To prevent re-replication of genomic segments, the eukaryotic cell cycle is divided into two non-overlapping phases. During late mitosis and G1 replication origins are 'licensed' by loading MCM2-7 double hexamers and during S phase licensed replication origins activate to initiate bidirectional replication forks. Replication forks can stall irreversibly, and if two converging forks stall with no intervening licensed origin - a 'double fork stall' (DFS) - replication cannot be completed by conventional means. We have been exploring how eukaryotic cells can esnure that the genome is precisely duplicated in light of these important constraints. This work is highly interdisciplinary, combining experimantal with mathemetical and computer modelling.
We have developed a simple mathematical model to describe the probability of replication failing due to the irreversible stalling of replication forks. We show that for yeasts, with genome sizes in the order of ~10 Mbp, the probability of complete genome replication is maximised if replication origins are evenly spaced, the largest inter-origin distances are minimised, and the end-most origins are positioned close to chromosome ends. In contrast, our analysis predicts that DFSs are highly likely to occur in large mammalian genomes. We have gone on to show that complementary strand synthesis in early mitosis, ultrafine anaphase bridges and G1-specific 53BP1 nuclear bodies provide a mechanism for resolving unreplicated DNA at DFSs in human cells. Our results provide a striking convergence of experimental and theoretical evidence that unreplicated DNA can pass through mitosis for resolution in the following cell cycle.
Blow, J.J. and Ge, X.Q. (2009). A model for DNA replication showing how inefficient origins safeguard against fork failure. EMBO Rep 10, 406-412. [abstract]
Newman, T.J., Mamun, M.A., Nieduszynski, CA. and Blow, J.J. (2013). Replisome stall events have shaped the distribution of replication origins in the genomes of yeasts. Nucleic Acids Res. 41, 9705-9718. [abstract] [pdf]
Moreno, A., Carrington, J.T., Albergante, L., Al Mamum, M., Haagensen, E.J., Komseli, E.-S., Gourgolis, V.G., Newman, T.J. and Blow, J.J. (2016). Unreplicated DNA remaining from unperturbed S phases passes through mitosis for resolution in daughter cells. Proc Natl Acad Sci USA, in press.