The 2-cell stage of mouse embryogenesis is characterized by two phases of α-amanitin-sensitive polypeptide synthetic activity, which appear to mark the first major expression of the embryonic genome, as assessed by examination of in vitro translates of mRNA. Using populations of embryos synchronized to the first cleavage division, we have established that DNA replication takes place over the period 1 to 5·5 h after the first cleavage division; the two bursts of putative transcription take place before and immediately after DNA replication, and the translation products are detectable in each case within 3–4 h. In addition, we have shown that suppression of cytokinesis and the second round of DNA replication does not affect synthesis of the α-amanitin-sensitive polypeptides, and that neither DNA replication nor the loss of maternal mRNA that take place during the 2-cell stage are dependent upon synthesis of the α-amanitin-sensitive polypeptides.

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