DYNAMICS OF THE GENOME DURING EARLY XENOPUS-LAEVIS DEVELOPMENT - KARYOMERES AS INDEPENDENT UNITS OF REPLICATION

During Xenopus laevis early development, the genome is replicated in l ess than 15 min every 30 min. We show that during this period, DNA rep lication proceeds in an atypical manner. Chromosomes become surrounded by a nuclear membrane lamina forming micronuclei or karyomeres, This genomic organization permits that prereplication centers gather on con densed chromosomes during anaphase and that DNA replication initiates autonomously in karyomeres at early telophase before nuclear reconstru ction and mitosis completion. The formation of karyomeres is not depen dent on DNA replication but requires mitotic spindle formation and the normal segregation of chromosomes. Thus. during early development, ch romosomes behave as structurally and functionally independent units. T he formation of a nuclear envelope around each chromosome provides an in vivo validation of its role in regulating initiation of DNA replica tion, enabling the rate of replication to accelerate and S phase to ov erlap M phase without illegitimate reinitiation. The abrupt disappeara nce of this atypical organization within one cell cycle after thirteen divisions defines a novel developmental transition at the blastula st age, which may affect both the replication and the transcription progr ams of development.