DNA synthesis by isolated embryonic nuclei of Xenopus

Nuclei, isolated in sucrose solutions, from adult and embryonic tissues of Xenopus laevis, incorporate TdR-³H when incubated with DNA precursors and tritiated thymidine. This incorporation is DNase-sensitive and requires all 4 deoxyribonucleoside triphosphates. The proportion of nuclei which synthesize DNA in a given time in vitro is very similar to the proportion which do so in vivo in the tissue or developmental stage from which the nuclei have been isolated. This suggests that nuclei which are in the S phase of the cell cycle at the time of isolation continue to synthesize DNA when incubated in vitro but that DNA synthesis is not initiated or prevented in nuclei by the process of isolating them.