Updated project metadata. Preblastoderm Drosophila embryos replicate their genomes at maximal speed. The assembly of chromatin on the newly synthesized chromosomes relies entirely on high concentrations of maternal nucleosome assembly and remodelling factors. Extracts from preblastoderm embryos can be turned into a system for highly efficient assembly of complex chromatin. We serendipitously discovered that this chromatin assembly system contains numerous activities that recognise unprotected DNA ends and initiate DNA damage signalling cascades. Within minutes the phosphorylation of H2A.V (homologous to H2A.X) initiates from DNA breaks and spreads over tens of thousands DNA base pairs. The phosphorylation of H2A.V remains tightly associated with the damaged DNA in cis and does not spread to intact DNA circles in the same reaction. The ultrafast cycles of syncytial nuclei divisions of cleavage-stage embryos do not allow sufficient time to repair DNA breaks. Rather, fatally damaged nuclei are eliminated by a programmed ‘mitotic catastrophe’. Our descriptions of the damage-related proteome and DNA break-dependent phospho-proteome suggest that certain aspects of the DNA damage checkpoint that leads to the elimination of nuclei can be studied in this cell-free system. Our data provide a rich resource for in-depth mechanistic analyses of the DNA break response in this model system.