Competence to replicate in the unfertilized egg is conferred by Cdc6 during meiotic maturation


Meiotic maturation, the final step of oogenesis, is a crucial stage of development in which an immature oocyte becomes a fertilizable egg1. In Xenopus, the ability to replicate DNA is acquired during maturation at breakdown of the nuclear envelope2 by translation of a DNA synthesis inducer that is not present in the oocyte2,3. Here we identify Cdc6, which is essential for recruiting the minichromosome maintenance (MCM) helicase to the pre-replication complex, as this inducer of DNA synthesis. We show that maternal cdc6 mRNA but not protein is stored in the oocyte. Cdc6 protein is synthesized during maturation, but this process can be blocked by degrading the maternal cdc6 mRNA by oligonucleotide antisense injections or by translation inhibition. Rescue experiments using recombinant Cdc6 protein show that Cdc6 is the only missing replication factor whose translation is necessary and sufficient to confer DNA replication competence to the egg before fertilization. The licence to replicate is given by Cdc6 at the end of meiosis I, but the cytostatic factor (CSF) pathway, which maintains large amounts of active Cdc2/Cyclin B2, prevents the entry into S phase until fertilization.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Cdc6 is missing in Xenopus oocytes but accumulates during oocyte maturation soon after GVBD.
Figure 2: cdc6 mRNA stored in the oocyte is translated after GVBD.
Figure 3: Cdc6 is the DNA replication induced activity acquired in vivo during maturation.
Figure 4: Cdc6 protein is the missing factor acquired soon after GVBD that confers to the egg the competence to replicate.
Figure 5: Interplay between replication induced by cdc6 and its negative control by MPF.


  1. 1

    Masui, Y. & Clarke, H. J. Oocyte maturation. Int. Rev. Cytol. 57, 185–282 (1979)

    CAS  Article  Google Scholar 

  2. 2

    Gurdon, J. B. On the origin and persistence of a cytoplasmic state inducing nuclear DNA synthesis in frogs eggs. Proc. Natl Acad. Sci. USA 58, 545–552 (1967)

    ADS  CAS  Article  Google Scholar 

  3. 3

    Furuno, N. et al. Suppression of DNA replication via Mos function during meiotic divisions in Xenopus oocytes. EMBO J. 13, 2399–2410 (1994)

    CAS  Article  Google Scholar 

  4. 4

    Kelly, T. J. & Brown, G. W. Regulation of chromosome replication. Annu. Rev. Biochem. 69, 829–880 (2000)

    CAS  Article  Google Scholar 

  5. 5

    Lei, M. & Tye, B. K. Initiating DNA synthesis: from recruiting to activating the MCM complex. J. Cell Sci. 114, 1447–1454 (2001)

    CAS  PubMed  Google Scholar 

  6. 6

    Coleman, T. R., Carpenter, P. B. & Dunphy, G. The Xenopus Cdc6 protein is essential for the initiation of a single round of DNA replication in cell-free extracts. Cell 87, 53–63 (1996)

    CAS  Article  Google Scholar 

  7. 7

    Pelizon, C., Madine, M. A., Romanowski, P. & Laskey, R. A. Unphosphorylatable mutants of Cdc6 disrupt its nuclear export but still support DNA replication once per cell cycle. Genes Dev. 14, 2526–2533 (2000)

    CAS  Article  Google Scholar 

  8. 8

    Okuno, Y., McNairn, A. J., den Elzen, N., Pines, J. & Gilbert, D. M. Stability, chromatin association and functional activity of mammalian pre-replication complex proteins during the cell cycle. EMBO J. 20, 4263–4277 (2001)

    CAS  Article  Google Scholar 

  9. 9

    Coue, M., Kearsey, S. E. & Mechali, M. Chromatin binding, nuclear localization and phosphorylation of Xenopus Cdc21 are cell-cycle dependent and associated with the control of initiation of DNA replication. EMBO J. 15, 1085–1097 (1996)

    CAS  Article  Google Scholar 

  10. 10

    Pereverzeva, I., Whitmire, E., Khan, B. & Coue, M. Distinct phosphoisoforms of the Xenopus Mcm4 protein regulate the function of the Mcm complex. Mol. Cell. Biol. 20, 3667–3676 (2000)

    CAS  Article  Google Scholar 

  11. 11

    Hara, K., Tydeman, P. & Kirschner, M. A cytoplasmic clock with the same period as the division cycle in Xenopus eggs. Proc. Natl Acad. Sci. USA 77, 462–466 (1980)

    ADS  CAS  Article  Google Scholar 

  12. 12

    Harland, R. M. & Laskey, R. A. Regulated replication of DNA microinjected into eggs of Xenopus laevis. Cell 21, 761–771 (1980)

    CAS  Article  Google Scholar 

  13. 13

    Blow, J. J. & Laskey, R. A. Initiation of DNA replication in nuclei and purified DNA by a cell-free extract of Xenopus eggs. Cell 47, 577–587 (1986)

    CAS  Article  Google Scholar 

  14. 14

    Menut, S., Lemaitre, J. M., Hair, A. & Méchali, M. in Advances in Molecular Biology: a Comparative Methods Approach to the Study of Ooocytes and Embryos (ed. Richter, J. D.) 196–226 (Oxford Univ. Press, Oxford, 1999)

    Google Scholar 

  15. 15

    Sagata, N., Watanabe, N., Vande Woude, G. F. & Ikawa, Y. The c-mos proto-oncogene product is a cytostatic factor responsible for meiotic arrest in vertebrate eggs. Nature 342, 512–518 (1989)

    ADS  CAS  Article  Google Scholar 

  16. 16

    Yew, N., Mellini, M. L. & Vande Woude, G. F. Meiotic initiation by the mos protein in Xenopus. Nature 355, 649–652 (1992)

    ADS  CAS  Article  Google Scholar 

  17. 17

    Nebreda, A. R., Gannon, J. V. & Hunt, T. Newly synthesized protein(s) must associate with p34cdc2 to activate MAP kinase and MPF during progesterone-induced maturation of Xenopus oocytes. EMBO J. 14, 5597–5607 (1995)

    CAS  Article  Google Scholar 

  18. 18

    Hochegger, H. et al. New B-type cyclin synthesis is required between meiosis I and II during Xenopus oocyte maturation. Development 128, 3795–3807 (2001)

    CAS  Google Scholar 

  19. 19

    Abrieu, A., Doree, M. & Fisher, D. The interplay between cyclin-B–Cdc2 kinase (MPF) and MAP kinase during maturation of oocytes. J. Cell Sci. 114, 257–267 (2001)

    CAS  PubMed  Google Scholar 

  20. 20

    Masui, Y. The elusive cytostatic factor in the animal egg. Nature Rev. Mol. Cell. Biol. 1, 228–232 (2000)

    CAS  Article  Google Scholar 

  21. 21

    Reimann, J. D. & Jackson, P. K. Emi1 is required for cytostatic factor arrest in vertebrate eggs. Nature 416, 850–854 (2002)

    ADS  CAS  Article  Google Scholar 

  22. 22

    Weinreich, M., Liang, C., Chen, H. H. & Stillman, B. Binding of cyclin-dependent kinases to ORC and Cdc6p regulates the chromosome replication cycle. Proc. Natl Acad. Sci. USA 98, 11211–11217 (2001)

    ADS  CAS  Article  Google Scholar 

  23. 23

    Calzada, A., Sacristan, M., Sanchez, E. & Bueno, A. Cdc6 cooperates with Sic1 and Hct1 to inactivate mitotic cyclin-dependent kinases. Nature 412, 355–358 (2001)

    ADS  CAS  Article  Google Scholar 

  24. 24

    Madine, M. A. et al. The roles of the MCM, ORC, and Cdc6 proteins in determining the replication competence of chromatin in quiescent cells. J. Struct. Biol. 129, 198–210 (2000)

    CAS  Article  Google Scholar 

  25. 25

    Mendez, J. & Stillman, B. Chromatin association of human origin recognition complex, cdc6, and minichromosome maintenance proteins during the cell cycle: assembly of prereplication complexes in late mitosis. Mol. Cell. Biol. 20, 8602–8612 (2000)

    CAS  Article  Google Scholar 

  26. 26

    Tian, J., Thomsen, G. H., Gong, H. & Lennarz, W. J. Xenopus Cdc6 confers sperm binding competence to oocytes without inducing their maturation. Proc. Natl Acad. Sci. USA 94, 10729–10734 (1997)

    ADS  CAS  Article  Google Scholar 

  27. 27

    Lemaitre, J. M., Geraud, G. & Mechali, M. Dynamics of the genome during early Xenopus laevis development: karyomeres as independent units of replication. J. Cell Biol. 142, 1159–1166 (1998)

    CAS  Article  Google Scholar 

  28. 28

    Taylor, M. V., Gusse, M., Evan, G. I., Dathan, N. & Mechali, M. Xenopus myc proto-oncogene during development: expression as a stable maternal mRNA uncoupled from cell division. EMBO J. 5, 3563–3570 (1986)

    CAS  Article  Google Scholar 

  29. 29

    Maiorano, D., Moreau, J. & Mechali, M. XCDT1 is required for the assembly of pre-replicative complexes in Xenopus laevis. Nature 404, 622–625 (2000)

    ADS  CAS  Article  Google Scholar 

  30. 30

    Whitmire, E., Khan, B. & Coué, M. Cdc6 synthesis regulates replication competence in Xenopus oocytes. Nature 419, 722–725 (2002)

    ADS  CAS  Article  Google Scholar 

Download references


We thank P. Françon for help; N. Montel for technical assistance; and B. Honda, D. Maiorano, D. Fisher and C. Jaulin for critically reading the manuscript. This work has been supported by grants from the Association pour la Recherche sur le Cancer and the Human Frontier Science Program.

Author information



Corresponding author

Correspondence to Marcel Méchali.

Ethics declarations

Competing interests

The authors declare that they have no competing financial interests.

Supplementary information

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Lemaître, J., Bocquet, S. & Méchali, M. Competence to replicate in the unfertilized egg is conferred by Cdc6 during meiotic maturation. Nature 419, 718–722 (2002).

Download citation

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Sign up for the Nature Briefing newsletter for a daily update on COVID-19 science.
Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing