Uncoordinated synthesis of histones and DNA by mouse eggs and preimplantation embryos

The temporal coordination of histone and DNA synthesis in mouse eggs and preimplantation embryos were examined. In unfertilized mouse eggs arrested at metaphase II, histone synthesis was observed, but at lower rates than previously reported for fertilized zygotes. Cytosine arabinoside, an inhibitor of DNA synthesis, was used to study the relationship between DNA and histone synthesis during cleavage stages of mouse development. Cytosine arabinoside (50 microM) inhibited by 90% the incorporation of [3H]thymidine into DNA by two-cell embryos and blastocysts. At this concentration, cytosine arabinoside inhibited incorporation of [3H]lysine into blastocyst histones by only 30% and had no effect at the two-cell stage. These results are interpreted to show that DNA synthesis and histone synthesis are not coordinated temporally during early development of the mouse, but may become more so as development proceeds.