Spatial organization and dynamics of genome replication: from forks to foci

Genome replication is without doubt the most complex and critical activity of all living cells. Any error may not only affect the cell itself but may lead to disease and, thus, kill the entire organism or be transmitted to subsequent generations via the germline. The task of replicating the genome "once and only once" becomes increasingly challenging with larger genomes and requires efficiency paired with coordination within the cell nucleus. While biochemical studies using frog egg extracts and genetic studies using bacteria and yeast have identified the basic machinery, it was the combination with cellular and molecular studies that yielded a comprehensive view of genome replication. Here, we focus on DNA replication in mammals as they pose a challenge in terms of genome size (and age) and total number of cell division cycles per lifetime, coupled with a variety of pathologies. We discuss how studies at the cellular level provided a framework for understanding the progression of DNA replication throughout the S phase. We highlight how pulse labeling and time lapse studies in combination with genome sequencing technologies are providing a comprehensive view of how large genomes are efficiently and precisely replicated every day in trillions of cells.