The DNA damage checkpoint in embryonic cell cycles is dependent on the DNA-to-cytoplasmic ratio.

In Xenopus, cell cycle checkpoints monitoring DNA damage, DNA replication, and spindle assembly do not appear until after the midblastula transition (MBT; 4000 cells). We show that a DNA damage checkpoint can slow the cell cycle even in 2-cell embryos when the DNA content is increased. Slowing follows caffeine-sensitive activation of the checkpoint kinase, Chk1; degradation of the cell cycle phosphatase, Cdc25A; and inhibitory phosphorylation of Cdc25C and cyclin-dependent kinases (Cdks). Alterations in the DNA-to-cytoplasmic ratio elicit a dose-dependent DNA damage checkpoint, and the ratio required to activate Chk1 for the damage response is lower than that associated with "developmental" activation of Chk1 shortly after the MBT. Our results indicate that a maternal damage response, independent of zygotic transcription, is present even in very early embryos, and requires both double-stranded DNA ends and a threshold DNA-to-cytoplasmic ratio to significantly affect the cell cycle.