Pre-replication complex proteins assemble at regions of low nucleosome occupancy within the Chinese hamster dihydrofolate reductase initiation zone

On the opportunistic nature of transcription and replication initiation in the metazoan genome

Cellular identity and its response to external or internal signalling variations are encoded in a cell's genome as regulatory information. The genomic regions that specify this type of information are highly variable and degenerated in their sequence determinants, as it is becoming increasingly evident through the application of genome-scale methods to study gene expression. Here, we speculate that the same scenario applies to the regulatory regions controlling where DNA replication starts in the metazoan genome. We propose that replication origins cannot be defined as unique genomic features, but rather that DNA synthesis initiates opportunistically from accessible DNA sites, making cells highly robust and adaptable to environmental or developmental changes.

Defining the replication program through the chromatin landscape

DNA replication is an essential cell cycle event required for the accurate and timely duplication of the chromosomes. It is essential that the genome is replicated accurately and completely within the confines of S-phase. Failure to completely copy the genome has the potential to result in catastrophic genomic instability. Replication initiates in a coordinated manner from multiple locations, termed origins of replication, distributed across each of the chromosomes. The selection of these origins of replication is a dynamic process responding to both developmental and tissue-specific signals. In this review, we explore the role of the local chromatin environment in regulating the DNA replication program at the level of origin selection and activation. Finally, there is increasing molecular evidence that the DNA replication program itself affects the chromatin landscape, suggesting that DNA replication is critical for both genetic and epigenetic inheritance.