Directing cell division during development.

Several evolutionarily conserved proteins constitute a universal mitotic trigger that is precisely controlled during the orderly cell divisions of embryogenesis. As development progresses, the mechanisms controlling this trigger change. Early divisions are executed by maternally synthesized gene products, and in Xenopus they are timed by the accumulation and periodic degradation of cyclin, a trigger component. Later, the zygotic genome assumes control, and in Drosophila, zygotic transcription is required for production of another trigger protein, the product of string. After this transition to zygotic control, pulses of string transcription define the timing of highly patterned embryonic cell divisions and cyclin accumulation is not rate limiting.