. Heat-denatured DNA stimulates 2-3 times as much incorporation as native DNA. Denatured DNA stimulates incorporation immediately after injection but a 20-min delay is observed before incorporation commences after injection of native DNA. A greater amount of incorporation is stimulated by increasing concentrations of injected DNA. The labelled DNA extracted from eggs which received denatured DNA as well as from those which received native DNA is 2-stranded. The amount of incorporation stimulated by native DNA is comparable to the amount of DNA synthesis that takes place in injected whole nuclei having an equivalent content of DNA. The average base composition of the labelled DNA extracted from injected eggs resembles that of the DNA injected and not that of egg chromosomal DNA. This was demonstrated by use of Xenopus DNA complementary to ribosomal RNA and of certain bacterial DNA's, all having a higher buoyant density than the average Xenopus chromosomal DNA. It is concluded that egg cytoplasm contains an enzyme system capable of replicating native DNA of vertebrate but not of bacterial origin, and that the components of egg or sperm nuclei other than their DNA are not essential for the initiation of the nuclear DNA synthesis that normally follows fertilization. © 1969.