© The Rockefeller University Press,
0021-9525/1998//1159 $5.00
The Journal of Cell Biology, Volume 142, Number 5,
, 1998 1159-1166
Dynamics of the Genome during Early Xenopus laevis Development: Karyomeres As Independent Units of Replication
Jean-Marc Lemaitre,
Gérard Géraud, and
Marcel Méchali
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.
Key Words: DNA replication • cell cycle • chromosomes • nuclear envelope • development
Abbreviations used in this paper: FB, fixation buffer; RPA, replication protein A.
This study has been supported by grants from the CNRS, Association pour la Recherche sur le Cancer, the Ligue Nationale contre le Cancer, the Fondation de la Recherche Médicale, and the Groupements des entreprises dans la lutte contre le cancer.
Address all correspondence to M. Méchali, Institut de Génétique Humaine, CNRS, Laboratoire Dynamique du Génome et Développement, 141 rue de la Cardonille, 34096 Montpellier, Cedex 05 France. Tel.: (33) 4-99-61-99-17. Fax: (33) 4-99-61-99-20. E-mail: mechali{at}igh.cnrs.fr
The present address of all authors is Institut de Génétique Humaine, CNRS, Laboratoire Dynamique du Génome et Développement, 141 rue de la Cardonille, 34096 Montpellier, Cedex 05 France.
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