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J. Cell Biol.,
Volume 142, Number 5, September 7, 1998 1159-1166
Institut Jacques Monod, Centre National de la Recherche Scientifique (CNRS), Laboratoire d'Embryologie Moléculaire, 75251 Paris Cedex 05, France
During Xenopus laevis early development,
the genome is replicated in less than 15 min every 30 min. We show that during this period, DNA replication
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 condensed chromosomes during anaphase and that DNA
replication initiates autonomously in karyomeres at
early telophase before nuclear reconstruction and mitosis completion. The formation of karyomeres is not
dependent on DNA replication but requires mitotic
spindle formation and the normal segregation of chromosomes. Thus, during early development, chromosomes behave as structurally and functionally independent units. The formation of a nuclear envelope around
each chromosome provides an in vivo validation of its
role in regulating initiation of DNA replication, enabling the rate of replication to accelerate and S phase
to overlap M phase without illegitimate reinitiation.
The abrupt disappearance of this atypical organization
within one cell cycle after thirteen divisions defines a
novel developmental transition at the blastula stage,
which may affect both the replication and the transcription programs of development.
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