To ensure that each DNA segment is only replicated once during the cell cycle, replication requires the 'licensing' of replication origins in late M and G1 phases. The origin recognition complex (ORC), Cdc6, Cdt1 and the Mcm2-7 helicase are needed for replication licensing in vitro, but little is known about the dynamic behaviour of licensing components in vivo. Blow and colleagues (J. Cell Biol. 196, 233–246; 2012) have now visualized replication licensing in living early Caenorhabditis elegans embryos by time-lapse microscopy using fluorescent protein tags.

The authors confirm that Mcm2-7 loading in late M phase depends on ORCs, Cdc6 and Cdt1, but also discover new features of these proteins. FRAP (fluorescence recovery after bleaching) experiments indicate that the dissociation of ORC and Cdc6 from chromatin is decreased in the absence of licensing, but promoted after loading of Mcm2-7, thus creating a negative feedback loop. They also identify the worm orthologues of ORC-3 and ORC-4. Interestingly, although Cdc6 is known to be exported from the nucleus, here ORC-2-5 and ORC-1 are also found to be excluded from the nucleus once the nuclear envelope has reformed after mitosis. Disrupting nuclear export by depleting the transport protein exportin causes extensive re-replication, consistent with the concept that nuclear exclusion of replication proteins is necessary to prevent DNA re-duplication in this system.