DNA synthesis in oocytes and eggs of Xenopus laevis injected with DNA

Abstract

Single-stranded calf thymus DNA injected into preovulation oocytes, postovulation oocytes or eggs of Xenopus laevis induces synthesis of double-stranded DNA of similar base composition. In contrast, native (double-stranded) calf thymus DNA injected into oocytes does not stimulate DNA synthesis, though it does do so in eggs. The buoyant density of normal or IUdR-substituted newly-synthesized DNA on neutral or alkaline CsCl gradients suggests that the injected DNA is replicated. The amount of synthesis induced by injecting single-stranded DNA is five times greater in eggs than in oocytes. The maximum synthesis observed in eggs injected with native DNA is 50 pg/hr; this is sufficient for nuclear DNA replication in uninjected fertilised eggs, but not in midcleavage. However in vitro studies (reported elsewhere) indicate the presence of a large store of DNA polymerase activity in eggs. We conclude that only a small proportion of the total DNA polymerase activity in an egg is available for DNA synthesis during the first 2 hr of development.