An unused license to fire

Not all available origins of replication fire during a normal S-phase. But when replication is perturbed, otherwise dormant origins go to work, Woodward et al. show on page 673.

Cells initially respond to slowed replication by turning on the ATR-dependent checkpoint, which prevents other origins from firing and thus getting into trouble too. But if the cell decides it is time to recover from that checkpoint, the mechanism discovered by Woodward et al. may ensure that there are enough origins to get the job done.

The excess supply of origins arises from an excess of sites that have the minichromosome maintenance protein complexes, Mcm2-7. These complexes are loaded onto chromatin before S-phase and are required to license replication origins for use. However, the number of complexes loaded is much higher than the number normally...