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During cell division, chromatin alternates between a condensed state to facilitate chromosome segregation and a decondensed form when DNA replicates. In most tissues, S phase and mitosis are separated by defined G1 and G2 gap phases, but early embryogenesis involves rapid oscillations between replication and mitosis. Using Caenorhabditis elegans embryos as a model system, we show that chromosome condensation and condensin II concentration on chromosomal axes require replicated DNA. Inaddition, we found that, during late telophase, replication initiates on condensed chromosomes and promotes the rapid decondensation of the chromatin.Upon replication initiation, the CDC-45-MCM-GINS (CMG) DNA helicase drives the release of condensin I complexes from chromatin and the activation or displacement of inactive MCM-2-7 complexes, which together with the nucleoporin MEL-28/ELYS tethers condensed chromatin to the nuclear envelope, thereby promoting chromatin decondensation. Our results show how, in an early embryo, the chromosome-condensation cycle is functionally linked with DNA replication. Chromatin organization changes dramatically within a cell cycle. Sonneville etal. find that in rapidly dividing C.elegans embryos, DNA replication initiation triggers rapid decondensation of chromatids. In addition, DNA replication promotes chromosome condensation in prophase. Therefore, the chromosome-condensation cycle and DNA replication are functionally linked.