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In certain situations, however, replication forks may experience replisome dissociation and collapse.Fork collapse may be caused by proteinDNA complexes that cannot be efciently removed or by the run off of the replisome at telomeres.In most cases, fork collapse should not represent a problem in eukaryotes using multiple replicons, as forks converging from adjacent replicons can complete replication. Furthermore, considering that only a fraction of replication origins are red at each round of DNA synthesis, it is reasonable to assume that the excess origins represent a reserve of replicons for cells experiencing extensive fork collapse.However, if forks collapse at subtelomeric regions, where there are no converging forks, then completion of replication under these circumstances will require the restart of the collapsed forks.It is becoming clear that it is not the damaged DNA per se that generates the checkpoint signal but rather the collision of the fork with the lesion. However, it is still unclear whether the RPA laments represent the primary signal or whether they are just required to boost the checkpoint response.This could explain why the rfa mutants isolated so far are only partially checkpointdefective. Besides, a twostep mechanism would have the advantage of preventing futile activation of the pathway when events cause only transient chromosomal stress.The idea that a threshold is indeed required for checkpoint activation was indicated by ndings showing that an amount of ssDNA above a certain level or threshold must be produced in order to activate the checkpoint response. One important implication of these results is that lesions that inhibit the helicase from unwinding may not generate a checkpoint response. Replication forks encountering genomic pausing sites or lesions on the template may stall, accumulating checkpoint signals represented by long stretches of ssDNA coated by RPA that result from uncoupling of leading and lagging strand synthesis; checkpoint activation does not occur when the replication block prevents the helicase progression.Recent ndings suggest that several events, probably resulting from replication accidents, might escape checkpoint surveillance or be dealt with in an inappropriate manner either because of adaptation or because of faulty repair.This would explain the genomic instability of cells operating with a limited pool of replication proteins, the absence of checkpoint system activation during certain events that generate fork collapse, and the segmental duplications of chromosomes that occur spontaneously, preferentially in the slow, late replicating zones of the chromosomes. Electron microscopic analysis of the budding yeast rad mutant showed that an important function of the replication checkpoint is to protect the stability of stalled forks. In checkpoint mutants, stalled forks rapidly degenerate, accumulating gapped and hemireplicated molecules. Furthermore, a large fraction of forks accumulate fourbranched molecules resembling reversed forks.Recent observations have suggested that these fourway junctions do not form as a consequence of fork reversal but rather as a result of an active process causing the run off of specialized sister chromatid junctions. Replication checkpoints are involved in modulating the replication fork response to intraS damage by stabilizing the stalled fork and by preventing the ring of late origins and the unscheduled ring of dormant origins.

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