Several anti-cancer agents like cisplatin and etoposide damage DNA by forming irreversible crosslinks with DNA nucleotides.The main reason for the anti-cancer effects of these drugs is inhibition of DNA replication at the places of DNA damage.However, DNA replication is a complex process that requires fine coordination of several individual events necessary for correct duplication of genetic information and maintenance of genome stability.Replicative synthesis is preceded by unwinding of the parental DNA duplex.During unperturbed replication, replicative unwinding is normally coupled to DNA synthesis.Synchronised synthesis of leading and lagging strand DNA immediately follows replicative unwinding.Whether attenuation of DNA synthesis in damaged regions influences DNA unwinding still remains to be elucidated.To investigate whether blocked DNA synthesis affects replieative unwinding we used temperature sensitive yeast polymerase/primase mutants cdc1 7-1, pri2-1 and pri1-m4 which fail to execute the early step of DNA synthesis.We report that some of the plasmid molecules in these mutant strains became extensively unwound when DNA synthesis isprevented.The limited number of unwound plasmid molecules and synthetic lethality ofpolymerase or primase with checkpoint mutants suggest a checkpoint regulation of the replicative unwinding.In concordance with this suggestion, we found that the Tofl/Csm3/Mrel checkpoint complex interacts directly with the MCM helicase during both replication fork progression and when the replication fork is stalled.