Introduction

Efficient duplication of eukaryotic genomes relies on the sequential activation of thousands of replication origins distributed along the chromosomes. This process follows a complex spatial and temporal program, which is under the control of epigenetic mechanisms and is tightly linked to the functional organization of the nucleus (Méchali, 2001). Recent evidence indicates that this replication program is also controlled by checkpoint kinases that monitor the correct execution of S phase, such as ATR in human and Mec1 in budding yeast (Tourriere and Pasero, 2007). Over the last twenty years, a wide variety of methods has been developed to map replication origins in eukaryotic cells (DePamphilis, 1997). These techniques have shown that eukaryotic genomes contain an excess of potential replication origins and that only a fraction of these origins fire within a given S phase. However, the dynamics of DNA replication remains poorly defined at the level of individual chromosomes, essentially because biochemical assays provide averaged replication profiles for a population of cells. [...]

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