High levels of origin licensing during Xenopus cleavage divisions ensures complete and timely genome duplication

Cells face several challenges to completing genome duplication. One challenge is the irreversible stalling of converging replication forks (‘double fork stalls’). Cell types that cannot delay mitotic entry must also ensure that no replication origins are too far apart (the ‘random gap problem’). We show how these challenges can be met in early Xenopus embryos by the very abundant licensing of replication origins: one MCM2-7 double hexamer every ∼250 bp. Licensing does not change nucleosome spacing, consistent with MCM2-7 being assembled onto inter-nucleosomal linker DNA. We show that later embryonic development can occur successfully with a per-cell cycle failure rate of <0.2% in early embryos. The high density of licensed origins in the early embryo reduces cell cycle failures from random gaps and from double fork stalls to levels compatible with subsequent development, suggesting that Xenopus early embryonic cells can ensure complete genome duplication without requiring unconventional replication mechanisms.