Noncoincidence of histone and DNA synthesis in cleavage cycles of early development.

The times of histone and DNA synthesis do not coincide in early cleavage of the sea urchin embryo. In fact, the production of histones increases during the interval after DNA synthesis (G2). Enucleate merogones, parthenogenetically activated, synthesize histones encoded upon maternal histone messenger RNA. The pattern of protein synthesis changes following fertilization, in part, but not solely, due to the increasing synthesis of histones relative to other proteins. Regulation of histone synthesis and the loading of newly replicated DNA with histones must themselves undergo change at the time of transition from cleavage cycles to cycles more typical of somatic cells.