Experimental evidence suggests that exposure of sea urchin gastrulae to 5-bromodeoxyuridine (BUdR), an analog of thymidine, causes a reduction in the rate of synthesis of some RNA species usually transcribed at this stage of development. In pulse-chase experiments, 3H (in gastrula stage 8–15S nuclear DNA labeled with BUdR-3H) could not be chased, with unlabeled BUdR, into 20–60S size-range DNA; in similar experiments in which gastrulae were pulsed with thymidine-3H and then chased with unlabeled thymidine, 3H in 8–15S nuclear DNA could be extensively chased into 20–60S DNA. DNA-RNA hybridization assays indicate that gastrula stage nuclear DNA in the range of 20–60S has greater sequence homology for gastrula stage RNA-3H than does nuclear DNA of similar size-range taken from gastrulae exposed to BUdR for 1 hr. An explanation is offered for the effect of BUdR on transcription and DNA replication in sea urchin embryos.

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